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**Fourier Representations of Signals & LTI Systems**

3 CHAPTER Fourier Representations of Signals & LTI Systems 3.1 Introduction 1. A signal can be represented as a weighted superposition of complex sinusoids. x(t) or x[n] LTI System y(t) or y[n] 2. LTI system: Output = A weighted superposition of the system response to each complex sinusoid. 3. Four distinct Fourier representations 3.2 Complex Sinusoids and Frequency Response of LTI Systems Frequency response The response of an LTI system to a sinusoidal input. Discrete-time LTI system 1. Impulse response of discrete-time LTI system = h[n], input = x[n] = e j n 2. Output:

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**Complex scaling factor A function of frequency **

3 CHAPTER Fourier Representations of Signals & LTI Systems Figure 3.1 (p. 196) The output of a complex sinusoidal input to an LTI system is a complex sinusoid of the same frequency as the input, multiplied by the frequency response of the system. Complex scaling factor 3. Frequency response: A function of frequency (3.1) Continuous-time LTI system 1. Impulse response of continuous-time LTI system = h(t), input = x(t) = e j t 2. Output: (3.2)

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**Fourier Representations of Signals & LTI Systems**

3 CHAPTER Fourier Representations of Signals & LTI Systems 3. Frequency response: (3.3) Polar form complex number c = a + jb: where Polar form for H (j): where Example 3.1 RC Circuit: Frequency response The impulse response of the system relating to the input voltage to the voltage across the capacitor in Fig. 3.2 is derived in Example 1.21 as Find an expression for the frequency response, and plot the magnitude and phase response.

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**3 Fourier Representations of Signals & LTI Systems <Sol.>**

CHAPTER Fourier Representations of Signals & LTI Systems <Sol.> Frequency response: Figure 3.2 (p. 197) RC circuit for Example 3.1. Magnitude response: Phase response: Fig. 3.6 (a) and (b)

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**Fourier Representations of Signals & LTI Systems**

3 CHAPTER Fourier Representations of Signals & LTI Systems Figure 3.3 (p. 198) Frequency response of the RC circuit in Fig (a) Magnitude response. (b) Phase response. Eigenvalue and eigenfunction of LTI system [A] Continuous-time case: 1. Eigenrepresentation: Fig. 3.4 (a)

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**Fourier Representations of Signals & LTI Systems**

3 CHAPTER Fourier Representations of Signals & LTI Systems H H H Figure 3.4 (p. 198) Illustration of the eigenfunction property of linear systems. The action of the system on an eigenfunction input is multiplication by the corresponding eigenvalue. (a) General eigenfunction (t) or [n] and eigenvalue . (b) Complex sinusoidal eigenfunction e jt and eigenvalue H(j). (c) Complex sinusoidal eigenfunction e jn and eigenvalue H(ej). Eigenfunction: Eigenvalue: 2. If ek is an eigenvector of a matrix A with eigenvalue k, then Arbitrary input = weighted superpositions of eigenfunctions Convolution operation Multiplication Ex. Input: Output:

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**Fourier Representations of Signals & LTI Systems**

3 CHAPTER Fourier Representations of Signals & LTI Systems [B] Discrete-time case: 1. Eigenrepresentation: Fig. 3.4 (c) Eigenfunction: Eigenvalue: 2. Input: Output: 3.3 Fourier Representations for Four classes of Signals Four distinct Fourier representations: Table 3.1.

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**3 Fourier Representations of Signals & LTI Systems**

CHAPTER Fourier Representations of Signals & LTI Systems 3.3.1 Periodic Signals: Fourier Series Representations 1. x[n] = discrete-time signal with fundamental period N. DTFS of x[n] is o = 2/N Fundamental frequency of x[n] (3.4) 2. x(t) = continuous-time signal with fundamental period T. FS of x(t) is o = 2/T Fundamental frequency of x(t) (3.5) “^” denotes approximate value. A[k] = the weight applied to the kth harmonic. 3. The complex sinusoids exp(jkon) are N-periodics in the frequency index k. <pf.> There are only N distinct complex sinusoids of the form exp(jk0n) should be used in Eq. (3.4). (3.6) Eq. (3.4) If x[n] is symmetric in odd or even, we can choose k as: k = (N 1)/2 to (N 1)/2

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**Fourier Representations of Signals & LTI Systems**

3 CHAPTER Fourier Representations of Signals & LTI Systems 4. Continuous-time complex sinusoid exp(jk0t) with distinct frequencies k0 are always distinct. FS of continuous-time periodic signal x(t) becomes (3.7) Mean-square error (MSE) between the signal and its series representation: Discrete-time case: (3.8) Continuous-time case: (3.9) 3.3.2 Nonperiodic Signals: Fourier-Transform Representations 1. FT of continuous-time signal: X(j)/(2) = the weight applied to a sinusoid of frequency in the FT representation.

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**3 3.4 Discrete-Time Periodic Signals: The Discrete-Time Fourier Series**

CHAPTER Fourier Representations of Signals & LTI Systems 2. DTFT of discrete-time signal: X(e j )/(2) = the weight applied to the sinusoid ej n in the DTFT representation. 3.4 Discrete-Time Periodic Signals: The Discrete-Time Fourier Series 1. DTFS pair of periodic signal x[n]: Fundamental period = N; Fundamental frequency = o = 2/N (3.10) Fourier coefficients; Frequency domain representation (3.11) Notation: Example 3.2 Determining DTFS Coefficients Find the frequency domain representation of the signal depicted in Fig. 3.5. <Sol.> 1. Period: N = 5 o = 2/5

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**3 Fourier Representations of Signals & LTI Systems**

CHAPTER Fourier Representations of Signals & LTI Systems Figure 3.5 (p. 203) Time-domain signal for Example 3.2. 2. Odd symmetry n = 2 to n = 2 3. Fourier coefficient: (3.12)

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**Fourier Representations of Signals & LTI Systems**

3 CHAPTER Fourier Representations of Signals & LTI Systems One period of the DTFS coefficients X[k], k = 2 to k = 2: Fig. 3.6. Eq. (3.11) 3. Calculate X[k] using n = 0 to n = 4:

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**Fourier Representations of Signals & LTI Systems**

3 CHAPTER Fourier Representations of Signals & LTI Systems Even function Odd function Figure 3.6 (p. 204) Magnitude and phase of the DTFS coefficients for the signal in Fig. 3.5.

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**Fourier Representations of Signals & LTI Systems**

3 CHAPTER Fourier Representations of Signals & LTI Systems Example 3.3 Computation of DTFS by Inspection Determine the DTFS coefficients of x[n] = cos (n/3 + ), using the method of inspection. <Sol.> 1. Period: N = 6 o = 2/6 = /3 2. Using Euler’s formula, x[n] can be expressed as (3.13) 3. Compare Eq. (3.13) with the DTFS of Eq. (3.10) with o = /3, written by summing from k = 2 to k = 3: (3.14) Equating terms in Eq.(3.13) with those in Eq. (3.14) having equal frequencies, k/3, gives

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**Fourier Representations of Signals & LTI Systems**

3 CHAPTER Fourier Representations of Signals & LTI Systems 4. Magnitude spectrum and phase spectrum of X[k]: Fig. 3.8. Figure 3.8 (p. 206) Magnitude and phase of DTFS coefficients for Example 3.3.

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**3 Fourier Representations of Signals & LTI Systems**

CHAPTER Fourier Representations of Signals & LTI Systems Example 3.4 DTFS Representation of An Impulse Train Find the DTFS coefficients of the N-periodic impulse train as shown in Fig. 3.9. <Sol.> It is convenient to evaluate Eq.(3.11) over the interval n = 0 to n = N 1 to obtain Figure 3.9 (p. 207) A discrete-time impulse train with period N.

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**Fourier Representations of Signals & LTI Systems**

3 CHAPTER Fourier Representations of Signals & LTI Systems The Inverse DTFS The similarity between Eqs. (3.10) and (3.11) indicates that the same mathematical methods can be used to find the time-domain signal corresponding to a set of DTFS coefficients. Example 3.5 The Inverse DTFS Use Eq. (3.10) to determine the time-domain signal x[n] from the DTFS coefficients depicted in Fig <Sol.> 1. Period of DTFS coefficients = 9 o = 2/9 2. It is convenient to evaluate Eq.(3.11) over the interval k = 4 to k = 4 to obtain Example 3.6 DTFS Representation of A Square Wave Find the DTFS coefficients for the N-periodic square wave given by

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**Fourier Representations of Signals & LTI Systems**

3 CHAPTER Fourier Representations of Signals & LTI Systems /3 /3 Figure (p. 208) Magnitude and phase of DTFS coefficients for Example 3.5.

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**3 Fourier Representations of Signals & LTI Systems**

CHAPTER Fourier Representations of Signals & LTI Systems That is, each period contains 2M + 1 consecutive ones and the remaining N (2M +1) values are zero, as depicted in Fig Note that this definition requires that N > 2M +1. Figure (p. 209) Square wave for Example 3.6. <Sol.> 1. Period of DTFS coefficients = N o = 2/N 2. It is convenient to evaluate Eq.(3.11) over the interval n = M to n = N M 1.

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**Change of variable on the index of summation: m = n + M**

3 CHAPTER Fourier Representations of Signals & LTI Systems Change of variable on the index of summation: m = n + M (3.15) 3. For k = 0, N, 2N, …, we have (3.15) 3. For k 0, N, 2N, …, we may sum the geometric series in Eq. (3.15) to obtain (3.16)

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**The numerator and denominator of above Eq. are divided by 2j.**

3 CHAPTER Fourier Representations of Signals & LTI Systems The numerator and denominator of above Eq. are divided by 2j. 4. Substituting o = 2/N, yields L’Hôpital’s Rule For this reason, the expression for X[k] is commonly written as The value of X[k] for k = 0, N, 2N, …, is obtained from the limit as k 0. Fig

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**Fourier Representations of Signals & LTI Systems**

3 CHAPTER Fourier Representations of Signals & LTI Systems Figure (p. 211) The DTFS coefficients for the square wave shown in Fig. 3.11, assuming a period N = 50: (a) M = 4. (b) M = 12.

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**e jn = cos(n) since sin(n) = 0 for integer n.**

3 CHAPTER Fourier Representations of Signals & LTI Systems Symmetry property of DTFS coefficient: X[k] = X[ k]. 1. DTFS of Eq. (3.10) can be written as a series involving harmonically related cosines. 2. Assume that N is even and let k range from ( N/2 ) + 1 to N/2. Eq. (3.10) becomes 3. Use X[m] = X[ m] and the identity No = 2 to obtain e jn = cos(n) since sin(n) = 0 for integer n. 4. Define the new set of coefficients

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**Fourier Representations of Signals & LTI Systems**

3 CHAPTER Fourier Representations of Signals & LTI Systems and write the DTFS for the square wave in terms of a series of harmonically related cosines as (3.17) A similar expression may be derived for N odd. Example 3.7 Building a Square Wave From DTFS Coefficients The contribution of each term to the square wave may be illustrated by defining the partial-sum approximation to x[n] in Eq. (3.17) as (3.18) where J N/2. This approximation contains the first 2J + 1 terms centered on k = 0 in Eq. (3.10). Assume a square wave has period N = 50 and M = 12. Evaluate one period of the Jth term in Eq. (3.18) and the 2J + 1 term approximation for J = 1, 3, 5, 23, and 25.

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**Fourier Representations of Signals & LTI Systems**

3 CHAPTER Fourier Representations of Signals & LTI Systems Figure 3.14a (p. 213) Individual terms in the DTFS expansion of a square wave (left panel) and the corresponding partial-sum approximations J[n] (right panel). The J = 0 term is 0[n] = ½ and is not shown. (a) J = 1. (b) J = 3. (c) J = 5. (d) J = 23. (e) J = 25.

