AMPLITUDE MODULATION.

AMPLITUDE MODULATION

INTRODUCTION Amplitude modulation (AM) radio is a commonplace technology today, and is standard in any type of commercial stereo device. Because of the low cost of the parts necessary to implement AM transmission and the simplicity of the underlying technology, using amplitude modulations is a cheap and effective way to perform many tasks that require wireless communication. The most well-known application of an AM transmitter is in radio. Am radio receivers are available in numerous devices, from automobile stereos to clock radios. However, the usage of AM transmitters is not restricted to professional radio stations

Suitable for radio/microwave and satellite communication.
Transmit information bearing (message) or baseband signal (voice music) through a communication channel Baseband = is a range of frequency signal to be transmitted. eg: Audio (0 - 4 kHz), Video (0 - 6 MHz). Communication channel: Transmission without frequency shifting. Transmission through twisted pair cable, coaxial cable and fiber optic cable. Significant power for whole range of frequencies. Not suitable for radio/microwave and satellite communication. Carrier communication Use technique of modulation to shift the frequency. Change the carrier signal characteristics (amplitude, frequency and phase) following the modulating signal amplitude. Suitable for radio/microwave and satellite communication.

the instantaneous amplitude of a carrier wave is varied in accordance with the instantaneous amplitude of the modulating signal. Main advantages of AM are small bandwidth and simple transmitter and receiver designs. Amplitude modulation is implemented by mixing the carrier wave in a nonlinear device with the modulating signal. This produces upper and lower sidebands, which are the sum and difference frequencies of the carrier wave and modulating signal. The carrier signal is represented by c(t) = A cos(wct) The modulating signal is represented by m(t) = B sin(wmt) Then the final modulated signal is [1 + m(t)] c(t) = A [1 + m(t)] cos(wct) = A [1 + B sin(wmt)] cos(wct) = A cos(wct) + A m/2 (cos((wc+wm)t)) + A m/2 (cos((wc-wm)t))

Single sideband suppressed carrier (ssbsc)
Form of amplitude modulation(AM); carrier is suppressed (typically 40 – 60dB below carrier) For short we call it SSB since the carrier is usually not transmitted Advantages Conservation of spectrum space (SSB signal occupies only half the band of DSB signal) The useful power output of the transmitter is greater since the carrier is not amplified SSB receiver is quieter due to the narrower bandwidth (receiver noise is a function of bandwidth) Disadvantages The carrier must be reinserted the receiver, to down convert the sideband back to the original modulating audio Carrier must be reinserted with the same frequency and phase it would have if it were still present This is why clarity control on a CB radio is so fiddly, if carrier not inserted correctly the received output will be like donald duck voice.

Single sideband modulation

SSB-LSB SIGNAL SSB-USB SIGNAL

SSB signal frequency spectrum
SSB-LSB signal SSB-USB signal

Hartley modulator A direct approach for creating a single sideband AM signal (SSB-AM) is to remove either the upper or lower SB by filtering the DSBSC- AM signal (frequency discriminator method) SSB modulator using the frequency discrimination approach Magnitude spectra : (a) baseband ; (b) DSBSC-AM; (c) upper SSB; (d) lower SSB

The hilbert transform is a wideband -90° phase shifter
Quadrature modulator can be used to create a SSB-AM signal by selecting the quadrature signal to coherently cancel either the upper /lower SB from the inphase channel SSB-AM signal given: Using the minus sign in equation (1) results in upper SSB,whereas selection of the plus sign yields lower SSB The hilbert transform is a wideband -90° phase shifter (1) (2)

Hartley modulator for SSB; -sign gives upper SSB, +sign gives lower SSB

Angle Modulation – Frequency Modulation
Consider again the general carrier represents the angle of the carrier. There are two ways of varying the angle of the carrier. By varying the frequency, c – Frequency Modulation. By varying the phase, c – Phase Modulation 1

Frequency Modulation In FM, the message signal m(t) controls the frequency fc of the carrier. Consider the carrier then for FM we may write: FM signal ,where the frequency deviation will depend on m(t). Given that the carrier frequency will change we may write for an instantaneous carrier signal where i is the instantaneous angle = and fi is the instantaneous frequency. 2

Frequency Modulation Since then
i.e. frequency is proportional to the rate of change of angle. If fc is the unmodulated carrier and fm is the modulating frequency, then we may deduce that fc is the peak deviation of the carrier. Hence, we have ,i.e. 3

Frequency Modulation After integration i.e. Hence for the FM signal, 4

Frequency Modulation The ratio
is called the Modulation Index denoted by  i.e. Note – FM, as implicit in the above equation for vs(t), is a non-linear process – i.e. the principle of superposition does not apply. The FM signal for a message m(t) as a band of signals is very complex. Hence, m(t) is usually considered as a 'single tone modulating signal' of the form 5

Frequency Modulation The equation may be expressed as Bessel
series (Bessel functions) where Jn() are Bessel functions of the first kind. Expanding the equation for a few terms we have: 6

FM Signal Spectrum. The amplitudes drawn are completely arbitrary, since we have not found any value for Jn() – this sketch is only to illustrate the spectrum. 7

Generation of FM signals – Frequency Modulation.
An FM demodulator is: a voltage-to-frequency converter V/F a voltage controlled oscillator VCO In these devices (V/F or VCO), the output frequency is dependent on the input voltage amplitude. 8

V/F Characteristics. Apply VIN , e.g. 0 Volts, +1 Volts, +2 Volts, -1 Volts, -2 Volts, ... and measure the frequency output for each VIN . The ideal V/F characteristic is a straight line as shown below. fc, the frequency output when the input is zero is called the undeviated or nominal carrier frequency. The gradient of the characteristic is called the Frequency Conversion Factor, denoted by  per Volt. 9

V/F Characteristics. Consider now, an analogue message input,
As the input m(t) varies from the output frequency will vary from a maximum, through fc, to a minimum frequency. 10

V/F Characteristics. For a straight line, y = c + mx, where c = value of y when x = 0, m = gradient, hence we may say and when VIN = m(t) ,i.e. the deviation depends on m(t). Considering that maximum and minimum input amplitudes are +Vm and -Vm respectively, then on the diagram on the previous slide. The peak-to-peak deviation is fmax – fmin, but more importantly for FM the peak deviation fc is Peak Deviation, Hence, Modulation Index, 11

Summary of the important points of FM
In FM, the message signal m(t) is assumed to be a single tone frequency, The FM signal vs(t) from which the spectrum may be obtained as where Jn() are Bessel coefficients and Modulation Index,  Hz per Volt is the V/F modulator, gradient or Frequency Conversion Factor,  per Volt  is a measure of the change in output frequency for a change in input amplitude. Peak Deviation (of the carrier frequency from fc) 12

FM Signal Waveforms. The diagrams below illustrate FM signal waveforms for various inputs At this stage, an input digital data sequence, d(t), is introduced – the output in this case will be FSK, (Frequency Shift Keying). 13

FM Signal Waveforms. Assuming the output ‘switches’ between f1 and f0.
14

FM Signal Waveforms. The output frequency varies ‘gradually’ from fc to (fc + Vm), through fc to (fc - Vm) etc. 15

