# Frequency Response and Filter Design By Poles and Zeros Positioning Dr. Mohamed Bingabr University of Central Oklahoma Slides For Lathi’s Textbook Provided.

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Frequency Response and Filter Design By Poles and Zeros Positioning Dr. Mohamed Bingabr University of Central Oklahoma Slides For Lathi’s Textbook Provided by Dr. P. Cheung

Frequency Response of a LTI System u We have seen that LTI system response to x(t)=e st is H(s)e st. We represent such input-output pair as: u Instead of using a complex frequency, let us set s = j , this yields: u It is often better to express H(j  ) in polar form: u Therefore Frequency Response Amplitude Response Phase Response

h(t) = 3e -5t |H(s)|

Frequency Response Example (1) u Find the frequency response of a system with transfer function: u Then find the system response y(t) for input x(t)=cos2t and x(t)=cos(10t-50  ) u Substitute s=j 

Frequency Response Example (2)

Frequency Response Example (3) u For input x(t)=cos2t, we have: u u Therefore

Frequency Response Example (4) u For input x(t)=cos(10t-50  ), we will use the amplitude and phase response curves directly: u u Therefore

Frequency Response of delay of T sec u H(s) of an ideal T sec delay is: u Therefore u That is, delaying a signal by T has no effect on its amplitude. u It results in a linear phase shift (with frequency), and a gradient of –T. u The quantity: u is known as Group Delay.

Frequency Response of an ideal differentiator u H(s) of an ideal differentiator is: u Therefore u This agrees with: u That’s why differentiator is not a nice component to work with – it amplifies high frequency component (i.e. noise!).

Frequency Response of an ideal integrator u H(s) of an ideal integrator is: u Therefore u This agrees with: u That’s why integrator is a nice component to work with – it suppresses high frequency component (i.e. noise!).

Filter Design By Poles and Zeros Positioning

Effects of Poles & Zeros on Frequency Response (1) u Consider a general system transfer function:  The value of the transfer function at some complex frequency s = p is:

Effects of Poles & Zeros on Frequency Response (2)  Therefore the magnitude and phase at s = p are given by:

Effects of Poles & Zeros on Frequency Response (3) u Frequency Response of a system is obtained by evaluating H(s) along the y-axis (i.e. taking all value of s=j  ). u Consider the effect of two complex system poles on the frequency response. Near to a pole ENHANCES amplitude

Effects of Poles & Zeros on Frequency Response (4) u Consider the effect of two complex system zeros on the frequency response. Near to a zero SUPPRESSES amplitude

Poles & Low-pass Filters u Use the enhancement and suppression properties of poles & zeros to design filters. u Low-pass filter (LPF) has maximum gain at  =0, and the gain decreases with . u Simplest LPF has a single pole on real axis, say at (s=-  c ). Then u To have a “brickwall” type of LPF (i.e. very sharp cut-off), we need a WALL OF POLE as shown, the more poles we get, the sharper the cut-off. Chebyshev Filter x x x x x x x jcjc -j  c Re  0 x x x x x x x Butterworth Filter N = 2

Poles & Band-pass Filter u Band-pass filter has gain enhanced over the entire passband, but suppressed elsewhere. u For a passband centred around  0, we need lots of poles opposite the imaginary axis in front of the passband centre at  0.

Notch Filter u Notch filter could in theory be realised with two zeros placed at ±j  0. However, such a filter would not have unity gain at zero frequency, and the notch will not be sharp. u To obtain a good notch filter, put two poles close to the two zeros on the semicircle as shown. Since both pole/zero pair are equal-distance to the origin, the gain at zero frequency is exactly one. Same for  =± .

Notch Filter Example u Design a second-order notch filter to suppress 60 Hz hum in a radio receiver. u Make  0 =120 . Place zeros are at s = ±j  0, and poles at -  0 cos  ± j  0 sin . u We get: HW7_Ch4: 4.8-1(a,b), 4.8-2 (a,b,d), 4.10-1, 4.10-3, 4.10-5, 4.10-8, 4.10-9

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