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**Fourier Representations of Signals & LTI Systems**

3 CHAPTER Fourier Representations of Signals & LTI Systems Figure 3.14b (p. 213)

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**Fourier Representations of Signals & LTI Systems**

3 CHAPTER Fourier Representations of Signals & LTI Systems <Sol.> 1. Fig depicts the J th term in the sum, B[J] cos(Jon), and one period of for the specified values of J. 2. Only odd values for J are considered, because the even-indexed coefficients B[k] are zero when N = 25 and M = 12. 3. The approximation improves as J increases, with x[n] represented exactly when J = N/2 = 25. 4. The coefficients B[k] associated with values of k near zero represent the low- frequency or slowly varying features in the signal, while the coefficients associated with values of k near N/2 represent the high frequency or rapidly varying features in the signal. Example 3.8 Numerical Analysis of the ECG Evaluate the DTFS representations of the two electrocardiogram (ECG) waveforms depicted in Figs (a) and (b). Fig (a) depicts a normal ECG, while Fig (b) depicts a heart experiencing ventricular tachycardia. The discrete-time signals are drawn as continuous functions, due to the difficulty of depicting all 2000 values in each case. Ventricular tachycardia is a series cardiac rhythm disturbance (i.e., an arrhythmia) that can result in death.

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**Fourier Representations of Signals & LTI Systems**

3 CHAPTER Fourier Representations of Signals & LTI Systems Figure (p. 214) Electrocardiograms for two different heartbeats and the first 60 coefficients of their magnitude spectra. (a) Normal heartbeat. (b) Ventricular tachycardia. (c) Magnitude spectrum for the normal heartbeat. (d) Magnitude spectrum for ventricular tachycardia.

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**Fourier Representations of Signals & LTI Systems**

3 CHAPTER Fourier Representations of Signals & LTI Systems Figure (p. 214) Electrocardiograms for two different heartbeats and the first 60 coefficients of their magnitude spectra. (c) Magnitude spectrum for the normal heartbeat. (d) Magnitude spectrum for ventricular tachycardia.

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**Fourier Representations of Signals & LTI Systems**

3 CHAPTER Fourier Representations of Signals & LTI Systems It is characterized by a rapid, regular heart rate of approximately 150 beats per minute. Ventricular complexes are wide (about 160 ms in duration) compared with normal complexes (less than 110 ms) and have an abnormal shape. Both signals appear nearly periodic, with only slight variations in the amplitude and length of each period. The DTFS of one period of each ECG may be computed numerically. The period of the normal ECG is N = 305, while the period of the ECG showing ventricular tachycardia is N = 421. One period of each waveform is available. Evaluate the DTFS coefficients of each, and plot their magnitude spectrum. <Sol.> 1. The magnitude spectrum of the first 60 DTFS coefficients is depicted in Fig. 3.15 (c) and (d). 2. The normal ECG is dominated by a sharp spike or impulsive feature. 3. The DTFS coefficients of the normal ECG are approximately constant, exhibiting a gradual decrease in amplitude as the frequency increase. Fairly small magnitude! 4. The ventricular tachycardia ECG contains smoother features in addition to sharp spikes, and thus the DTFS coefficients have a greater dynamic range, with the low-frequency coefficients containing a large proportion of the total power.

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**Frequency domain representation of x(t)**

3 CHAPTER Fourier Representations of Signals & LTI Systems 3.5 Continuous-Time Periodic Signals: The Fourier Series Fourier series pair Exponential FS 1. Continuous-time periodic signals are represented by the Fourier series (FS). 2. A signal with fundamental period T and fundamental frequency o = 2/T, the FS pair of x(t) is (3.19) Frequency domain representation of x(t) (3.20) The variable k determines the frequency of the complex sinusoid associated with X[k] in Eq. (3.19). 3. Notation: Mean-square error (MSE) 1. Suppose we define and choose the coefficients X[k] according to Eq.(3.20.)

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**Fourier Representations of Signals & LTI Systems**

3 CHAPTER Fourier Representations of Signals & LTI Systems 2. If x(t) is square integrable, then the MSE between x(t) and is zero, or, mathematically, at all values of t; it simply implies that there is zero power in their difference. 3. Pointwise convergence of is guaranteed at all values of t except those corresponding to discontinuities if the Dirichlet conditions are satisfied: x(t) is bounded. x(t) has a finite number of maximum and minima in one period. x(t) has a finite number of discontinuities in one period. If a signal x(t) satisfies the Dirichlet conditions and is not continuous, then converges to the midpoint of the left and right limits of x(t) at each discontinuity.

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**3 Fourier Representations of Signals & LTI Systems**

CHAPTER Fourier Representations of Signals & LTI Systems Example 3.9 Direct Calculation of FS Coefficients Determine the FS coefficients for the signal x(t) depicted in Fig <Sol.> 1. The period of x(t) is T = 2, so o = 2/2 = . 2. One period of x(t): x(t) = e 2t, 0 t 2. 3. FS of x(t): Figure (p. 216) Time-domain signal for Example 3.9. Fig e jk2 = 1 The Magnitude of X[k] the magnitude spectrum of x(t); the phase of X[k] the phase spectrum of x(t).

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**Fourier Representations of Signals & LTI Systems**

3 CHAPTER Fourier Representations of Signals & LTI Systems Figure (p. 217) Magnitude and phase spectra for Example 3.9.

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**Fourier Representations of Signals & LTI Systems**

3 CHAPTER Fourier Representations of Signals & LTI Systems Example 3.10 FS Coefficients for An Impulse Train Determine the FS coefficients for the signal x(t) defined by <Sol.> 1. Fundamental period of x(t) is T = 4, each period contains an impulse. 2. By integrating over a period that is symmetric about the origin 2 < t 2, to obtain X[k]: 3. The magnitude spectrum is constant and the phase spectrum is zero. Inspection method for finding X[k]: Whenever x(t) is expressed in terms of sinusoid, it is easier to obtain X[k] by inspection. The method of inspection is based on expanding all real sinusoids in terms of complex sinusoids and comparing each term in the resulting expansion to the corresponding terms of Eq. (3.19).

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**Fourier Representations of Signals & LTI Systems**

3 CHAPTER Fourier Representations of Signals & LTI Systems Example 3.11 Calculation of FS Coefficients By Inspection Determine the FS representation of the signal <Sol.> 1. Fundamental period of x(t) is T = 4, and o = 2/4 = /2. Eq. (3.19) is written as (3.21) 2. Equating each term in this expression to the terms in Eq. (3.21) gives the FS coefficients: Magnitude and phase spectrum: Fig (3.22)

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**3 Fourier Representations of Signals & LTI Systems /4 /4**

CHAPTER Fourier Representations of Signals & LTI Systems /4 /4 Figure (p. 219) Magnitude and phase spectra for Example 3.11. Time-domain representation obtained from FS coefficients Example 3.12 Inverse FS Find the time-domain signal x(t) corresponding to the FS coefficients Assume that the fundamental period is T = 2. <Sol.> 1. Fundamental frequency: o = 2/T = . From Eq. (3.19), we obtain

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**3 Fourier Representations of Signals & LTI Systems**

CHAPTER Fourier Representations of Signals & LTI Systems 2. Putting the fraction over a common denominator results in Example 3.13 FS for A Square Wave Determine the FS representation of the square wave depicted in Fig Figure (p. 221) Square wave for Example 3.13. <Sol.>

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**Fourier Representations of Signals & LTI Systems**

3 CHAPTER Fourier Representations of Signals & LTI Systems 1. Fundamental frequency: o = 2/T. 2. X[k]: by integrating over T/2 t T/2 For k = 0, we have 3. By means of L’Hôpital’s rule, we have

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**Fourier Representations of Signals & LTI Systems**

3 CHAPTER Fourier Representations of Signals & LTI Systems 4. X[k] is real valued. Using o = 2/T gives X[k] as a function of the ratio To/T: (3.23) 5. Fig (a)-(c) depict X[k], 50 k 50, for To/T =1/4, To/T =1/16, and To/T = 1/64, respectively. Definition of Sinc function: (3.24) Fig 1) Maximum of sinc function is unity at u = 0, the zero crossing occur at integer values of u, and the amplitude dies off as 1/u. 2) The portion of this function between the zero crossings at u = 1 is manilobe of the sinc function.

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**Fourier Representations of Signals & LTI Systems**

3 CHAPTER Fourier Representations of Signals & LTI Systems Figure 3.22a&b (p. 222) The FS coefficients, X[k], –50 k 50, for three square waves. (see Fig ) (a) To/T = 1/4 . (b) To/T = 1/16. (c) To/T = 1/64.

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**Fourier Representations of Signals & LTI Systems**

3 CHAPTER Fourier Representations of Signals & LTI Systems Figure 3.22c (p. 222)

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**3 Fourier Representations of Signals & LTI Systems**

CHAPTER Fourier Representations of Signals & LTI Systems Figure (p. 223) Sinc function sinc(u) = sin(u)/(u)

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**B[0] = X[0] represents the time-averaged value of the signal.**

3 CHAPTER Fourier Representations of Signals & LTI Systems 3) The smaller ripples outside the mainlobe are termed sidelobe. 4) The FS coefficients in Eq. (3.23) are expressed as Fourier series pair Trigonometric FS 1. Trigonometric FS of real-valued signal x(t): (3.25) FS coefficients: B[0] = X[0] represents the time-averaged value of the signal. (3.26)

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**Because x(t) is an even function**

3 CHAPTER Fourier Representations of Signals & LTI Systems 2. Relation between exponential FS and trigonometric FS coefficients: Euler’s Formula for k 0 (3.27) Ex. Find trigonometric FS coefficients of the square wave studied in Example 3.13. <Sol.> 1. Substituting Eq. (3.20) into Eq. (3.27), gives (3.28) Because x(t) is an even function 2. Trigonometric FS expression of x(t): (3.29)

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**Fourier Representations of Signals & LTI Systems**

3 CHAPTER Fourier Representations of Signals & LTI Systems Example 3.14 Square Wave Partial-Sum Approximation Let the partial-sum approximation to the FS in Eq. (3.29), be given by This approximation involves the exponential FS coefficients with indices J k J. Consider a square wave with T = 1 and To/T = ¼. Depict one period of the Jth term in this sum, and find <Sol.> 1. The individual terms and partial-sum approximation are depicted in Fig 2. Each partial-sum approximation passes through the average value (1/2) of the discontinuity, the approximation exhibits ripple. 3. This ripple near discontinuities in partial-sum FS approximations is termed the Gibbs phenomenon. 4. As J increase, the ripple in the partial-sum approximations becomes more and more concentrated near the discontinuities.

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**Fourier Representations of Signals & LTI Systems**

3 CHAPTER Fourier Representations of Signals & LTI Systems Figure 3.25a (p. 225) Individual terms (left panel) in the FS expansion of a square wave and the corresponding partial-sum approximations J(t) (right panel). The square wave has period T = 1 and Ts/T = ¼. The J = 0 term is 0(t) = ½ and is not shown. (a) J = 1.

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**3 Fourier Representations of Signals & LTI Systems**

CHAPTER Fourier Representations of Signals & LTI Systems Figure 3.25b-3 (p. 226) (b) J = 3. (c) J = 7. (d) J = 29.

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**3 Fourier Representations of Signals & LTI Systems**

CHAPTER Fourier Representations of Signals & LTI Systems Figure 3.25e (p. 226) (e) J = 99. Example 3.15 RC Circuit: Calculating The Output By Means of FS Let us find the FS representation for the output y(t) of the RC circuit depicted in Fig. 3.2 in response to the square-wave input depicted in Fig. 3.21, assuming that To/T = ¼, T = 1 s, and RC = 0.1 s. Figure (p. 221) Square wave for Example 3.13. Figure 3.2 (p. 197) RC circuit for Example 3.1.