FM Signal Waveforms. If we plot fOUT as a function of VIN:
In general, m(t) will be a ‘band of signals’, i.e. it will contain amplitude and frequency variations. Both amplitude and frequency change in m(t) at the input are translated to (just) frequency changes in the FM output signal, i.e. the amplitude of the output FM signal is constant. Amplitude changes at the input are translated to deviation from the carrier at the output. The larger the amplitude, the greater the deviation. 16

FM Signal Waveforms. Frequency changes at the input are translated to rate of change of frequency at the output. An attempt to illustrate this is shown below: 17

FM Spectrum – Bessel Coefficients.
The FM signal spectrum may be determined from The values for the Bessel coefficients, Jn() may be found from graphs or, preferably, tables of ‘Bessel functions of the first kind’. 18

Jn()   = 2.4  = 5 In the series for vs(t), n = 0 is the carrier component, i.e. , hence the n = 0 curve shows how the component at the carrier frequency, fc, varies in amplitude, with modulation index . 19

Hence for a given value of modulation index , the values of Jn() may be read off the graph and hence the component amplitudes (VcJn()) may be determined. A further way to interpret these curves is to imagine them in 3 dimensions 20

Examples from the graph
 = 0: When  = 0 the carrier is unmodulated and J0(0) = 1, all other Jn(0) = 0, i.e.  = 2.4: From the graph (approximately) J0(2.4) = 0, J1(2.4) = 0.5, J2(2.4) = 0.45 and J3(2.4) = 0.2 21

Significant Sidebands – Spectrum.
As may be seen from the table of Bessel functions, for values of n above a certain value, the values of Jn() become progressively smaller. In FM the sidebands are considered to be significant if Jn()  0.01 (1%). Although the bandwidth of an FM signal is infinite, components with amplitudes VcJn(), for which Jn() < 0.01 are deemed to be insignificant and may be ignored. Example: A message signal with a frequency fm Hz modulates a carrier fc to produce FM with a modulation index  = 1. Sketch the spectrum. 22

As shown, the bandwidth of the spectrum containing significant components is 6fm, for  = 1. 23

The table below shows the number of significant sidebands for various modulation indices () and the associated spectral bandwidth. e.g. for  = 5, 16 sidebands (8 pairs). 24

Carson’s Rule for FM Bandwidth.
An approximation for the bandwidth of an FM signal is given by BW = 2(Maximum frequency deviation + highest modulated frequency) Carson’s Rule 25

Narrowband and Wideband FM
Narrowband FM NBFM From the graph/table of Bessel functions it may be seen that for small , (  0.3) there is only the carrier and 2 significant sidebands, i.e. BW = 2fm. FM with   0.3 is referred to as narrowband FM (NBFM) (Note, the bandwidth is the same as DSBAM). Wideband FM WBFM For  > 0.3 there are more than 2 significant sidebands. As  increases the number of sidebands increases. This is referred to as wideband FM (WBFM). 26

VHF/FM VHF/FM (Very High Frequency band = 30MHz – 300MHz) radio transmissions, in the band 88MHz to 108MHz have the following parameters: Max frequency input (e.g. music) 15kHz fm Deviation 75kHz Modulation Index  5 For  = 5 there are 16 sidebands and the FM signal bandwidth is 16fm = 16 x 15kHz = 240kHz. Applying Carson’s Rule BW = 2(75+15) = 180kHz. 27

Comments FM The FM spectrum contains a carrier component and an infinite number of sidebands at frequencies fc  nfm (n = 0, 1, 2, …) FM signal, In FM we refer to sideband pairs not upper and lower sidebands. Carrier or other components may not be suppressed in FM. The relative amplitudes of components in FM depend on the values Jn(), where thus the component at the carrier frequency depends on m(t), as do all the other components and none may be suppressed. 28

Comments FM Components are significant if Jn()  For <<1 (  0.3 or less) only J0() and J1() are significant, i.e. only a carrier and 2 sidebands. Bandwidth is 2fm, similar to DSBAM in terms of bandwidth - called NBFM. Large modulation index means that a large bandwidth is required – called WBFM. The FM process is non-linear. The principle of superposition does not apply. When m(t) is a band of signals, e.g. speech or music the analysis is very difficult (impossible?). Calculations usually assume a single tone frequency equal to the maximum input frequency. E.g. m(t)  band 20Hz  15kHz, fm = 15kHz is used. 29

Power in FM Signals. From the equation for FM
we see that the peak value of the components is VcJn() for the nth component. Single normalised average power = then the nth component is Hence, the total power in the infinite spectrum is Total power 30

Power in FM Signals. By this method we would need to carry out an infinite number of calculations to find PT. But, considering the waveform, the peak value is Vc, which is constant. Since we know that the RMS value of a sine wave is and power = (VRMS)2 then we may deduce that Hence, if we know Vc for the FM signal, we can find the total power PT for the infinite spectrum with a simple calculation. 31

Power in FM Signals. Now consider – if we generate an FM signal, it will contain an infinite number of sidebands. However, if we wish to transfer this signal, e.g. over a radio or cable, this implies that we require an infinite bandwidth channel. Even if there was an infinite channel bandwidth it would not all be allocated to one user. Only a limited bandwidth is available for any particular signal. Thus we have to make the signal spectrum fit into the available channel bandwidth. We can think of the signal spectrum as a ‘train’ and the channel bandwidth as a tunnel – obviously we make the train slightly less wider than the tunnel if we can. 32

Power in FM Signals. However, many signals (e.g. FM, square waves, digital signals) contain an infinite number of components. If we transfer such a signal via a limited channel bandwidth, we will lose some of the components and the output signal will be distorted. If we put an infinitely wide train through a tunnel, the train would come out distorted, the question is how much distortion can be tolerated? Generally speaking, spectral components decrease in amplitude as we move away from the spectrum ‘centre’. 33

= carrier power + sideband powers.
Power in FM Signals. In general distortion may be defined as With reference to FM the minimum channel bandwidth required would be just wide enough to pass the spectrum of significant components. For a bandlimited FM spectrum, let a = the number of sideband pairs, e.g. for  = 5, a = 8 pairs (16 components). Hence, power in the bandlimited spectrum PBL is = carrier power + sideband powers. 34

Power in FM Signals. Since Distortion
Also, it is easily seen that the ratio = 1 – Distortion i.e. proportion pf power in bandlimited spectrum to total power = 35

Example Consider NBFM, with  = 0.2. Let Vc = 10 volts. The total power in the infinite spectrum = 50 Watts, i.e. = 50 Watts. From the table – the significant components are i.e. the carrier + 2 sidebands contain or 99% of the total power 36

Example Distortion = or 1%.
Actually, we don’t need to know Vc, i.e. alternatively Distortion = (a = 1) D = Ratio 37

FM Demodulation –General Principles.
An FM demodulator or frequency discriminator is essentially a frequency-to-voltage converter (F/V). An F/V converter may be realised in several ways, including for example, tuned circuits and envelope detectors, phase locked loops etc. Demodulators are also called FM discriminators. Before considering some specific types, the general concepts for FM demodulation will be presented. An F/V converter produces an output voltage, VOUT which is proportional to the frequency input, fIN. 38

If the input is FM, the output is m(t), the analogue message signal. If the input is FSK, the output is d(t), the digital data sequence. In this case fIN is the independent variable and VOUT is the dependent variable (x and y axes respectively). The ideal characteristic is shown below. We define Vo as the output when fIN = fc, the nominal input frequency. 39