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**Waveform of y(t): Fig. 3.26(c).**

CHAPTER Fourier Representations of Signals & LTI Systems <Sol.> 1. If the input to an LTI system is expressed as a weighted sum of sinusoid, then the output is also a weighted sum of sinusoids. 2. Input: 4. Frequency response of the system H(j): 5. Frequency response of the RC circuit: 3. Output: 6. Substituting for H(jko) with RC = 0.1 s and o = 2, and To/T = ¼, gives 7. We determine y(t) using the approximation The magnitude and phase spectra for the range 25 k 25: Fig (a) and (b). Waveform of y(t): Fig. 3.26(c). (3.30)

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**Fourier Representations of Signals & LTI Systems**

3 CHAPTER Fourier Representations of Signals & LTI Systems Figure (p. 228) The FS coefficients Y[k], –25 k 25, for the RC circuit output in response to a square-wave input. (a) Magnitude spectrum. (b) Phase spectrum. c) One period of the input signal x(t) dashed line) and output signal y(t) (solid line). The output signal y(t) is computed from the partial-sum approximation given in Eq. 3.30).

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**Fourier Representations of Signals & LTI Systems**

3 CHAPTER Fourier Representations of Signals & LTI Systems Figure (p. 228) The FS coefficients Y[k], –25 k 25, for the RC circuit output in response to a square-wave input. (a) Magnitude spectrum. (b) Phase spectrum. c) One period of the input signal x(t) dashed line) and output signal y(t) (solid line). The output signal y(t) is computed from the partial-sum approximation given in Eq. (3.30).

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**Fourier Representations of Signals & LTI Systems**

3 CHAPTER Fourier Representations of Signals & LTI Systems 8. The circuit attenuates the amplitude of X[k] when k 1. The degree of attenuation increases as the frequency ko increases. The circuit also introduces a frequency-dependent phase dependent shift. Example 3.16 DC-to-AC Conversion A simple scheme for converting direct current (dc) to alternating current (ac) is based on applying a periodic switch to a dc power source and filtering out or removing the higher order harmonics in the switched signal. The switch in Fig changes position every 1/20 second. We consider two cases: (a) The switch in either open or closed; (b) the switch reverses polarity. Fig (a) and (b) depict the output waveforms for these two cases. Define the conversion efficiency as the ratio of the power in the 60-Hz component of the output waveform x(t) to the available dc power at the input. Evaluate the conversion efficiency for each case. <Sol.> 1. The FS for the square wave x(t) with T = 1/60 second and o = 2/T = 120 rad/s: Fig. 3.28(a), where

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**3 Fourier Representations of Signals & LTI Systems**

CHAPTER Fourier Representations of Signals & LTI Systems Figure (p. 229) Switching power supply for DC-to-AC conversion. Figure (p. 229) Switching power supply output waveforms with fundamental frequency 0 = 2/T = 120. (a) On-off switch. (b) Inverting switch.

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**Fourier Representations of Signals & LTI Systems**

3 CHAPTER Fourier Representations of Signals & LTI Systems 2. The harmonic at 60 Hz in the trigonometric FS representation of x(t) has amplitude given by B[1] and contains power B[1]2/2. The dc input has power A2. 3. Conversion efficiency for Fig (a): 4. The FS for the signal x(t) shown in Fig. 3.28(b): 5. Conversion efficiency for Fig (b):

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**Frequency-domain representation x[n]**

3 CHAPTER Fourier Representations of Signals & LTI Systems 3.6 Discrete-Time Nonperiodic Signals: The Discrete-Time Fourier Transform (DTFT) 1. The DTFT is used to represent a discrete-time nonperiodic signal as a superposition of complex sinusoids. 2. DTFT representation of time-domain signal x[n]: DTFT pair (3.31) (3.32) Frequency-domain representation x[n] 3. Notation: 4. Condition for convergence of DTFT: If x[n] is of infinite duration, then the sum converges only for certain classes of signals. If x[n] is absolutely summable, i.e., The sum in Eq. (3.32) converges uniformly to a continuous function of .

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**Fourier Representations of Signals & LTI Systems**

3 CHAPTER Fourier Representations of Signals & LTI Systems If x[n] is not absolutely summable, but does satisfy (i.e., if x[n] has finite energy), It can be shown that the sum in Eq. (3.32) converges in a mean-square error sense, but does not converge pointwise. Example 3.17 DTFT of An Exponential Sequence Find the DTFT of the sequence x[n] = nu[n]. <Sol.> This sum diverges for 1. DTFT of x[n]: 2. For < 1, we have (3.33) 3. If is real valued, Eq. (3.33) becomes Euler’s Formula

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**Fourier Representations of Signals & LTI Systems**

3 CHAPTER Fourier Representations of Signals & LTI Systems 4. Magnitude and phase spectra: Even function Odd function Fig for = 0.5 and = 0.9. Example 3.18 DTFT of A Rectangular Pulse Let as depicted in Fig (a). Find the DTFT of x[n]. <Sol.> 1. DTFT of x[n]:

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**Fourier Representations of Signals & LTI Systems**

3 CHAPTER Fourier Representations of Signals & LTI Systems Figure (p.232) The DTFT of an exponential signal x[n] = ()nu[n]. (a) Magnitude spectrum for = 0.5. (b) Phase spectrum for = 0.5. (c) Magnitude spectrum for = 0.9. (d) Phase spectrum for = 0.9.

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**Fourier Representations of Signals & LTI Systems**

3 CHAPTER Fourier Representations of Signals & LTI Systems Figure (p. 233) Example (a) Rectangular pulse in the time domain. (b) DTFT in the frequency domain. p

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**Change of variable m = n + M**

3 CHAPTER Fourier Representations of Signals & LTI Systems Change of variable m = n + M 2. The expression for X(e j ) when L’Hôptital’s Rule With understanding that X(e j ) for is obtained as limit.

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**Fourier Representations of Signals & LTI Systems**

3 CHAPTER Fourier Representations of Signals & LTI Systems 3. Graph of X(e j ): Fig. 3.30(b). Example 3.19 Inverse DTFT of A Rectangular Pulse Find the inverse DTFT of Figure (p. 234) Example (a) Rectangular pulse in the frequency domain. (b) Inverse DTFT in the time domain. which is depicted in Fig (a). <Sol.> 1. Note that X(e j ) is specified only for < . p 2. Inverse DTFT x[n]:

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**Fourier Representations of Signals & LTI Systems**

3 CHAPTER Fourier Representations of Signals & LTI Systems 3. For n = 0, the integrand is unity and we have x[0] = W/. Using L’Hôpital’s rule, we easily show that and thus we usually write as the inverse DTFT of X(e j ), with the understanding that the value at n = 0 is obtained as limit. 4. We may also write Fig (b).

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**Fourier Representations of Signals & LTI Systems**

3 CHAPTER Fourier Representations of Signals & LTI Systems Example 3.20 DTFT of The Unit Impulse Find the DTFT of x[n] = [n]. <Sol.> 1. DTFT of x[n]: Figure (p. 235) Example (a) Unit impulse in the time domain. (b) DTFT of unit impulse in the frequency domain. 2. This DTFT pair is depicted in Fig Example 3.21 Inverse DTFT of A Unit Impulse Spectrum p Find the inverse DTFT of X(e j ) = (), < . <Sol.> 1. Inverse DTFT of X(e j ):

65
**Sifting property of impulse function**

3 CHAPTER Fourier Representations of Signals & LTI Systems Sifting property of impulse function We can define X(e j ) over all by writing it as an infinite sum of delta functions shifted by integer multiples of 2. p 2. This DTFT pair is depicted in Fig Figure (p. 236) Example (a) Unit impulse in the frequency domain. (b) Inverse DTFT in the time domain. Dilemma: The DTFT of x[n] = 1/(2) does not converge, since it is not a square summable signal, yet x[n] is a valid inverse DTFT!

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**Fourier Representations of Signals & LTI Systems**

3 CHAPTER Fourier Representations of Signals & LTI Systems Example 3.22 Moving-Average Systems: Frequency Response Consider two different moving-average systems described by the input-output equations and The first system averages successive inputs, while the second forms the difference. The impulse responses are and Find the frequency response of each system and plot the magnitude responses. <Sol.> 1. The frequency response is the DTFT of the impulse response. 2. For system # 1 h1[n] : Frequency response:

67
**Fourier Representations of Signals & LTI Systems**

3 CHAPTER Fourier Representations of Signals & LTI Systems Magnitude response: Phase response: 3. For system # 2 h2[n] : Frequency response: Magnitude response: Phase response: 4. Fig (a) and (b) depict the magnitude responses of the two systems on the interval < .

68
**Fourier Representations of Signals & LTI Systems**

3 CHAPTER Fourier Representations of Signals & LTI Systems p Figure (p. 239) The magnitude responses of two simple discrete-time systems. (a) A system that averages successive inputs tends to attenuate high frequencies. (b) A system that forms the difference of successive inputs tends to attenuate low frequencies. Example 3.23 Multipath Communication Channel: Frequency Response The input-output equation introduced in Section 1.10 describing a discrete-time model of a two-path propagation channel is In Example 2.12, we identified the impulse response of this system as h[n] = [n] + a[n 1] and determined that the impulse response of the inverse system was hinv[n] = ( a)nu[n]. The inverse system is stable, provided that a< 1.

69
**Fourier Representations of Signals & LTI Systems**

3 CHAPTER Fourier Representations of Signals & LTI Systems Compare the magnitude responses of both systems for a = 0.5 e j/3 and a = 0.9 e j2/3. <Sol.> 1. The frequency response is the DTFT of the impulse response. 2. Frequency response of the system modeling two-path propagation: Using Apply Euler’s formula 3. Magnitude response: 4. The frequency response of the inverse system may be obtained by replacing with a in Eq. (3.33).

70
**Expressing a in polar form**

3 CHAPTER Fourier Representations of Signals & LTI Systems Expressing a in polar form The frequency response of the inverse system is the inverse of the frequency response of the original system. 5. Magnitude response of the inverse system: 6. Fig depicts the magnitude response of H(e j ) for both a = 0.5 e j/3 and a = 0.9 e j2/3 on the interval < . 7. The magnitude response approaches a maximum of 1 + a when = arg{a} and a minimum of 1 a when = arg{a} . 8. The magnitude response of the corresponding inverse system: Fig

71
**Fourier Representations of Signals & LTI Systems**

3 CHAPTER Fourier Representations of Signals & LTI Systems Figure (p. 241) Magnitude response of the system in Example 3.23 describing multipath propagation. (a) Echo coefficient a = 0.5ej/3. (b) Echo coefficient a = 0.9ej2/3.

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**Fourier Representations of Signals & LTI Systems**

3 CHAPTER Fourier Representations of Signals & LTI Systems Figure (p. 241) Magnitude response of the inverse system for multipath propagation in Example (a) Echo coefficient a = 0.5ej/3. (b) Echo coefficient a = 0.9ej/3 If a = 1, then the multipath model applies zero gain to any sinusoid with frequency = arg{a} . The multipath system cannot be inverted when a= 1.

73
**Frequency-domain representation of the signal x(t)**

3 CHAPTER Fourier Representations of Signals & LTI Systems 3.7 Continuous-Time Nonperiodic Signals: The Fourier Transform (FT) 1. The Fourier transform (FT) is used to represent a continuous-time non- periodic signal as a superposition of complex sinusoids. 2. FT representation of time-domain signal x(t): (3.35) Frequency-domain representation of the signal x(t) (3.36) 3. Notation for FT pair: 4. Convergence condition for FT: 1) Approximation: 2) Squared error between x(t) and : the error energy, given by

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**Fourier Representations of Signals & LTI Systems**

3 CHAPTER Fourier Representations of Signals & LTI Systems is zero if x(t) is square integrable, i.e., if Zero squared error does not imply pointwise convergence [i.e., ] at all values of t. There is zero energy in the difference of 3. Pointwise convergence of is guaranteed at all values of t except those corresponding to discontinuities if x(t) satisfies the Dirichlet conditions for nonperiodic signals: x(t) is absolutely integrable: x(t) has a finite number of maximum, minima, and discontinuities in any finite interval. The size of each discontinuity is finite.

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**Fourier Representations of Signals & LTI Systems**

3 CHAPTER Fourier Representations of Signals & LTI Systems Example 3.24 FT of A Real Decaying Exponential Find the FT of x(t) = e a t u(t), shown in Fig. 3.39(a). <Sol.> 1. For a 0, since x(t) is not absolutely integrable, i.e., The FT of x(t) does not converge for a 0. 2. For a > 0, the FT of x(t) is 3. Magnitude and phase of X(j): Fig (b) and (c).

76
**Fourier Representations of Signals & LTI Systems**

3 CHAPTER Fourier Representations of Signals & LTI Systems arg{X(j)} /4 /2 /4 X(j) Figure (p. 243) Example (a) Real time-domain exponential signal. (b) Magnitude spectrum. (c) Phase spectrum.

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**Fourier Representations of Signals & LTI Systems**

3 CHAPTER Fourier Representations of Signals & LTI Systems Example 3.25 FT of A Rectangular Pulse Consider the rectangular pulse depicted in Fig (a) and defined as Find the FT of x(t). <Sol.> 1. The rectangular pulse x(t) is absolutely integrable, provided that To < . 2. FT of x(t): With understanding that the value at = 0 is obtained by evaluating a limit. 3. For = 0, the integral simplifies to 2To. 4. Magnitude spectrum:

78
**Fourier Representations of Signals & LTI Systems**

3 CHAPTER Fourier Representations of Signals & LTI Systems Fig (b). 5. Phase spectrum: 6. X(j) in terms of sinc function: As To increases, the nonzero time extent of x(t) increases, while X(j) becomes more concentrated about the frequency origin. v ; p Figure (p. 244) Example (a) Rectangular pulse in the time domain. (b) FT in the frequency domain.

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**Fourier Representations of Signals & LTI Systems**

3 CHAPTER Fourier Representations of Signals & LTI Systems Example 3.26 Inverse FT of A Rectangular Pulse Find the inverse FT of the rectangular spectrum depicted in Fig (a) and given by Figure (p. 246) Example (a) Rectangular spectrum in the frequency domain. (b) Inverse FT in the time domain. <Sol.> v ; p 1. Inverse FT of x(t): 2. When t = 0, the integral simplifies to W/, i.e.,

80
**With understanding that the value at t = 0 is obtained as a limit.**

3 CHAPTER Fourier Representations of Signals & LTI Systems With understanding that the value at t = 0 is obtained as a limit. 3. Inverse FT is usually written as or Fig (b). As W increases, the frequency-domain reprsentation becomes less concentrated about = 0, while the time-domain representation X(j) becomes more concentrated about t = 0. Duality between Example 3.25 and 3.26. Example 3.27 FT of The Unit Impulse Find the FT of x(t) = (t). <Sol.> 1. x(t) does not satisfy the Dirichlet conditions, since the discontinuity at the origin is infinite. 2. FT of x(t):

81
**Duality between Example 3.27 and 3.28**

CHAPTER Fourier Representations of Signals & LTI Systems Notation for FT pair: Example 3.28 Inverse FT of An Impulse Spectrum Find the inverse FT of X(j) = 2(). <Sol.> 1. Inverse FT of X(j): Duality between Example 3.27 and 3.28 2. Notation of FT pair: Example 3.29 Characteristic of Digital Communication Signals In a simple digital communication system, one signal or “symbol” is transmitted for each “1” in the binary representation of the message, while a different signal or symbol is transmitted for each “0”. One common scheme, binary phase-shift keying (BPSK), assumes that the signal representing “0” is the negative of the signal and the signal representing “1” is the positive of the signal. Fig depicts two candidate signals for this approach: a rectangular pulse defined as

82
**Fourier Representations of Signals & LTI Systems**

3 CHAPTER Fourier Representations of Signals & LTI Systems Figure (p. 249) Pulse shapes used in BPSK communications. (a) Rectangular pulse. (b) Raised cosine pulse. and a raised-cosine pulse defined as The transmitted BPSK signals for communicating a sequence of bits using each pulse shape are illustrated in Fig Note that each pulse is To seconds long,

83
**Fourier Representations of Signals & LTI Systems**

3 CHAPTER Fourier Representations of Signals & LTI Systems so this scheme has a transmission rate of 1/To bits per second. Each user’s signal is transmitted within an assigned frequency band, as depicted in Chapter 5. In order to prevent interference with users of other frequency bands, governmental agencies place limits on the energy of a signal that any user transmits into adjacent frequency bands. Suppose the frequency band assigned to each user is 20 kHz wide. Then, to prevent interference with adjacent channels, we assume that the peak value of the magnitude spectrum of the transmitted signal outside the 20-kHz band is required to be 30 dB below the peak in-band magnitude spectrum. Choose the constant Ar and Ac so that both BPSK signals have unit power. Use the FT to determine the maximum number of bits per second that can be transmitted when the rectangular and raised-cosine pulse shapes are utilized. <Sol.> 1. BPSK signals are not periodic, but their magnitude squared is To periodic. 2. Powers in rectangular pulse and raised-cosine pulse: and

84
**Fourier Representations of Signals & LTI Systems**

3 CHAPTER Fourier Representations of Signals & LTI Systems Figure (p. 249) BPSK signals constructed by using (a) rectangular pulse shapes and (b) raised-cosine pulse shapes.

85
**Fourier Representations of Signals & LTI Systems**

3 CHAPTER Fourier Representations of Signals & LTI Systems Hence, unity transmission power is obtained by choosing Ar = 1 and Ac = 3. FT of the rectangular pulse xr(t): In terms of Hz rather than rad/sec ( = 2f): Fig

86
**Fourier Representations of Signals & LTI Systems**

3 CHAPTER Fourier Representations of Signals & LTI Systems Figure (p. 250) Spectrum of rectangular pulse in dB, normalized by T0. The normalized by To removed the dependence of the magnitude on To.

87
**Substituting the appropriate value of for each integral**

3 CHAPTER Fourier Representations of Signals & LTI Systems 4. From Fig. 3.46, we must choose To so that the 10 th zero crossing is at 10 kHz in order to satisfy the constraint that the peak value of the magnitude spectrum of the transmitted signal outside the 20-kHz band allotted to this user be less than 30 dB. f = k/To 1000 = 10/To To = 10 3 Data transmission rate = 1000 bits/sec. 5. FT of raised-cosine pulse xc(t): Euler’s formula Each of three integrals is of the form Substituting the appropriate value of for each integral

88
**Fourier Representations of Signals & LTI Systems**

3 CHAPTER Fourier Representations of Signals & LTI Systems In terms of f (Hz), the above equation becomes First term corresponds to the spectrum of the rectangular pulse. The second and third terms are the versions of 1st term shifted by 1/To. Fig for To = 1. Fig The normalized by To removed the dependence of the magnitude on To. 3) In this case, the peak value of the first sidelobe is below 30 dB, so we may satisfy the adjacent channel interference specifications by choosing the mainlobe to be 20 kHz wide: f = k/To 10,000 = 2/To To = 2 10 4 seconds. Data transmission rate = 5000 bits/sec.

89
**Fourier Representations of Signals & LTI Systems**

3 CHAPTER Fourier Representations of Signals & LTI Systems Figure (p. 252) The spectrum of the raised-cosine pulse consists of a sum of three frequency-shifted sinc functions.

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**Fourier Representations of Signals & LTI Systems**

3 CHAPTER Fourier Representations of Signals & LTI Systems Figure (p. 252) Spectrum of the raised-cosine pulse in dB, normalized by T0.

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**Fourier Representations of Signals & LTI Systems**

3 CHAPTER Fourier Representations of Signals & LTI Systems 3.8 Properties of Fourier Representation 1. Four Fourier representations: Table 3.2.

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**Fourier Representations of Signals & LTI Systems**

3 CHAPTER Fourier Representations of Signals & LTI Systems 2. Signals that are periodic in time have discrete frequency-domain representations, while nonperiodic time signals have continuous frequency- domain representations: Table 3.3. 3.9 Linearity and Symmetry Properties 1. Linearity property for four Fourier representations:

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**Fourier Representations of Signals & LTI Systems**

3 CHAPTER Fourier Representations of Signals & LTI Systems Example 3.30 Linearity in The FS Suppose z(t) is the periodic signal depicted in Fig. 3.49(a). Use the linearity and the results of Example 3.13 to determine the FS coefficients Z[k]. <Sol.> 1. Signal z(t): where x(t) and y(t): Fig. 3.49(b) and (c). 2. X[k] and Y[k]:

94
**X(j) is complex-conjugate symmetric**

3 CHAPTER Fourier Representations of Signals & LTI Systems From Example 3.13. Linearity 3. FS of z(t): 3.9.1 Symmetry Properties: Real and Imaginary Signals Symmetry property for real-valued signal x(t): 1. FT of x(t): (3.37) 2. Since x(t) is real valued, x(t) = x(t). Eq. (3.37) becomes X(j) is complex-conjugate symmetric (3.38)

95
**Fourier Representations of Signals & LTI Systems**

3 CHAPTER Fourier Representations of Signals & LTI Systems The conjugate symmetry property for DTFS: Because the DTFS coefficients are N periodic, and thus The symmetry conditions in all four Fourier representations of real-valued signals are indicated in Table 3.4.

96
**Applying Eq. (3.2) and linearity**

CHAPTER Fourier Representations of Signals & LTI Systems Complex-conjugate symmetry in FT: A simple characterization of the output of an LTI system with a real-valued impulse response when the input is a real-valued sinusoid. Continuous-time case: 1. Input signal of LTI system: Euler’s formula 2. Real-valued impulse response of LTI system is denoted by h(t). 3. Output signal of LTI system: Exploiting the symmetry conditions: Applying Eq. (3.2) and linearity

97
**Fourier Representations of Signals & LTI Systems**

3 CHAPTER Fourier Representations of Signals & LTI Systems The LTI system modifies the amplitude of the input sinusoid by H(j)and the phase by arg{H(j)}. v ; f Fig Figure (p. 257) A sinusoidal input to an LTI system results in a sinusoidal output of the same frequency, with the amplitude and phase modified by the system’s frequency response. Discrete-time case: 1. Input signal of LTI system: x[n] = Acos( n )

98
**Fourier Representations of Signals & LTI Systems**

3 CHAPTER Fourier Representations of Signals & LTI Systems 2. Real-valued impulse response of LTI system is denoted by h[n]. 3. Output signal of LTI system: The LTI system modifies the amplitude of the input sinusoid by H(e j)and the phase by arg{H(e j)}. x(t) is purely imaginary: 1. x(t) = x(t). 2. Eq.(3.37) becomes (3.39) 3.9.2 Symmetry Properties: Even and Odd Signals 1. x(t) is real valued and has even symmetry. x(t) = x(t) and x ( t) = x(t) x(t) = x( t)

99
**Change of variable = t**

3 CHAPTER Fourier Representations of Signals & LTI Systems 2. Eq.(3.37) becomes Change of variable = t 3. Conclusion: 1) The imaginary part of X(j) = 0: If x(t) is real and even, then X(j) is real. 2) If x(t) is real and odd, then X(j) = X(j) and X(j) is imaginary. 3.10 Convolution Properties The convolution property is a consequence of complex sinusoids being eigenfunctions of LTI system. Convolution of Nonperiodic Signals — Continuous-time case 1. Convolution of two nonperiodic continuous-time signals x(t) and h(t): 2. FT of x(t ):

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**Fourier Representations of Signals & LTI Systems**

3 CHAPTER Fourier Representations of Signals & LTI Systems 3. Substituting this expression into the convolution integral yields The inner integral over as the FT of h(), or H(j). Hence, y(t) may be written as and we identify H(j)X(j) as the FT of y(t). (3.40) Example 3.31 Solving A Convolution Problem in The Frequency Domain Let x(t) = (1/(t))sin(t) be the input to a system with impulse response h(t) = (1/(t))sin(2t). Find the output y(t) = x(t) h(t).