The gradient is called the voltage conversion factor i.e. Gradient = Voltage Conversion Factor, K volts per Hz. Considering y = mx + c etc. then we may say VOUT = V0 + KfIN from the frequency modulator, and since V0 = VOUT when fIN = fc then we may write where V0 represents a DC offset in VOUT. This DC offset may be removed by level shifting or AC coupling, or the F/V may be designed with the characteristic shown next 40

The important point is that VOUT = KVIN. If VIN = m(t) then the output contains the message signal m(t), and the FM signal has been demodulated. 41

Often, but not always, a system designed so that , so that K = 1 and VOUT = m(t). A complete system is illustrated. 42

43

Methods Tuned Circuit – One method (used in the early days of FM) is to use the slope of a tuned circuit in conjunction with an envelope detector. 44

Methods The tuned circuit is tuned so the fc, the nominal input frequency, is on the slope, not at the centre of the tuned circuits. As the FM signal deviates about fc on the tuned circuit slope, the amplitude of the output varies in proportion to the deviation from fc. Thus the FM signal is effectively converted to AM. This is then envelope detected by the diode etc to recover the message signal. Note: In the early days, most radio links were AM (DSBAM). When FM came along, with its advantages, the links could not be changed to FM quickly. Hence, NBFM was used (with a spectral bandwidth = 2fm, i.e. the same as DSBAM). The carrier frequency fc was chosen and the IF filters were tuned so that fc fell on the slope of the filter response. Most FM links now are wideband with much better demodulators. A better method is to use 2 similar circuits, known as a Foster-Seeley Discriminator 45

Foster-Seeley Discriminator
This gives the composite characteristics shown. Diode D2 effectively inverts the f2 tuned circuit response. This gives the characteristic ‘S’ type detector. 46

Phase Locked Loops PLL A PLL is a closed loop system which may be used for FM demodulation. A full analytical description is outside the scope of these notes. A brief description is presented. A block diagram for a PLL is shown below. Note the similarity with a synchronous demodulator. The loop comprises a multiplier, a low pass filter and VCO (V/F converter as used in a frequency modulator). 47

Phase Locked Loops PLL The input fIN is applied to the multiplier and multiplied with the VCO frequency output fO, to produce  = (fIN + fO) and  = (fIN – fO). The low pass filter passes only (fIN – fO) to give VOUT which is proportional to (fIN – fO). If fIN  fO but not equal, VOUT = VIN, fIN – fO is a low frequency (beat frequency) signal to the VCO. This signal, VIN, causes the VCO output frequency fO to vary and move towards fIN. When fIN = fO, VIN (fIN – fO) is approximately constant (DC) and fO is held constant, i.e locked to fIN. As fIN changes, due to deviation in FM, fO tracks or follows fIN. VOUT = VIN changes to drive fO to track fIN. VOUT is therefore proportional to the deviation and contains the message signal m(t). 48

Chapter 5. Noise in CW Modulation System
5.1 Introduction - Receiver Noise (Channel Noise) : additive, White, and Gaussian 으로 가정 5.2 Receiver Model 1. RX Model N0 = KTe where K = Boltzmann’s constant Te = equivalent noise Temp. Average noise power per unit bandwidth Sw(f) Rw() f 

- Band Pass Filter (Ideal case)
w(t) n(t) - filtered noise as narrow-band noise n(t) = nI(t)cos(2fCt) - nQ(t)sin(2fCt) where nI(t) is inphase, nQ(t) is quadrature component -  filtered signal x(t) x(t) = s(t) + n(t) - Average Noise Power = N0BT 2B N0 SNI, SNQ 2B BPF

< DSB와 SSB의 Signal과 Noise Power Spectral Density 정리 > - DSB
1 SM(f) f SS(f) SY(f) f SS(f) 1 f f

5.3 Noise in DSB-SC Receivers 1. Model of DSB-SC Receivers
2. 변복조간의 비교 1) 서로 다른 변복조 system을 비교하기 위한 조건 - s(t) by each system has the same average power - noise w(t) has the same average power measured in the message BW =W 2) Channel SNR 3) 5.3 Noise in DSB-SC Receivers 1. Model of DSB-SC Receivers

2. (SNR)O

5.4 Noise in SSB Receivers - SSB Modulated wave

5.4 Noise in AM Receiver

ex1) Single-Tone Modulation
- By envelope detector ex1) Single-Tone Modulation

Threshold Effect Carrier-to-noise < 1 narrow-band noise n(t)
Threshold Effect : loss of message in an envelope detector that operates at a low CNR.

5.6 Noise in FM Receivers - BPF : [fC - BT/2 ~ fC + BT/2]
w(t) : zero mean white Gaussian noise with psd = No/2 s(t) : carrier =fc, BW = BT 즉 (fC  BT/2) - BPF : [fC - BT/2 ~ fC + BT/2] - Amplitude limiter : remove amplitude variations by clipping and BPF - Discriminator slope network or differentiator : varies linearly with frequency envelope detector - Baseband LPF : message BW에 맞추어서. - FM signal - Filtered noise n(t)

- Discriminator output
at discriminator output at Rx output

<결론> BT Noise Performance
- Figure of merit of FM where P = message power W = message bandwidth kf = frequency sensitivity <결론> BT Noise Performance ex) Single-tone modulation trade off

5.7 Pre-emphasis and de-emphasis in FM
P.S.D. of noise at FM Rx output P.S.D. of typical message signal => 모든 band를 효과적으로 사용 못함 Commercial FM radio에서 사용 signal noise

Applications) FM radio, audio-tape-recording
Dolby-A, Dolby-B, DBX : filtering+dynamic range compression

5.8 Summary and Discussion
<H.W.> chap chap , 6.7, 6.15, 6.17

EEE2009 Communications Contents Introduction to Communication Systems
Analogue Modulation AM, DSBSC, VSB, SSB, FM, PM, Narrow band FM, PLL Demodulators, and FLL Loops Sampling Systems Time and Frequency Division multiplexing systems, Nyquist Principle, PAM, PPM, and PWM. Principles of Noise Random variables, White Noise, Shot, Thermal and Flicker Noise, Noise in cascade amplifiers Pulse Code Modulation PCM and its derivatives, Quantising Noise, and Examples Digital Communication Techniques ASK, FSK, PSK, QPSK, QAM, and M-ary QAM. Case Studies Spread Spectrum Systems, Mobile radio concepts, GSM and Multiple Access Schemes Mobile radio

Recommended Text Books
“An introduction to Analogue and Digital communication”, Haykin (Wiley) “Communication Systems”, Carlson (McGraw & Hill) “Information, Transmission, Modulation and Noise”, Schwartz (McGraw & Hill) “Analogue and Digital Communication Systems”, Raden (Prentice- Hall) “Communication Systems”, Haykin (Wiley) “Electronic Communication Techniques”, Young (Merril-Publ)

Introduction to Modulation and Demodulation
The purpose of a communication system is to transfer information from a source to a destination. In practice, problems arise in baseband transmissions, the major cases being: Noise in the system – external noise and circuit noise reduces the signal-to-noise (S/N) ratio at the receiver (Rx) input and hence reduces the quality of the output. Such a system is not able to fully utilise the available bandwidth, for example telephone quality speech has a bandwidth ≃ 3kHz, a co-axial cable has a bandwidth of 100's of Mhz. Radio systems operating at baseband frequencies are very difficult. Not easy to network.