101
**Fourier Representations of Signals & LTI Systems**

3 CHAPTER Fourier Representations of Signals & LTI Systems <Sol.> x(t) = (1/(t))sin(t) h(t) 1. Solving problem: y(t) = x(t) h(t) Time domain Extremely Difficult Frequency domain Quite Simple Convolution property 2. From Example 3.26, we have and Since y(t) = (1/(t))sin(t) Example 3.32 Finding Inverse FT’s by Means of The Convolution Property Use the convolution property to find x(t), where

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**Fourier Representations of Signals & LTI Systems**

3 CHAPTER Fourier Representations of Signals & LTI Systems <Sol.> 1. Write X(j) = Z(j) Z(j), where 2. Convolution property: Fig (a) 3. Using the result of Example 3.25: 4. Performing the convolution with itself gives the triangular waveform depicted in Fig (b) as the solution for x(t). Figure (p. 261) Signals for Example (a) Rectangular pulse z(t). (b) Convolution of z(t) with itself gives x(t).

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**Fourier Representations of Signals & LTI Systems**

3 CHAPTER Fourier Representations of Signals & LTI Systems Convolution of Nonperiodic Signals — Continuous-time case If and (3.41) Filtering 1. Multiplication in frequency domain Filtering. 2. The terms “filtering” implies that some frequency components of the input are eliminated while others are passed by the system unchanged. 3. System Types of filtering: Fig (a), (b), and (c) Low-pass filter High-pass filter Band-pass filter The characterization of DT filter is based on its behavior in the frequency range < because its frequency response is 2 -periodic. 4. Realistic filter: Gradual transition band Nonzero gain of stop band 5. Magnitude response of filter: or [dB]

104
**Fourier Representations of Signals & LTI Systems**

3 CHAPTER Fourier Representations of Signals & LTI Systems v ; p Figure (p. 263) Frequency response of ideal continuous- (left panel) and discrete-time (right panel) filters. (a) Low-pass characteristic. (b) High-pass characteristic. (c) Band-pass characteristic.

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**Fourier Representations of Signals & LTI Systems**

3 CHAPTER Fourier Representations of Signals & LTI Systems 6. The edge of the passband is usually defined by the frequencies for which the response is 3 dB, corresponding to a magnitude response of (1/ ). Unity gain = 0 dB 7. Energy spectrum of filter: The 3 dB point corresponds to frequencies at which the filter passes only half of the input power. 3 dB point Cutoff frequency Example 3.33 RC Circuit: Filtering For the RC circuit depicted in Fig. 3.54, the impulse response for the case where yC(t) is the output is given by Since yR(t) = x(t) yC(t), the impulse response for the case where yR(t) is the output is given by Figure (p. 264) RC circuit with input x(t) and outputs yc(t) and yR(t).

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**Fourier Representations of Signals & LTI Systems**

3 CHAPTER Fourier Representations of Signals & LTI Systems Plot the magnitude responses of both systems on a linear scale and in dB, and characterize the filtering properties of the systems. <Sol.> 1. Frequency response corresponding to hC(t): 2. Frequency response corresponding to hR(t): 3. Magnitude response in linear scale: Fig (a) and (b). Magnitude response in dB scale: Fig (c) and (d). 4. System corresponding to yC(t): Low-pass filter Cutoff frequency = c = 1/(RC) System corresponding to yR(t): High-pass filter Cutoff frequency = c = 1/(RC)

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**Fourier Representations of Signals & LTI Systems**

3 CHAPTER Fourier Representations of Signals & LTI Systems v Figure (p. 265) RC circuit magnitude responses as a function of normalized frequency RC. (a) Frequency response of the system corresponding to yC(t), linear scale. (b) Frequency response of the system corresponding to yR(t), linear scale.

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**Fourier Representations of Signals & LTI Systems**

3 CHAPTER Fourier Representations of Signals & LTI Systems v Figure 3.55a&c (p. 265) RC circuit magnitude responses as a function of normalized frequency RC. (c) Frequency response of the system corresponding to yC(t), dB scale. (d) Frequency response of the system corresponding to yR(t), dB scale, shown on the range from 0 dB to –25 dB.

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**Fourier Representations of Signals & LTI Systems**

3 CHAPTER Fourier Representations of Signals & LTI Systems The convolution property implies that the frequency response of a system may be expressed as the ratio of the FT or DTFT of the output to the input . For CT system: (3.42) Frequency response For DT system: (3.43) Example 3.34 Identifying a System, Given Its Input and Output The output of an LTI system in response to an input x(t) = e 2 t u(t) is y(t) = e t u(t) . Find the frequency response and the impulse response of this system. <Sol.> 1. FT of x(t) and y(t): and 2. Frequency response:

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**Fourier Representations of Signals & LTI Systems**

3 CHAPTER Fourier Representations of Signals & LTI Systems 3. Impulse response: Recover the input of the system from the output: CT case DT case and where and ◆ An inverse system is also known as an equalizer, and the process of recovering the input from the output is known as equalization. ◆ Causality restriction An exact inverse system is difficult or impossible to be implemented! Ex. Time delay in communication system need an equalizer to introduce a time advance. Equalizer is a noncausal system and cannot in fact be implemented! Approximation!

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**Fourier Representations of Signals & LTI Systems**

3 CHAPTER Fourier Representations of Signals & LTI Systems Example 3.35 Multipath Communication Channel: Equalization Consider again the problem addressed in Example In this problem, a distorted received signal y[n] is expressed in terms of a transmitted signal x[n] as Use the convolution property to find the impulse response of an inverse system that will recover x[n] from y[n]. <Sol.> 1. In Example 2.13, we have y[n] = x[n] h[n], where the impulse response is 2. The impulse response of an inverse system, hinv[n], must satisfy

112
**Frequency response of inverse system**

3 CHAPTER Fourier Representations of Signals & LTI Systems Frequency response of inverse system 3. DTFT of h[n]: The frequency response of the inverse system is then obtained as Convolution of Periodic Signals Basic Concept: Periodic h Unstable system

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**Fourier Representations of Signals & LTI Systems**

3 CHAPTER Fourier Representations of Signals & LTI Systems Define the periodic convolution of two CT signals x(t) and z(t), each having period T, as where the symbol denotes that integration is performed over a single period of the signals involved. (3.44) ◆ Convolution in Time-Domain Multiplication in Frequency-Domain Example 3.36 Convolution of Two Periodic Signals Evaluate the periodic convolution of the sinusoidal signal with the periodic square wave x(t) depicted in Fig <Sol.> 1. Both x(t) and z(t) have fundamental period T = 1. 2. Let

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**Fourier Representations of Signals & LTI Systems**

3 CHAPTER Fourier Representations of Signals & LTI Systems Figure (p. 268) Square wave for Example 3.36. 3. Coefficients of FS representation of z(t): Coefficients of FS representation of x(t): Example 3.13

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**Fourier Representations of Signals & LTI Systems**

3 CHAPTER Fourier Representations of Signals & LTI Systems 3. FS Coefficients of y(t): Explanation for the origin of the Gibbs phenomenon: Example 3.14: 1. A partial-sum approximation to the FS representation for x(t) may be obtained by using the FS coefficients T = 1 where 2. In the time domain, is the periodic convolution of x(t) and T = 1 (3.45)

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**Fourier Representations of Signals & LTI Systems**

3 CHAPTER Fourier Representations of Signals & LTI Systems 3. One period of the signal w(t) is depicted in Fig for J = 10. 4. The periodic convolution of x(t) and w(t) is the area under time-shifted version of w(t) on t< ½ .Time- shifting the sidelobes of w(t) into and out of the interval t< ½ introduces ripples the partial-sum approximation 5. As J increases, the mainlobe and sidelobe widths in w(t) decrease, but the size of the sidelobe does not change. Figure (p. 270) The signal z(t) defined in Eq. (3.45) when J = 10. This is why the ripples in becomes more concentrated near discontinuity of x(t), but remain the same magnitude.

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**Fourier Representations of Signals & LTI Systems**

3 CHAPTER Fourier Representations of Signals & LTI Systems Define the discrete-time convolution of two N-periodic sequences x[n] and z[n] as DTFS of y[n] (3.46) ◆ Convolution in Time-Domain Multiplication in Frequency-Domain ◆ The convolution properties of all four Fourier representation are summarized in Table 3.5. Table 3.5 Convolution Properties

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**Fourier Representations of Signals & LTI Systems**

3 CHAPTER Fourier Representations of Signals & LTI Systems 3.11 Differentiation and Integration Properties Differentiation and integration are operations that apply to continuous functions. Differentiation in Time 1. A nonperiodic signal x(t) and its FT, X(j), is related by Differentiating both sides with respect to t ◆ Differentiation of x(t) in Time-Domain (j) X(j) in Frequency-Domain Example 3.37 Verifying the Differentiation Property The differentiation property implies that Verify this result by differentiating and taking the FT of the result.

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**Fourier Representations of Signals & LTI Systems**

3 CHAPTER Fourier Representations of Signals & LTI Systems <Sol.> 1. Using the product rule for differentiation, we have 2. Taking the FT of each term and using linearity, we may write Frequency response of a continuous system 1. System equation in terms of differential equation: Taking FT of both sides (3.47) 2. Frequency response of the system:

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**Fourier Representations of Signals & LTI Systems**

3 CHAPTER Fourier Representations of Signals & LTI Systems Example 3.38 MEMS Accelerometer: Frequency Response and Resonance The MEMS accelerometer introduced in Section 1.10 is described by the differential equation Find the frequency response of this system and plot the magnitude response in dB for n = 10,000 rads/s for (a) Q = 2/5, (b) Q = 1, and (c) Q = 200. <Sol.> 1. Frequency response: 2. Magnitude response for (a) Q = 2/5, (b) Q = 1, and (c) Q = 200: Fig Discussion: Refer to textbook, p. 273. n = 10,000 rads/s Resonant condition!

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**Fourier Representations of Signals & LTI Systems**

3 CHAPTER Fourier Representations of Signals & LTI Systems v Figure (p. 273) Magnitude of frequency response in dB for MEMS accelerometer for n = 10,000 rad/s, Q = 2/5, Q = 1, and Q = 200.

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**Fourier Representations of Signals & LTI Systems**

3 CHAPTER Fourier Representations of Signals & LTI Systems The Differentiation property for a periodic signal 1. The FS representation of a periodic signal x(t): Differentiating both sides with respect to t 2. Differentiation property of a periodic signal: ◆ Differentiation of x(t) in Time-Domain (jk0) X[k] in Frequency-Domain Example 3.39 Differentiation Property Use the differentiation property to find the FS representation of the triangular wave depicted in Fig (a). Fig (b) <Sol.> 1. Define a waveform:

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**Fourier Representations of Signals & LTI Systems**

3 CHAPTER Fourier Representations of Signals & LTI Systems Figure (p. 274) Signals for Example (a) Triangular wave y(t). (b) The derivative of y(t) is the square wave z(t). 2. Comparing z(t) in Fig (b) and the sqare wave x(t) with T0/T = ¼ in Example 3.13, we have

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**Fourier Representations of Signals & LTI Systems**

3 CHAPTER Fourier Representations of Signals & LTI Systems 3. The differentiation property implies that Except for k = 0 The quantity Y[0] is the average value of y(t) and is determined by inspection Fig (b) to be T/2. Therefore, Differentiation in Frequency 1. FT of signal x(t): Differentiating both sides with respect to 2. Differentiation property in frequency domain:

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**Fourier Representations of Signals & LTI Systems**

3 CHAPTER Fourier Representations of Signals & LTI Systems ◆ Differentiation of X(j) in Frequency-Domain ( jt) x(t) in Time-Domain Example 3.40 FT of a Gaussian Pulse Use the differentiation-in-time and differentiation-in-frequency properties to determine the FT of the Gaussian pulse, depicted in Fig and defined by D.E. of g(t) p <Sol.> 1. The derivative of g(t) with respect to t: (3.48) 2. Differentiation-in-time property: Figure (p. 275) Gaussian pulse g(t). (3.48) (3.49) 3. Differentiation-in-time property:

126
3 CHAPTER Fourier Representations of Signals & LTI Systems (3.50) Since the left-hand sides of Eqs. (3.49) and (3.50) are equal, then we have D.E. of G(j) 4. The integration constant c is determined by noting that (see Appendix A-4) The FT of a Gaussian pulse is also a Gaussian pulse! Integration The operation of integration applies only to continuous dependent variables. In both FT and FS, we may integrate with respect to time. In both FT and DTFT, we may integrate with respect to frequency.

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**c can be determined by the average value of x(t)**

3 CHAPTER Fourier Representations of Signals & LTI Systems Case for nonperiodic signal 1. Define (3.51) 2. By differentiation property, we have Indeterminate at = 0 X(j0) = 0 (3.52) 3. When the average value of x(t) is not zero, then it is possible that y(t) is not square integrable. FT of y(t) may not converge! W can get around this problem by including impulse in the transform, i.e. c can be determined by the average value of x(t) C = X(j0) 4. General form of integration property:

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**Fourier Representations of Signals & LTI Systems**

3 CHAPTER Fourier Representations of Signals & LTI Systems (3.53) Ex. Demonstration for the integration property by deriving the FT of the unit step 1. Unit step signal: 2. FT of (t): 3. Eq. (3.53) suggests that: Check: 1. Unit step: (3.54) Fig. 3.61 2. Signum function:

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**Fourier Representations of Signals & LTI Systems**

3 CHAPTER Fourier Representations of Signals & LTI Systems Figure (p. 279) Representation of a step function as the sum of a constant and a signum function. 3. Using the results of Example 3.28: The transform of sgn(t) may be derived using the differentiation property because it has a zero time-average value. Let We know that S(j0) = 0. This knowledge removes the indeterminacy at = 0 associated with differentiation property, and we conclude that

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**Fourier Representations of Signals & LTI Systems**

3 CHAPTER Fourier Representations of Signals & LTI Systems This relationship can be written as S(j) = 2/j with the understanding that S(j0) = 0. 4. Applying the linearity property to Eq. (3.54) gives the FT of u(t): This agrees exactly with the transform of the unit step obtained by using the integration property. Table 3.6 Commonly Used Differentiation and Integration Properties. ◆ The differentiation and integration properties of Fourier representation are summarized in Table 3.6.

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**Fourier Representations of Signals & LTI Systems**

3 CHAPTER Fourier Representations of Signals & LTI Systems 3.12 Time- and Frequency-Shift Properties Time-Shift Property 1. Let z(t) = x(t t0) be a time-shifted version of x(t). 2. FT of z(t): 3. Change variable by = t t0: ◆ Time-shifting of x(t) by t0 in Time-Domain (e j t0) X(j) in Frequency-Domain and ◆ The time-shifting properties of four Fourier representation are summarized in Table 3.7.

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**Fourier Representations of Signals & LTI Systems**

3 CHAPTER Fourier Representations of Signals & LTI Systems Table 3.7 Time-Shift Properties of Fourier Representations Example 3.41 Finding an FT Using the Time-Shift Property Use the FT of the rectangular pulse x(t) depicted in Fig (a) to determine the FT of the time-shift rectangular pulse z(t) depicted in Fig (b). <Sol.> 1. Note that z(t) = x(t T1). 2. Time-shift property:

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**Fourier Representations of Signals & LTI Systems**

3 CHAPTER Fourier Representations of Signals & LTI Systems 3. In Example 3.25, we obtained Frequency response of a discrete-time system 1. Difference equation: Taking DTFTof both sides of this equation

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**Fourier Representations of Signals & LTI Systems**

3 CHAPTER Fourier Representations of Signals & LTI Systems 2. Frequency response: (3.55) The problem is to express the inverse FT of Z(j) = X(j( )) in terms of x(t). Frequency-Shift Property 1. Suppose that: and 2. By the definition of the inverse FT, we have Substituting variables = into above Eq. gives ◆ Frequency-shifting of X(j) by in Frequency-Domain [i.e. X(j( ))] (e t) x(t) in Time-Domain

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**Fourier Representations of Signals & LTI Systems**

3 CHAPTER Fourier Representations of Signals & LTI Systems ◆ The frequency-shift properties of Fourier representations are summarized in Table 3.8. Table 3.8 Frequency-Shift Properties of Fourier Representations Example 3.42 Finding an FT by Using the Frequency-Shift Property Use the frequency-shift property to determine the FT of the complex sinusoidal pulse: <Sol.> 1. Express z(t) as the product of a complex sinusoid ej10t and a rectangular pulse

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**Fourier Representations of Signals & LTI Systems**

3 CHAPTER Fourier Representations of Signals & LTI Systems 2. Use the results of Example 3.25, we have Frequency-shift property 3. FT of z(t): Example 3.43 Using multiple Properties to Find an FT Find the FT of the signal <Sol.> 1. To solve this problem, it needs to use three properties: differentiation in time, convolution, and time shifting.

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**Fourier Representations of Signals & LTI Systems**

3 CHAPTER Fourier Representations of Signals & LTI Systems 2. Let and Convolution and differentiation property 3. From the transform pair: 4. Use the same transform pair and the time-shift property to find V(j) by first writing 5. FT of x(t):

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**3 3.13 Finding Inverse Fourier Transforms by Using Partial-**

CHAPTER Fourier Representations of Signals & LTI Systems 3.13 Finding Inverse Fourier Transforms by Using Partial- Fraction Expansions A ratio of two polynomials in j or e j Frequency response Inverse Fourier Transform 1. FT X(j) in terms of a ratio of polynomials in j: Assume that M < N. If M N, then we may use long division to express X(j) in the form Partial-fraction expansion is applied to this term Applying the differentiation property and the pair to these terms 2. Let the roots of the denominator A(j) be dk, k = 1, 2, …, N. These roots are found by replacing j with a generic variable v and determining the roots of the polynomial

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**3 Fourier Representations of Signals & LTI Systems**

CHAPTER Fourier Representations of Signals & LTI Systems 5. For M < N, we may the write Ck, k = 1, 2, …, N are determined by the method of residues Assuming that all the roots dk, k = 1, 2, …, N, are distinct, we may write Appendix B In Example 3.24, we derived the transform pair This pair is valid even if d is complex, provided that Re{d} < 0. Assuming that the real part of each dk, k = 1, 2, …, N, is negative, we use linearity to write

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**Fourier Representations of Signals & LTI Systems**

3 CHAPTER Fourier Representations of Signals & LTI Systems Example 3.44 MEMS Accelerometer: Impulse Response Find the impulse response for the MEMS accelerometer introduced in Sec. 1.10, assuming that n = 10,000 rads/s, and (a) Q = 2/5, (b) Q = 1, and (c) Q = 200. <Sol.> Frequency response for the MEMS accelerometer: Case (a): n = 10,000 rads/s and Q = 2/5 1. Frequency response:

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**Fourier Representations of Signals & LTI Systems**

3 CHAPTER Fourier Representations of Signals & LTI Systems 2. Partial-fraction expansion: The roots of the denominator polynomial are d1 = 20,000 and d2 = 5,0000. Coefficients C1 and C2:

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**Fourier Representations of Signals & LTI Systems**

3 CHAPTER Fourier Representations of Signals & LTI Systems 3. Impulse response: Case (b): n = 10,000 rads/s and Q = 1 1. Frequency response: 2. Partial-fraction expansion: The roots of the denominator polynomial are 3. Impulse response:

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**Fourier Representations of Signals & LTI Systems**

3 CHAPTER Fourier Representations of Signals & LTI Systems Case (c): n = 10,000 rads/s and Q = 200 1. Frequency response: The roots of the denominator polynomial are 2. Impulse response: Impulse response for (a) Q = 2/5, (b) Q = 1, and (c) Q = 200: Fig (a) ~ (c). 1. For both Q = 2/5 and Q = 1, the impulse response is approximately zero for t > 1 ms. 2. When Q = 200, the accelerometer exhibits resonant nature. A sinusoidal oscillation of n = 10,000 rads/sec.

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**Fourier Representations of Signals & LTI Systems**

3 CHAPTER Fourier Representations of Signals & LTI Systems Figure (p. 289) Impulse response of MEMS accelerometer.

145
**Find dk, the roots of this polynomial**

3 CHAPTER Fourier Representations of Signals & LTI Systems Inverse Discrete-Time Fourier Transform 1. Suppose X(e j ) is given by a ratio of polynomial in e j , i.e. Normalized to 1 2. Factor the denominator polynomial as Replace e j with the generic variable v: Find dk, the roots of this polynomial 3. Partial-fraction expansion: Assuming that M < N and all the dk are distinct, we may express X(e j )as Since

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**Expansions for repeated roots are treated in Appendix B.**

3 CHAPTER Fourier Representations of Signals & LTI Systems Expansions for repeated roots are treated in Appendix B. The linearity property implies that Example 3.45 Inverse by Partial-Fraction Expansion Find the inverse DTFT of <Sol.> 1. Characteristic polynomial: 2. The roots of above polynomial: d1 = 1/2 and d2 = 1/3. 3. Partial-Fraction Expansion:

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**Fourier Representations of Signals & LTI Systems**

3 CHAPTER Fourier Representations of Signals & LTI Systems Coefficients C1 and C2

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**Inner Part: Z(j) X(j)**

3 CHAPTER Fourier Representations of Signals & LTI Systems 3.14 Multiplication Property Non-periodic continuous-time signals 1. Non-periodic signals: x(t), z(t), and y(t) = x(t)z(t). Find the FT of y(t). 2. FT of x(t) and z(t): and Inner Part: Z(j) X(j) Change variable: = v 3. FT of y(t): Outer Part: FT of y(t) (3.56) Scaled by 1/2

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**Fourier Representations of Signals & LTI Systems**

3 CHAPTER Fourier Representations of Signals & LTI Systems where ◆ Multiplication of two signals in Time-Domain Convolution in Frequency-Domain (1/2) Non-periodic continuous-time signals Find the DTFT of y[n]. 1. Non-periodic DT signals: x[n], z[n], and y[n] = x[n]z[n]. 2. DTFT of y[n]: (3.57) where the symbol denotes periodic convolution. Here, X(e j ) and X(e j ) are 2-periodic, so we evaluate the convolution over a 2 interval: ◆ Multiplication of two signals in Time-Domain Convolution in Frequency-Domain (1/2)

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**Fourier Representations of Signals & LTI Systems**

3 CHAPTER Fourier Representations of Signals & LTI Systems Multiplication property can be used to study the effects of truncating a time-domain signal on its frequency- domain. Windowing ! Truncate signal x(t) by a window function w(t) is represented by y(t) = x(t)w(t) Fig (a) Figure 3.65a (p. 293) The effect of windowing. (a) Truncating a signal in time by using a window function w(t).

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**Fourier Representations of Signals & LTI Systems**

3 CHAPTER Fourier Representations of Signals & LTI Systems 1. FT of y(t): 2. If w(t) is the rectangular window depicted in Fig (b), we have p ; v Smoothing the details in X(j) and introducing oscillation near discontinuities in X(j). Figure 3.65b (p. 293) (b) Convolution of the signal and window FT’s resulting from truncation in time.