Multiplexing Multiplexing is a modulation method which improves channel bandwidth utilisation. For example, a co-axial cable has a bandwidth of 100's of Mhz. Baseband speech is a only a few kHz

1) Frequency Division Multiplexing FDM
This allows several 'messages' to be translated from baseband, where they are all in the same frequency band, to adjacent but non overlapping parts of the spectrum. An example of FDM is broadcast radio (long wave LW, medium wave MW, etc.)

2) Time Division Multiplexing TDM
TDM is another form of multiplexing based on sampling which is a modulation technique. In TDM, samples of several analogue message symbols, each one sampled in turn, are transmitted in a sequence, i.e. the samples occupy adjacent time slots.

Radio Transmission Aerial dimensions are of the same order as the wavelength, , of the signal (e.g. quarter wave /4, /2 dipoles).  is related to frequency by where c is the velocity of an electromagnetic wave, and c = 3x108 m/sec in free space. For baseband speech, with a signal at 3kHz, (3x103Hz) = 105 metres or 100km. Aerials of this size are impractical although some transmissions at Very Low Frequency (VLF) for specialist applications are made. A modulation process described as 'up-conversion' (similar to FDM) allows the baseband signal to be translated to higher 'radio' frequencies. Generally 'low' radio frequencies 'bounce' off the ionosphere and travel long distances around the earth, high radio frequencies penetrate the ionosphere and make space communications possible. The ability to 'up convert' baseband signals has implications on aerial dimensions and design, long distance terrestrial communications, space communications and satellite communications. Background 'radio' noise is also an important factor to be considered. In a similar content, optical (fibre optic) communications is made possible by a modulation process in which an optical light source is modulated by an information source.

Networks A baseband system which is essentially point-to- point could be operated in a network. Some forms of access control (multiplexing) would be desirable otherwise the performance would be limited. Analogue communications networks have been in existence for a long time, for example speech radio networks for ambulance, fire brigade, police authorities etc. For example, 'digital speech' communications, in which the analogue speech signal is converted to a digital signal via an analogue-to-digital converter give a form more convenient for transmission and processing.

What is Modulation? The Carrier
In modulation, a message signal, which contains the information is used to control the parameters of a carrier signal, so as to impress the information onto the carrier. The Messages The message or modulating signal may be either: analogue – denoted by m(t) digital – denoted by d(t) – i.e. sequences of 1's and 0's The message signal could also be a multilevel signal, rather than binary; this is not considered further at this stage. The Carrier The carrier could be a 'sine wave' or a 'pulse train'. Consider a 'sine wave' carrier: If the message signal m(t) controls amplitude – gives AMPLITUDE MODULATION AM If the message signal m(t) controls frequency – gives FREQUENCY MODULATION FM If the message signal m(t) controls phase- gives PHASE MODULATION PM or M

Considering now a digital message d(t):
If the message d(t) controls amplitude – gives AMPLITUDE SHIFT KEYING ASK. As a special case it also gives a form of Phase Shift Keying (PSK) called PHASE REVERSAL KEYING PRK. If the message d(t) controls frequency – gives FREQUENCY SHIFT KEYING FSK. If the message d(t) controls phase – gives PHASE SHIFT KEYING PSK. In this discussion, d(t) is a binary or 2 level signal representing 1's and 0's The types of modulation produced, i.e. ASK, FSK and PSK are sometimes described as binary or 2 level, e.g. Binary FSK, BFSK, BPSK, etc. or 2 level FSK, 2FSK, 2PSK etc. Thus there are 3 main types of Digital Modulation: ASK, FSK, PSK.

Multi-Level Message Signals
As has been noted, the message signal need not be either analogue (continuous) or binary, 2 level. A message signal could be multi-level or m levels where each level would represent a discrete pattern of 'information' bits. For example, m = 4 levels

4 level PSK is also called QPSK (Quadrature Phase Shift Keying).
In general n bits per codeword will give 2n = m different patterns or levels. Such signals are often called m-ary (compare with binary). Thus, with m = 4 levels applied to: Amplitude gives 4ASK or m-ary ASK Frequency gives 4FSK or m-ary FSK Phase gives 4PSK or m-ary PSK 4 level PSK is also called QPSK (Quadrature Phase Shift Keying).

Consider Now A Pulse Train Carrier
where and The 3 parameters in the case are: Pulse Amplitude E Pulse width vt Pulse position T Hence: If m(t) controls E – gives PULSE AMPLITUDE MODULATION PAM If m(t) controls t - gives PULSE WIDTH MODULATION PWM If m(t) controls T - gives PULSE POSITION MODULATION PPM In principle, a digital message d(t) could be applied but this will not be considered further.

What is Demodulation? Demodulation is the reverse process (to modulation) to recover the message signal m(t) or d(t) at the receiver.

Summary of Modulation Techniques 1

Summary of Modulation Techniques 2

Summary of Modulation Techniques with some Derivatives and Familiar Applications

Summary of Modulation Techniques with some Derivatives and Familiar Applications 2

Modulation Types AM, FM, PAM

Modulation Types AM, FM, PAM 2

Modulation Types (Binary ASK, FSK, PSK)

Modulation Types (Binary ASK, FSK, PSK) 2

Modulation Types – 4 Level ASK, FSK, PSK

Modulation Types – 4 Level ASK, FSK, PSK 2

Analogue Modulation – Amplitude Modulation
Consider a 'sine wave' carrier. vc(t) = Vc cos(ct), peak amplitude = Vc, carrier frequency c radians per second. Since c = 2fc, frequency = fc Hz where fc = 1/T. Amplitude Modulation AM In AM, the modulating signal (the message signal) m(t) is 'impressed' on to the amplitude of the carrier.

Message Signal m(t) In general m(t) will be a band of signals, for example speech or video signals. A notation or convention to show baseband signals for m(t) is shown below

Message Signal m(t) In general m(t) will be band limited. Consider for example, speech via a microphone. The envelope of the spectrum would be like:

Message Signal m(t) In order to make the analysis and indeed the testing of AM systems easier, it is common to make m(t) a test signal, i.e. a signal with a constant amplitude and frequency given by

Schematic Diagram for Amplitude Modulation
VDC is a variable voltage, which can be set between 0 Volts and +V Volts. This schematic diagram is very useful; from this all the important properties of AM and various forms of AM may be derived.

Equations for AM From the diagram where VDC is the DC voltage that can
be varied. The equation is in the form Amp cos ct and we may 'see' that the amplitude is a function of m(t) and VDC. Expanding the equation we get:

Equations for AM Now let m(t) = Vm cos mt, i.e. a 'test' signal, Using the trig identity we have Components: Carrier upper sideband USB lower sideband LSB Amplitude: VDC Vm/2 Vm/2 Frequency: c c + m c – m fc fc + fm fc + fm This equation represents Double Amplitude Modulation – DSBAM

Spectrum and Waveforms
The following diagrams represent the spectrum of the input signals, namely (VDC + m(t)), with m(t) = Vm cos mt, and the carrier cos ct and corresponding waveforms.

Spectrum and Waveforms
The above are input signals. The diagram below shows the spectrum and corresponding waveform of the output signal, given by

Double Sideband AM, DSBAM
The component at the output at the carrier frequency fc is shown as a broken line with amplitude VDC to show that the amplitude depends on VDC. The structure of the waveform will now be considered in a little more detail. Waveforms Consider again the diagram VDC is a variable DC offset added to the message; m(t) = Vm cos mt

This is multiplied by a carrier, cos ct. We effectively multiply (VDC + m(t)) waveform by +1, -1, +1, -1, ... The product gives the output signal

Modulation Depth Consider again the equation , which may be written as
The ratio is defined as the modulation depth, m, i.e. Modulation Depth From an oscilloscope display the modulation depth for Double Sideband AM may be determined as follows:

Modulation Depth 2 Modulation Depth This may be shown to equal
2Emax = maximum peak-to-peak of waveform 2Emin = minimum peak-to-peak of waveform Modulation Depth This may be shown to equal as follows: = =

Double Sideband Modulation 'Types'
There are 3 main types of DSB Double Sideband Amplitude Modulation, DSBAM – with carrier Double Sideband Diminished (Pilot) Carrier, DSB Dim C Double Sideband Suppressed Carrier, DSBSC The type of modulation is determined by the modulation depth, which for a fixed m(t) depends on the DC offset, VDC. Note, when a modulator is set up, VDC is fixed at a particular value. In the following illustrations we will have a fixed message, Vm cos mt and vary VDC to obtain different types of Double Sideband modulation.

Graphical Representation of Modulation Depth and Modulation Types.

Graphical Representation of Modulation Depth and Modulation Types 2.

Graphical Representation of Modulation Depth and Modulation Types 3
Note then that VDC may be set to give the modulation depth and modulation type. DSBAM VDC >> Vm, m  1 DSB Dim C 0 < VDC < Vm, m > 1 (1 < m < ) DSBSC VDC = 0, m =  The spectrum for the 3 main types of amplitude modulation are summarised

Bandwidth Requirement for DSBAM
In general, the message signal m(t) will not be a single 'sine' wave, but a band of frequencies extending up to B Hz as shown Remember – the 'shape' is used for convenience to distinguish low frequencies from high frequencies in the baseband signal.

Bandwidth Requirement for DSBAM
Amplitude Modulation is a linear process, hence the principle of superposition applies. The output spectrum may be found by considering each component cosine wave in m(t) separately and summing at the output. Note: Frequency inversion of the LSB the modulation process has effectively shifted or frequency translated the baseband m(t) message signal to USB and LSB signals centred on the carrier frequency fc the USB is a frequency shifted replica of m(t) the LSB is a frequency inverted/shifted replica of m(t) both sidebands each contain the same message information, hence either the LSB or USB could be removed (because they both contain the same information) the bandwidth of the DSB signal is 2B Hz, i.e. twice the highest frequency in the baseband signal, m(t) The process of multiplying (or mixing) to give frequency translation (or up- conversion) forms the basis of radio transmitters and frequency division multiplexing which will be discussed later.

Power Considerations in DSBAM
Remembering that Normalised Average Power = (VRMS)2 = we may tabulate for AM components as follows: Component Carrier USB LSB Amplitude pk VDC Power Total Power PT = Carrier Power Pc + PUSB + PLSB

From this we may write two equivalent equations for the total power PT, in a DSBAM signal and The carrier power i.e. or Either of these forms may be useful. Since both USB and LSB contain the same information a useful ratio which shows the proportion of 'useful' power to total power is

For DSBAM (m  1), allowing for m(t) with a dynamic range, the average value of m may be assumed to be m = 0.3 Hence, Hence, on average only about 2.15% of the total power transmitted may be regarded as 'useful' power. ( 95.7% of the total power is in the carrier!) Even for a maximum modulation depth of m = 1 for DSBAM the ratio i.e. only 1/6th of the total power is 'useful' power (with 2/3 of the total power in the carrier).

Example Suppose you have a portable (for example you carry it in your ' back pack') DSBAM transmitter which needs to transmit an average power of 10 Watts in each sideband when modulation depth m = 0.3. Assume that the transmitter is powered by a 12 Volt battery. The total power will be where = 10 Watts, i.e. = Watts Hence, total power PT = = Watts. Hence, battery current (assuming ideal transmitter) = Power / Volts = amps! i.e. a large and heavy 12 Volt battery. Suppose we could remove one sideband and the carrier, power transmitted would be 10 Watts, i.e amps from a 12 Volt battery, which is more reasonable for a portable radio transmitter.

Single Sideband Amplitude Modulation
One method to produce signal sideband (SSB) amplitude modulation is to produce DSBAM, and pass the DSBAM signal through a band pass filter, usually called a single sideband filter, which passes one of the sidebands as illustrated in the diagram below. The type of SSB may be SSBAM (with a 'large' carrier component), SSBDimC or SSBSC depending on VDC at the input. A sequence of spectral diagrams are shown on the next page.

Note that the bandwidth of the SSB signal B Hz is half of the DSB signal bandwidth. Note also that an ideal SSB filter response is shown. In practice the filter will not be ideal as illustrated. As shown, with practical filters some part of the rejected sideband (the LSB in this case) will be present in the SSB signal. A method which eases the problem is to produce SSBSC from DSBSC and then add the carrier to the SSB signal.

with m(t) = Vm cos mt, we may write: The SSB filter removes the LSB (say) and the output is Again, note that the output may be SSBAM, VDC large SSBDimC, VDC small SSBSC, VDC = 0 For SSBSC, output signal =

Power in SSB From previous discussion, the total power in the DSB signal is = for DSBAM. Hence, if Pc and m are known, the carrier power and power in one sideband may be determined. Alternatively, since SSB signal = then the power in SSB signal (Normalised Average Power) is Power in SSB signal =

Demodulation of Amplitude Modulated Signals
There are 2 main methods of AM Demodulation: Envelope or non-coherent Detection/Demodulation. Synchronised or coherent Demodulation.

Envelope or Non-Coherent Detection
An envelope detector for AM is shown below: This is obviously simple, low cost. But the AM input must be DSBAM with m << 1, i.e. it does not demodulate DSBDimC, DSBSC or SSBxx.

Large Signal Operation
For large signal inputs, ( Volts) the diode is switched i.e. forward biased  ON, reverse biased  OFF, and acts as a half wave rectifier. The 'RC' combination acts as a 'smoothing circuit' and the output is m(t) plus 'distortion'. If the modulation depth is > 1, the distortion below occurs

Small Signal Operation – Square Law Detector
For small AM signals (~ millivolts) demodulation depends on the diode square law characteristic. The diode characteristic is of the form i(t) = av + bv2 + cv , where i.e. DSBAM signal.

Small Signal Operation – Square Law Detector
i.e. = = = 'LPF' removes components. Signal out = i.e. the output contains m(t)

Synchronous or Coherent Demodulation
A synchronous demodulator is shown below This is relatively more complex and more expensive. The Local Oscillator (LO) must be synchronised or coherent, i.e. at the same frequency and in phase with the carrier in the AM input signal. This additional requirement adds to the complexity and the cost. However, the AM input may be any form of AM, i.e. DSBAM, DSBDimC, DSBSC or SSBAM, SSBDimC, SSBSC. (Note – this is a 'universal' AM demodulator and the process is similar to correlation – the LPF is similar to an integrator).