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**Fourier Representations of Signals & LTI Systems**

3 CHAPTER Fourier Representations of Signals & LTI Systems The smoothing is a consequence of the 2/T0 width of the mainlobe of W(j), while the oscillations near the discontinuities are due to the oscillations in the sidelobes of W(j). Example 3.46 Truncating the Impulse Response The frequency response H(e j) of an ideal discrete-time system is depicted in Fig (a). Describe the frequency response of a system whose impulse response is the ideal system impulse response truncated to the interval M n M. <Sol.> 1. The ideal impulse response is the inverse DTFT of H(e j ). Using the result of Example 3.19, we write 2. Let ht[n] be the truncated impulse response:

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**Fourier Representations of Signals & LTI Systems**

3 CHAPTER Fourier Representations of Signals & LTI Systems v , u Figure (p. 294) The effect of truncating the impulse response of a discrete-time system. (a) Frequency response of ideal system. (b) F() for near zero.

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**Fourier Representations of Signals & LTI Systems**

3 CHAPTER Fourier Representations of Signals & LTI Systems v , u Ht(e j ) is the area under F( ) between = /2 and = /2. Figure (p. 294) The effect of truncating the impulse response of a discrete-time system. (c) F() for slightly greater than /2.(d) Frequency response of system with truncated impulse response.

155
**Discussion: refer to p. 295 in textbook.**

3 CHAPTER Fourier Representations of Signals & LTI Systems 3. Let Using the multiplication property in Eq. (3.57), we have 4. Since On the basis of Example 3.18 and Discussion: refer to p. 295 in textbook. where we have defined

156
**Non-periodic convolution of the FS coefficients**

3 CHAPTER Fourier Representations of Signals & LTI Systems Multiplication of periodic time-domain signals corresponds to convolution of the Fourier representations. Same fundamental periods of x(t) and z(t) (3.58) where Non-periodic convolution of the FS coefficients x(t) 與 z(t)基本週期不同時，FS係數 X[k]與 Z[k]需取兩信號基本週期的最小 公倍數(Least common multiple)週期。 Example 3.47 Radar Range Measurement: Spectrum of RF Pulse Train The RF pulse train used to measure the range and introduced in Section 1.10 may be defined as the product of a square wave p(t) and a sine wave s(t), as shown in Fig Assume that s(t) = sin (1000t/T). Find the FS coefficients of x(t). <Sol.> 1. Suppose that s(t) = p(t) x(t). Multiplication property

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**Fourier Representations of Signals & LTI Systems**

3 CHAPTER Fourier Representations of Signals & LTI Systems Figure (p. 297) The RF pulse is expressed as the product of a periodic square wave and a sine wave.

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**3 Fourier Representations of Signals & LTI Systems**

CHAPTER Fourier Representations of Signals & LTI Systems Fundamental frequency 2. Fundamental period: For p(t): 0 = 2/T. s(t) = sin (1000t/T) = sin(5000t ). 3. FS coefficients of s(t): 4. FS coefficients of p(t): Result of Example 3.13 Time-shift property 5. FS coefficients of x(t): 6. Fig depicts the magnitude spectrum for 0 k 1000.

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**Fourier Representations of Signals & LTI Systems**

3 CHAPTER Fourier Representations of Signals & LTI Systems Figure (p. 298) FS magnitude spectrum for 0 k The result is depicted as a continuous curve, due to the difficulty of displaying 1000 stems

160
**Table 3.9 Multiplication Properties of Fourier Representations**

CHAPTER Fourier Representations of Signals & LTI Systems Multiplication property for discrete-time periodic signals: (3.59) where Periodic convolution of DTFS coefficients ◆ Fundamental period = N. ◆ The multiplication properties of all four Fourier representations are summarized in Table 3.9. Table 3.9 Multiplication Properties of Fourier Representations

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3 CHAPTER Fourier Representations of Signals & LTI Systems 3.15 Scaling Property 1. Let z(t) = x(at). 2. FT of z(t) : Changing variable: = at (3.60) ◆ Scaling in Time-Domain Inverse Scaling in Frequency-Domain Signal expansion or compression! Refer to Fig

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**Fourier Representations of Signals & LTI Systems**

3 CHAPTER Fourier Representations of Signals & LTI Systems v Figure (p. 300) The FT scaling property. The figure assumes that 0 < a < 1.

163
**Scaling property and a = 1/2**

3 CHAPTER Fourier Representations of Signals & LTI Systems Example 3.48 Scaling a Rectangular Pulse Let the rectangular pulse Use the FT of x(t) and the scaling property to find the FT of the scaled rectangular pulse <Sol.> 1. Substituting T0 = 1 into the result of Example gives Scaling property and a = 1/2 2. Note that y(t) = x(t/2). Fig. 3.71 Substituting T0 = 2 into the result of Example 3.25 can also give the answer! Example 3.49 Using Multiple Performance to Find an Inverse FT Find x(t) if

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**Fourier Representations of Signals & LTI Systems**

3 CHAPTER Fourier Representations of Signals & LTI Systems Figure 3.71 (p. 301) Application of the FT scaling property in Example (a) Original time signal. (b) Original FT. (c) Scaled time signal y(t) = x(t/2). (d) Scaled FT Y(j) = 2X(j2). v

165
**Differentiation property**

3 CHAPTER Fourier Representations of Signals & LTI Systems <Sol.> 1. Differentiation in frequency, time shifting, and scaling property to be used to solve the problem. 2. Transform pair: 3. Treatment procedure: scaling time-shift differentiation Define Y(j) = S(j/3). Define W(j) = e j2 Y(j/3). Differentiation property Since

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**Fourier Representations of Signals & LTI Systems**

3 CHAPTER Fourier Representations of Signals & LTI Systems Scaling property in FS: 1. If x(t) is a periodic signal, then z(t) = x(at) is also periodic. 2. If x(t) has fundamental period T, then z(t) has fundamental period T/a. 3. If the fundamental frequency of x(t) is 0, then the fundamental frequency of z(t) is 0/a. 4. FS coefficients of z(t): (3.61) Scaling property in Discrete-Time Domain: z[n] = x[pn] 1. First of all, z[n] = x[pn] is defined only for integer values of p. 2. If p> 1, then scaling operation discards information. Refer to Problem 3.80.

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**The integral inside the braces is the FT of x(t).**

3 CHAPTER Fourier Representations of Signals & LTI Systems 3.16 Parseval Relationships The energy or power in the time-domain representation of a signal is equal to the energy or power in the frequency-domain representation. ◆ Case for CT nonperiodic signal: x(t) 1. Energy in x(t): The integral inside the braces is the FT of x(t). 2. Note that Express x*(t) in terms of its FT X(j):

168
**Table 3.10 Parseval Relationships for the Four Fourier Representations**

CHAPTER Fourier Representations of Signals & LTI Systems (3.62) Normalization! ◆ Energy in Time-Domain Representation Energy in Frequency-Domain Representation (1/2) ◆ The Parseval Relationships of all four Fourier representations are summarized in Table 3.10. Table 3.10 Parseval Relationships for the Four Fourier Representations

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**3 Fourier Representations of Signals & LTI Systems**

CHAPTER Fourier Representations of Signals & LTI Systems Example 3.50 Calculating Energy in a Signal Let Use Parseval’s theorem to evaluate Direct calculation in time-domain is very difficult! <Sol.> 1. Using the DTFT Parseval relationship in Table 3.10, we have 2. Since

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**Fourier Representations of Signals & LTI Systems**

3 CHAPTER Fourier Representations of Signals & LTI Systems 3.17 Time-Bandwidth Product 1. Preview: Fig. 3.72 v , p Figure (p. 305) Rectangular pulse illustrating the inverse relationship between the time and frequency extent of a signal. 2. x(t) has time extent 2T0. X(j) is actually of infinite extent frequency. 3. The mainlobe of sinc function is fallen in the interval of < /T0, in which contains the majority of its energy.

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**Fourier Representations of Signals & LTI Systems**

3 CHAPTER Fourier Representations of Signals & LTI Systems 4. The product of the time extent T0 and mainlobe width 2/T0 is a constant. 5. Compressing a signal in time leads expansion in the frequency domain and vice versa. Signla’s time-bandwidth product ! 6. Bandwidth: The extent of the signal’s significant frequency content. A mainlobe bounded by nulls. Ex. Lowpass filter One half the width of mainlobe. 2) The frequency at which the magnitude spectrum is 1/2 times its peak values. 7. Effective duration of signal x(t): (3.63) 8. Effective bandwidth of signal x(t): (3.64)

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**Fourier Representations of Signals & LTI Systems**

3 CHAPTER Fourier Representations of Signals & LTI Systems 9. The time-bandwidth product for any signal is lower bounded according to the relationship (3.65) We cannot simultaneously decrease the duration and bandwidth of a signal. Uncertainty principle ! Example 3.51 Bounding the Bandwidth of a Rectangular Pulse Let Use the uncertainty principle to place a lower bound on the effective bandwidth of x(t). <Sol.> 1. Td of x(t):

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**3 3.18 Duality Fourier Representations of Signals & LTI Systems**

CHAPTER Fourier Representations of Signals & LTI Systems 2. The uncertainty principle given by Eq. (3.65) states that Rectangular in time-domain sinc function in frequency-domain 3.18 Duality The Duality Property of the FT Difference in the factor 2 and the sign change in the complex sinusoid. 1. FT pair: and 2. General equation: (3.66) Choose = t and = the Eq. (3.66) implies that

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**Fourier Representations of Signals & LTI Systems**

3 CHAPTER Fourier Representations of Signals & LTI Systems v ; p Figure (p. 307) Duality of rectangular pulses and sinc functions.

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**Fourier Representations of Signals & LTI Systems**

3 CHAPTER Fourier Representations of Signals & LTI Systems (3.67) 3. Interchange the role of time and frequency by letting = and = t, then Eq. (3.66) implies that (3.68) Fig. 3.74 4. If we are given by an FT pair (3.69) (3.70) Example 3.52 Applying Duality Find the FT of

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**Fourier Representations of Signals & LTI Systems**

3 CHAPTER Fourier Representations of Signals & LTI Systems v ; p Figure (p. 309) The FT duality property.

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**3 Fourier Representations of Signals & LTI Systems <Sol.>**

CHAPTER Fourier Representations of Signals & LTI Systems <Sol.> 1. Note that: 2. Replacing by t, we obtain x(t) has been expressed as F(jt). 3. Duality property: Difference in the factor N and the sign change in the complex sinusoidal frequency. The Duality Property of the DTFS 1. DTFS Pair for x[n]: and 2. The DTFS duality is stated as follows: if

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**3 Fourier Representations of Signals & LTI Systems**

CHAPTER Fourier Representations of Signals & LTI Systems N = time index; k = frequency index (3.71) (3.72) The Duality Property of the DTFT and FS 1. FS of a periodic continuous time signal z(t): 2. DTFT of an nonperiodic discrete-time signal x[n]: Require 3. Duality relationship between z(t) and X(e j) T = 2 In the DTFT t in the FS Assumption: 0 = 1 n In the DTFT k in the FS

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**Fourier Representations of Signals & LTI Systems**

3 CHAPTER Fourier Representations of Signals & LTI Systems 4. FS coefficients Z[k]: In the DTFT t in the FS n In the DTFT k in the FS 5. DTFT representation of x[n]: 6. Duality property between the FS and the DTFT: if (3.73) (3.74) ◆ The Duality Properties of Fourier representations are summarized in Table 3.11. Example 3.53 FS-DTFT Duality Use the duality property and the results of Example 3.39 to determine the inverse DTFT of the triangular spectrum X(e j) depicted in Fig (a). <Sol.>

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**Table 3.11 Duality Properties of Fourier Representations**

CHAPTER Fourier Representations of Signals & LTI Systems Table 3.11 Duality Properties of Fourier Representations p Figure (p. 311) Example (a) Triangular spectrum. (b) Inverse DTFT. 1. Define a time function z(t) =X(e jt). 2. Duality property of Eq. (3.74): if

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**Fourier Representations of Signals & LTI Systems**