Synchronous or Coherent Demodulation
If the AM input contains a small or large component at the carrier frequency, the LO may be derived from the AM input as shown below.

Synchronous (Coherent) Local Oscillator
If we assume zero path delay between the modulator and demodulator, then the ideal LO signal is cos(ct). Note – in general the will be a path delay, say , and the LO would then be cos(c(t – ), i.e. the LO is synchronous with the carrier implicit in the received signal. Hence for an ideal system with zero path delay Analysing this for a DSBAM input =

VX = AM input x LO = = = We will now examine the signal spectra from 'modulator to Vx'

(continued on next page)

and and and and and Note – the AM input has been 'split into two' – 'half' has moved or shifted up to and half shifted down to baseband, and

The LPF with a cut-off frequency  fc will pass only the baseband signal i.e. In general the LO may have a frequency offset, , and/or a phase offset, , i.e. The AM input is essentially either: DSB (DSBAM, DSBDimC, DSBSC) SSB (SSBAM, SSBDimC, SSBSC)

1. Double Sideband (DSB) AM Inputs
The equation for DSB is where VDC allows full carrier (DSBAM), diminished carrier or suppressed carrier to be set. Hence, Vx = AM Input x LO Since

The LPF with a cut-off frequency  fc Hz will remove the components at 2c (i.e. components above c) and hence Obviously, if and we have, as previously Consider now if  is equivalent to a few Hz offset from the ideal LO. We may then say The output, if speech and processed by the human brain may be intelligible, but would include a low frequency 'buzz' at , and the message amplitude would fluctuate. The requirement  = 0 is necessary for DSBAM.

Consider now if  is equivalent to a few Hz offset from the ideal LO. We may then say The output, if speech and processed by the human brain may be intelligible, but would include a low frequency 'buzz' at , and the message amplitude would fluctuate. The requirement  = 0 is necessary for DSBAM. Consider now that  = 0 but   0, i.e. the frequency is correct at c but there is a phase offset. Now we have 'cos()' causes fading (i.e. amplitude reduction) of the output.

The 'VDC' component is not important, but consider for m(t), if (900), i.e. if (1800), i.e. The phase inversion if  =  may not be a problem for speech or music, but it may be a problem if this type of modulator is used to demodulate PRK However, the major problem is that as  increases towards the signal strength output gets weaker (fades) and at the output is zero

If the phase offset varies with time, then the signal fades in and out. The variation of amplitude of the output, with phase offset  is illustrated below Thus the requirement for  = 0 and  = 0 is a 'strong' requirement for DSB amplitude modulation.

2. Single Sideband (SSB) AM Input
The equation for SSB with a carrier depending on VDC is i.e. assuming Hence =

The LPF removes the 2c components and hence Note, if  = 0 and  = 0, ,i.e. has been recovered. Consider first that   0, e.g. an offset of say 50Hz. Then If m(t) is a signal at say 1kHz, the output contains a signal a 50Hz, depending on VDC and the 1kHz signal is shifted to 1000Hz - 50Hz = 950Hz.

The spectrum for Vout with  offset is shown Hence, the effect of the offset  is to shift the baseband output, up or down, by . For speech, this shift is not serious (for example if we receive a 'whistle' at 1kHz and the offset is 50Hz, you hear the whistle at 950Hz ( = +ve) which is not very noticeable. Hence, small frequency offsets in SSB for speech may be tolerated. Consider now that  = 0,  = 0, then

This indicates a fading VDC and a phase shift in the output. If the variation in  with time is relatively slow, thus phase shift variation of the output is not serious for speech. Hence, for SSB small frequency and phase variations in the LO are tolerable. The requirement for a coherent LO is not as a stringent as for DSB. For this reason, SSBSC (suppressed carrier) is widely used since the receiver is relatively more simple than for DSB and power and bandwidth requirements are reduced.

Comments In terms of 'evolution', early radio schemes and radio on long wave (LW) and medium wave (MW) to this day use DSBAM with m < 1. The reason for this was the reduced complexity and cost of 'millions' of receivers compared to the extra cost and power requirements of a few large LW/MW transmitters for broadcast radio, i.e. simple envelope detectors only are required. Nowadays, with modern integrated circuits, the cost and complexity of synchronous demodulators is much reduced especially compared to the additional features such as synthesised LO, display, FM etc. available in modern receivers. Amplitude Modulation forms the basis for: Digital Modulation – Amplitude Shift Keying ASK Digital Modulation – Phase Reversal Keying PRK Multiplexing – Frequency Division Multiplexing FDM Up conversion – Radio transmitters Down conversion – Radio receivers

Carriers and Modulation
CS442

Baseband Transmission
Digital transmission is the transmission of electrical pulses. Digital information is binary in nature in that it has only two possible states 1 or 0. Sequences of bits encode data (e.g., text characters). Digital signals are commonly referred to as baseband signals. In order to successfully send and receive a message, both the sender and receiver have to agree how often the sender can transmit data (data rate). Data rate often called bandwidth – but there is a different definition of bandwidth referring to the frequency range of a signal!

With unipolar signaling techniques, the voltage is always positive or negative (like a dc current). In bipolar signaling, the 1’s and 0’s vary from a plus voltage to a minus voltage (like an ac current). In general, bipolar signaling experiences fewer errors than unipolar signaling because the signals are more distinct.

Manchester encoding is a special type of unipolar signaling in which the signal is changed from a high to low (0) or low to high (1) in the middle of the signal. More reliable detection of transition rather than level consider perhaps some constant amount of dc noise, transitions still detectable but dc component could throw off NRZ-L scheme Transitions still detectable even if polarity reversed Manchester encoding is commonly used in local area networks (ethernet, token ring).

Manchester Encoding

ANALOG TRANSMISSION OF DIGITAL DATA
Analog Transmission occurs when the signal sent over the transmission media continuously varies from one state to another in a wave-like pattern. e.g. telephone networks, originally built for human speech rather than data. Advantage for long distance communications: much less attenuation for analog carrier than digital

Digital Data to Analog Transmission
Before we get further into Analog to Digital, we need to understand various characteristics of analog transmission.