3 CHAPTER Fourier Representations of Signals & LTI Systems Hence, we seek the FS coefficients Z[k] associated with z(t). 3. Assuming that T = 2, z(t) is a time-shifted version of the triangular wave y(t), i.e. 4. From x[n] = Z [ n], we obtain Fig (b)

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**Fourier Representations of Signals & LTI Systems**

3 CHAPTER Fourier Representations of Signals & LTI Systems 3.19 Exploring Concepts with MATLAB Frequency Response of LTI Systems from Impulse Response Discrete-time system LTI System h[n] x[n] y[n] 1. The frequency response of a discrete-time LTI system with finite-duration impulse response may be determined with the use of a finite-duration input sinusoid that is sufficiently long drive the system to a steady state. 2. Suppose that h[n] = 0 for n < kh and n > lh. Let input be the finite-duration sinusoid: 3. System output:

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**Fourier Representations of Signals & LTI Systems**

3 CHAPTER Fourier Representations of Signals & LTI Systems Hence, the system output in response to a finite-duration sinusoidal input corresponds to the output in response to an infinite-duration sinusoidal input on the interval lh n < kh + lv. 4. The magnitude and phase response of the system are determined from y[n], lh n < kh + lv, by noting that 1) Magnitude: 2) Phase: Example Consider the system with impulse response Let us determine the frequency response and 50 values of the steady-state output of this system for the input frequencies = /4 and 3/4. <Sol.>

184
**Obtaining the output signals**

3 CHAPTER Fourier Representations of Signals & LTI Systems 1. kh = 0 and lh = 1. to obtain 50 values of the sinusoidal steady-state response, we require that lv 51. 2. MATLAB commands: Obtaining the output signals >> omega1=pi/4;omega2=3*pi/4; >> v1=exp(j*omega1*[0:50]); >> v2=exp(j*omega2*[0:50]); >> h=[0.5,-0.5]; >> y1=conv(v1,h);y2=conv(v2,h); 3. Plots for the output signals: 1) MATLAB commands for plotting the real and imaginary components of y1: >> subplot(2,1,1) >> stem([0:51],real(y1)) >> xlabel('Time');ylabel('Amplitude'); >> title('real(y1)') >> subplot(2,1,2) >> stem([0:51],imag(y1)) >> title('imag(y1)') Fig (a) and (b)

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**Fourier Representations of Signals & LTI Systems**

3 CHAPTER Fourier Representations of Signals & LTI Systems Figure 3.76 (p. 314) Sinusoidal steady-state response computed with the use of MATLAB. The values at times 1 through 50 represent the sinusoidal steady-state response.

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**3 Fourier Representations of Signals & LTI Systems**

CHAPTER Fourier Representations of Signals & LTI Systems 2) MATLAB commands for obtaining the magnitude and phase components from any element of the vectors of y1 and y2: Ex. Find the fifth element: >> H1mag=abs(y1(5)) H1mag = 0.3827 >> H2mag=abs(y2(5)) H2mag = 0.9239 >> H1phs=angle(y1(5))-omega1*5 H1phs = >> H2phs=angle(y2(5))-omega2*5 H2phs = The phase response is measured in radians. The angle command always returns a value between and radians. Command: angle(y1(n))-omega1*n may result in answers that differ by integer multiples of 2 when different values of n are used.

187
**Fast Fourier transform**

3 CHAPTER Fourier Representations of Signals & LTI Systems The DTFS Fast Fourier transform 1. DTFS pair: (3.10) and (3.11) 2. MATLAB commands fft and ifft may be used to evaluate the DTFS. 1) Given a length-N vector x representing one period of an N periodic signal x[n], the command MATLAB run from 0 to N 1 >> X=fft(x)/N produces a length-N vector X containing the DTFS coefficients X[k]. ◆ The division by N is necessary because fft evaluates the sum in Eq. (3.11) without dividing by N. 2) Given DTFS coefficients in a vector X, the command >> x=ifft(X)*N produces a length-N vector x that represents one period of the time-domain waveform.

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**Fourier Representations of Signals & LTI Systems**

3 CHAPTER Fourier Representations of Signals & LTI Systems Example Consider using MATLAB to solve Problem 3.3 (a) for the DTFS coefficients. The signal is <Sol.> >> x=ones(1,24)+sin([0:23]*pi/12+3*pi/8); >> X=fft(x)/24 1. Period = 24. 2. MATLAB command: Output: X = Columns 1 through 5 i i i i Columns 6 through 10 i i i i i Columns 11 through 15 i i i i Columns 16 through 20 i i i i i Columns 21 through 24 i i i i

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**Fourier Representations of Signals & LTI Systems**

3 CHAPTER Fourier Representations of Signals & LTI Systems 3. Explicit answer in rectangular form: Answer to Problem 3.3 (a) Using indices 11 k 12, we may also write the answer by specifying one period as 4. Using ifft, we may reconstruct the time-domain signal and evaluate the first four values of the reconstructed signal with the command: >> xrecon=ifft(X)*24; >> xrecon(1:4);

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**Fourier Representations of Signals & LTI Systems**

3 CHAPTER Fourier Representations of Signals & LTI Systems ans = i i i i Example The partial-sum approximation used in Example 3.7 is easily evaluated in MATLAB: >> k=1:24; >> n=-24:25; >> B(1)=25/50; % coeff for k=0 >> B(2:25)=2*sin(k*pi*25/50)./(50*sin(k*pi/50)); >> B(26)=sin(25*pi*25/50)./(50*sin(25*pi/50)); % coeff for k =N/2 >> xjhat(1,:)=B(1)*cos(n*0*pi/25); % term in sum for k=0 %accumulate partial sums >> for k=2:26 xjhat(k,:)=xjhat(k-1,:)+B(k)*cos(n*(k-1)*pi/25); end This set of commands produces a matrix xjhat whose (J + 1)st row corresponds to

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**Fourier Representations of Signals & LTI Systems**

3 CHAPTER Fourier Representations of Signals & LTI Systems The FS Trigonometric FS expression of x(t) in Example 3.14: (3.29) where (3.28) Let the partial-sum approximation to the FS in Eq. (3.29), be given by 1. We must use sufficiently closely spaced samples to capture the details in 2. Requirement: The highest-frequency term in sum: is well approximated by the sampled signal:

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**Fourier Representations of Signals & LTI Systems**

3 CHAPTER Fourier Representations of Signals & LTI Systems 3. With plot command, a good approximation to continuous cosine can be achieved by the order of 20 samples per period. Note that the total number of samples in one period is then 20 Jmax. 4. Assuming Jmax = 99 and T = 1, the MATLAB command for computing the partial- sum from the given B[k] is as below: >> t=[-(10*jmax-1):10*jmax]*(1/(20*99)); >> xjhat(1,:) = B(1)*cos(t*0*2*pi/T); >> for k=2:100 xjhat(k,:) = xjhat(k-1,:)+B(k)*cos(t*(k-1)*2*pi/T); end 維度有一些錯誤! 5. The row of xjhat represents samples of a continuous-valued function. We should use plot instead of stem to display xjhat. The partial-sum for J = 5 can be displayed by the command: plot(t, xjhat(6,:))

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**Fourier Representations of Signals & LTI Systems**

3 CHAPTER Fourier Representations of Signals & LTI Systems Frequency Response of LTI Systems Described by Differential or Difference Equations 1. MATLAB commands: freqs Frequency response of LTI system described by differential eq. freqz Frequency response of LTI system described by difference eq. 2. MATLAB command: h=freqs(b,a,w) returns the values of the CT system frequency response given by Eq. (3.47) at the frequencies specified in the vector w, where Coefficients of the differential equation. (3.47)

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**Fourier Representations of Signals & LTI Systems**

3 CHAPTER Fourier Representations of Signals & LTI Systems Example The frequency response of the MEMS accelerometer depicted in Fig for Q =1 is obtained via the MATLAB commands: >> w=[0:100:100000]; >> b=1; >> a=[ *10000]; >> H=freqs(b,a,w); >> plot(w,20*log10(abs(H))) 3. MATLAB command: h=freqz(b,a,w) returns the values of the DT system frequency response given by Eq. (3.55) at the frequencies specified in the vector w, where Coefficients of the difference equation.

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**Fourier Representations of Signals & LTI Systems**

3 CHAPTER Fourier Representations of Signals & LTI Systems The entries of w must lie between 0 and 2. (3.55) DT periodic signals Time-Bandwidth Product 1. Command fft can be used to evaluate DTFS and explore the time-bandwidth product property. 2. Example 3.6 1) Time-domain signal: 2) DTFS coefficients: 3) Parameter definitions: Td = 2M +1 =the nonzero portion of one period of x[n]

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**Fourier Representations of Signals & LTI Systems**

3 CHAPTER Fourier Representations of Signals & LTI Systems Bw N/(2M +1) = Bandwidth = the “frequency” of the first null of X[k] The time-bandwidth product for the square wave, Td Bw N, is independent of M. 4) MATLAB command: Fig (b) >> N=50; >> M=12; >> x=[ones(1,M+1),zeros(1,N-2*M-1),ones(1,M)]; >> X=fft(x)/N; >> k=[0:N-1]; %frequency index >> stem(k,real(fftshift(X))) fft find DTFS coefficients stem display the results real suppress any small imaginary components fftshift reorder the elements of the vector X to generate the DTFS coefficients centered on k = 0

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**Fourier Representations of Signals & LTI Systems**

3 CHAPTER Fourier Representations of Signals & LTI Systems >> N=50; >> M=4; >> x=[ones(1,M+1),zeros(1,N-2*M-1),ones(1,M)]; >> X=fft(x)/N; >> k=[0:N-1]; %frequency index >> stem(k,real(fftshift(X))) Fig (a)

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**Fourier Representations of Signals & LTI Systems**

3 CHAPTER Fourier Representations of Signals & LTI Systems 3. Effective duration and bandwidth for the discrete-time periodic signals: (3.75) (3.76) MATLAB function for computing the product TdBw: >> function TBP=TdBw(x) >> %Compute the Time-Bandwidth product using the DTFS >> %One period must be less than 1025 points >> N=1025; >> M=(N-max(size(x)))/2; >> xc=[zeros(1,M),x,zeros(1,M)]; >> % center pulse within a period >> n=[-(N-1)/2:(N-1)/2]; >> n2=n.*n; >> Td=sqrt((xc.*xc)*n2'/(xc*xc')); >> X=fftshift(fft(xc)/N); % evaluate DTFS and center >> Bw=sqrt(real((X.*conj(X))*n2'/(X*X'))); >> TBP=Td*Bw; 1) The length of input vector x = odd and centers on middle 2) .* = element-by-element product 3) * = inner product 4) ' = complex-conjugate transpose

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**Fourier Representations of Signals & LTI Systems**

3 CHAPTER Fourier Representations of Signals & LTI Systems 4. Use the function TdBw to evaluate the time-bandwidth product for two rectangular, raised cosine, and Gaussian pulse train as follows: >> x=ones(1,101); % 101 point rectangular pulse >> TdBw(x) ans = >> x=ones(1,301); % 301 point rectangular pulse 1.3604e+003 >> x=0.5*ones(1,101)+cos(2*pi*[-50:50]/101); % 101 point rectangular pulse >> x=0.5*ones(1,301)+cos(2*pi*[-150:150]/301); % 301 point rectangular pulse

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**Fourier Representations of Signals & LTI Systems**

3 CHAPTER Fourier Representations of Signals & LTI Systems >> n=[-500:500]; >> x=exp(-0.001*(n.*n)); % Narrow Gaussian pulse >> TdBw(x) ans = >> x=exp( *(n.*n)); % Broad Gaussian pulse Note that the Gaussian pulse trains have the smallest time-bandwidth product. Time-bandwidth product is identical for both the narrow and broad Gaussian pulse trains .

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**Fourier Representations of Signals & LTI Systems**

3 CHAPTER Fourier Representations of Signals & LTI Systems

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