Periodic Signals

Sine Wave Peak Amplitude (A) Frequency (f) Phase () General Sine wave
maximum strength of signal volts Frequency (f) Rate of change of signal Hertz (Hz) or cycles per second Period = time for one repetition (T) T = 1/f Phase () Relative position in time, from 0-2*pi General Sine wave

Varying Sine Waves

Wavelength Distance occupied by one cycle
Distance between two points of corresponding phase in two consecutive cycles  = Wavelength Assuming signal velocity v  = vT f = v c = 3*108 ms-1 (speed of light in free space)

Frequency Domain Concepts
Signal usually made up of many frequencies Components are sine (or cosine) waves Can be shown (Fourier analysis) that any continuous signal is made up of component sine waves Can plot frequency domain functions

Addition of Frequency Components
Notes: 2nd freq a multiple of 1st 1st called fundamental freq Others called harmonics Period of combined = Period of the fundamental Fundamental = carrier freq

Frequency Domain Discrete Freq Rep:
Any continuous signal can be represented as the sum of sine waves! (May need an infinite number..) Discrete signals result in Continuous, Infinite Frequency Rep: s(t)=1 from –X/2 to X/2

Data Rate and Bandwidth
Any transmission system has a limited band of frequencies This limits the data rate that can be carried Spectrum range of frequencies contained in signal Absolute bandwidth width of spectrum Effective bandwidth Often just bandwidth Narrow band of frequencies containing most of the energy

Example of Data Rate/Bandwidth
Want to transmit: Let’s say that f=1Mhz or 106 cycles/second, so T= 1microsecond Let’s approximate the square wave with a few sine waves:

Ex(1): Sine Wave 1 Bandwidth=5f-f =4f
If f=1Mhz, then the bandwidth = 4Mhz T=1 microsecond; we can send two bits per microsecond so the data rate = 2 * 106 = 2Mbps

Ex(2): Sine Wave 1, Higher freq
Bandwidth=5f-f =4f If f=2Mhz, then the bandwidth = 8Mhz T=0.5 microsecond; we can send two bits per 0.5 microseconds or 4 bits per microsecond, so the data rate = 4 * 106 = 4Mbps Double the bandwidth, double the data rate!

Ex(3): Sine Wave 2 Bandwidth=3f-f =2f
If f=2Mhz, then the bandwidth = 4Mhz T=0.5 microsecond; we can send two bits per 0.5 microseconds or 4 bits per microsecond, so the data rate = 4 * 106 = 4Mbps Still possible to get 4Mbps with the “lower” bandwidth, but our receiver must be able to discriminate from more distortion!

Bandwidth / Representation
2000 bps B=500 Hz B=1000 Hz B=1700 Hz B=4000 Hz Increasing bandwidth improves the representation of the data signal. 500Hz too low to reproduce the signal. Want to maximize the capacity of the available bandwidth.

Multiplexers A multiplexer puts two or more simultaneous transmissions on a single communications circuit. Generally speaking, the multiplexed circuit must have the same capacity as the sum of the circuits it combines. The primary benefit of multiplexing is to save money.

Multiplexed Circuit

Multiplexing There are three major types of multiplexers
Frequency division multiplexers (FDM) E.g. AM/FM Radio, Telephone Time division multiplexers (TDM) ISDN Statistical time division multiplexers (STDM) We’ll cover later (maybe) Wavelength division multiplexing (WDM) Used in optical carriers (colors carry signals)

Frequency Division Multiplexing (FDM)
Frequency division multiplexers can be described as dividing the circuit “horizontally” so that many signals can travel a single communication circuit simultaneously. The circuit is divided into a series of separate channels, each transmitting on a different frequency. Guardbands are employed to keep one channel from leaking over into another channel. Frequency division multiplexers are somewhat inflexible because once you determine how many channels are required, it may be difficult to add more channels without purchasing an entirely new multiplexer.

Frequency Division Multiplexing (FDM)

Time Division Multiplexing (TDM)
Time division multiplexing shares a circuit among two or more terminals by having them take turns, dividing the circuit “vertically.” Time on the circuit is allocated even when data are not transmitted, so that some capacity is wasted when a terminal is idle. Time division multiplexing is generally more efficient and less expensive to maintain than frequency division multiplexing, because it does not need guardbands.

Time Division Multiplexing (TDM)

Transmission Impairments
Signal received may differ from signal transmitted Analog - degradation of signal quality Digital - bit errors Caused by Attenuation and attenuation distortion Delay distortion Noise

Attenuation Signal strength falls off with distance Depends on medium
Received signal strength: must be enough to be detected must be sufficiently higher than noise to be received without error Attenuation is an increasing function of frequency; higher frequencies suffer from more attenuation. Can distort the signal. Solution: Equalization. Boost higher frequency components.

Delay Distortion Only in guided media
Propagation velocity varies with frequency Velocity highest near center frequency Results in phase shift at different frequencies “Overlapping” bits Solution: Equalization

Noise (1) Additional signals inserted between transmitter and receiver
Thermal Due to thermal agitation of electrons Uniformly distributed White noise Intermodulation Signals that are the sum and difference of original frequencies sharing a medium

Noise (2) Crosstalk Impulse
A signal from one line is picked up by another Impulse Irregular pulses or spikes e.g. External electromagnetic interference Short duration High amplitude

What Causes Errors? Summary of Errors and Noise:
Source of Error What Causes It How to Prevent It. Line Outages White Noise Impulse Noise Cross-Talk Echo Attenuation Intermodulation Noise Jitter Harmonic Distortion Storms, Accidents Movement of electrons Sudden increases in electricity (e.g. lightning) Multiplexer guardbands too small, or wires too close together Poor connections Graduate decrease in signal over distance Signals from several circuits combine Analog signals change phase Amplifier changes phase Increase signal strength Shield or move the wires Increase the guardbands, or move or shield the wires Fix the connections, or tune equipment Use repeaters or amps Move or shield the wires Tune equipment

Error Prevention There are many ways to prevent errors:
Shielding (adding insulation) Moving cables away from noise sources Changing multiplexing type (FDMTDM) Tuning transmission equipment and improving connection quality Using amplifiers and repeaters Equalization Leasing conditioned circuits

Modulation - Digital Data, Analog Signal
Public telephone system 300Hz to 3400Hz Guardband from 0-300, Hz Use modem (modulator-demodulator) Amplitude shift keying (ASK) Frequency shift keying (FSK) Phase shift keying (PSK)

Amplitude Modulation and ASK

Frequency Modulation and FSK

Phase Modulation and PSK

Amplitude Shift Keying
Values represented by different amplitudes of carrier Usually, one amplitude is zero i.e. presence and absence of carrier is used Susceptible to sudden gain changes Inefficient Typically used up to 1200bps on voice grade lines Used over optical fiber

Frequency Shift Keying
Values represented by different frequencies (near carrier) Less susceptible to error than ASK Typically used up to 1200bps on voice grade lines High frequency radio Even higher frequency on LANs using co-ax

FSK on Voice Grade Line Bell Systems 108 modem

Phase Shift Keying Phase of carrier signal is shifted to represent data Differential PSK Phase shifted relative to previous transmission rather than some reference signal

Sending Multiple Bits Simultaneously
Each of the three modulation techniques can be refined to send more than one bit at a time. It is possible to send two bits on one wave by defining four different amplitudes. This technique could be further refined to send three bits at the same time by defining 8 different amplitude levels or four bits by defining 16, etc. The same approach can be used for frequency and phase modulation.

In practice, the maximum number of bits that can be sent with any one of these techniques is about five bits. The solution is to combine modulation techniques. One popular technique is quadrature amplitude modulation (QAM) involves splitting the signal into eight different phases, and two different amplitude for a total of 16 different possible values, giving us lg(16) or 4 bits per value.

2-D Diagram of QAM

Trellis coded modulation (TCM) is an enhancement of QAM that combines phase modulation and amplitude modulation. The problem with high speed modulation techniques such as TCM is that they are more sensitive to imperfections in the communications circuit.

Bits Rate Versus Baud Rate Versus Symbol Rate
The terms bit rate (the number of bits per second) and baud rate are used incorrectly much of the time. They are not the same. A bit is a unit of information, a baud is a unit of signaling speed, the number of times a signal on a communications circuit changes. ITU-T now recommends the term baud rate be replaced by the term symbol rate.

Bits Rate Versus Baud Rate Versus Symbol Rate
The bit rate and the symbol rate (or baud rate) are the same only when one bit is sent on each symbol. If we use QAM or TCM, the bit rate would be several times the baud rate.

Modem Standards There are many different types of modems available today. Most modems support several standards so that they can communicate with a variety of different modems. Better modems can change data rates during transmission to reduce the rate in case of noisy transmission (fast retrain).

Modem Standards

Modem Standards V.22 V.32 and V.32bis V.34 and V.34bis
baud/bps, FSK V.32 and V.32bis full duplex at 9600 bps (2400 baud at QAM) bis uses TCM to achieve 14,400 bps. V.34 and V.34bis Works best for phone networks using digital transmission beyond the local loop to reduce noise. Up to 28,800 bps (TCM) bis up to 36,600 with TCM

Modem Standards V.42bis data compression modems, accomplished by run length encoding, code book compression, Huffman encoding and adaptive Huffman encoding MNP5 - uses Huffman encoding to attain 1.3:1 to 2:1 compression. bis uses Lempel-Ziv encoding and attains 3.5:1 to 4:1. V.42bis compression can be added to almost any modem standard effectively tripling the data rate.

Analog Data, Digital Signal
Digitization Conversion of analog data into digital data Digital data can then be transmitted using digital signaling (e.g. Manchester) Or, digital data can then be converted to analog signal Analog to digital conversion done using a codec (coder/decoder) Two techniques to convert analog to digital Pulse code modulation / Pulse amplitude modulation Delta modulation

Pulse Amplitude Modulation
Analog voice data must be translated into a series of binary digits before they can be transmitted. With Pulse Amplitude Modulation, the amplitude of the sound wave is sampled at regular intervals and translated into a binary number. The difference between the original analog signal and the translated digital signal is called quantizing error.

For standard voice grade circuits, the sampling of 3300 Hz at an average of 2 samples/second would result in a sample rate of 6600 times per second. There are two ways to reduce quantizing error and improve the quality of the PAM signal. Increase the number of amplitude levels Sample more frequently (oversampling).

Pulse Code Modulation Pulse Code Modulation is the most commonly used technique in the PAM family and uses a sampling rate of samples per second. Each sample is an 8 bit sample resulting in a digital rate of 64,000 bps (8 x 8000). Sampling Theorem: If a signal is sampled at a rate higher than twice the highest signal frequency, then the samples contain all the information of the original signal. E.g.: For voice capped at 4Khz, can sample at 8000 times per second to regenerate the original signal.

Performance of A/D techniques
Good voice reproduction via PCM PCM levels (7 bit) Voice bandwidth 4khz Should be 8000 x 7 = 56kbps for PCM (Actually 8000 x 8 with control bit) Data compression can improve on this e.g. Interframe coding techniques for video Why digital? Repeaters instead of amplifiers; don’t amplify noise Allows efficient and flexible Time Division Multiplexing over Frequency Division Multiplexing Conversion to digital allows use of more efficient digital switching techniques

Analog Data, Analog Signals
Why modulate analog signals? Higher frequency can give more efficient transmission Permits frequency division multiplexing Types of modulation Amplitude Frequency Phase Ex. Of analog/analog modulation : Spread Spectrum

Spread Spectrum Analog or digital data Analog signal
Spread data over wide bandwidth Makes jamming and interception harder Frequency hopping Signal broadcast over pseudorandom series of frequencies Direct Sequence Each bit is represented by multiple bits in transmitted signal Chipping code

Analog/Digital Modems (56k Modems)
The V.34+ modem is probably the fastest analog modem that will be developed. The basic idea behind 56K modems (V.90) is to take the basic concepts of PCM both forwards and backwards. The PSTN is already digitizing analog data, and sending it at 64Kbps. However, not all of these bit patterns are actually available for data (one bit used for control), so the maximum data rate becomes 56K. The problem becomes the Analog to Digital conversion; assuming a 4Khz bandwidth coupled with quantization error limits us to the 33.6Kbps when performing an ADC conversion! Solution: Eliminate one ADC conversions going downstream from the ISP; we still have to do a DAC conversion but this doesn’t introduce quantization error (our V.90 modem must have the fine resolution to reproduce the original signal)

V.90 56K Modems

Analog/Digital Modems (56k Modems)
Noise is a critical issue. Tests found 56K modems to connect at less than 40 Kbps 18% of the time, Kbps 80% of the time, and 50+ Kbps only 2 % of the time. It is easier to control noise in the channel transmitting from the server to the client than in the opposite direction. Because the current 56K technology is based on the PCM standard, it cannot be used on services that do not use this standard.

Cable Modems Much more complicated than normal modems
Tuner, decoder, modulator, demodulator, router, hub, etc. Bus architecture – scalability issues Downstream: 27Mbs per 6Mhz channel, QAM Upstream: more noise, 3Mbps per 2Mhz channel, QPSK (2-3 bits per symbol)

xDSL Digital Subscriber Line – High bandwidth via ordinary copper lines Typically range from Mbps to 512 Kbps downstream and around 128 Kbps upstream but could have 8Mbps downstream, 768Kbps upstream for ADSL Many variants: ADSL, ADSL-Lite, CDSL, HDSL, IDSL, SDSL, VDSL … Need to be about <15000 feet from a CO More scalable than cable modems No real standards yet? (DSL-Lite somewhat, no splitter)

DSL Installation

ADSL Splitter

FREQUENCY AND PHASE MODULATION (ANGLE MODULATION)

ANGLE MODULATION – When frequency or phase of the carrier is varied by the modulating signal , then it is called angle modulation. Frequency Modulation – When the frequency of the carrier varies as per amplitude of modulating signal, then it is called frequency modulation (FM). Phase Modulation - When the phase of the carrier varies as per amplitude of modulating signal, then it is called phase modulation (PM). Amplitude of the modulated carrier remains constant in both modulation systems. BDG(xx)

An important feature of angle modulation:
It can provide a better discrimination (robustness) against noise and interference than AM. This improvement is achieved at the expense of incr eased transmission bandwidth. In case of angle modulation, channel bandwidth may be exchanged for improved noise performance Such trade-oﬀ is not possible with AM BDG(xx)

Sinusoidal carrier c(t) =Ac cos[θi(t)] Angle of carrier θi(t)[rad]
BASIC DEFINITIONS -Relationship between the angle and frequency of a sinusoidal signal Sinusoidal carrier c(t) =Ac cos[θi(t)] Angle of carrier θi(t)[rad] Instantaneous frequency of carrier fi(t) =(1/2π)ωi(t) =(1/2π)di(t)/dt =(1/2π)˙ θi(t)[Hz]. In the case of an un-modulated carrier, the angle becomes θi(t) = 2πfct + θc BDG(xx)

Time domain representation
BDG(xx)

In FM, the frequency of carrier is varied directly.
Compare FM-PM – The basic difference between FM & PM lies in which property of the carrier is directly varied by modulating signal. In FM, the frequency of carrier is varied directly. In PM, phase of the carrier is varied directly. Instantaneous phase deviation is represented by θ(t). Instantaneous phase= ωct + θ(t) rad. BDG(xx)

Instantaneous frequency deviation = d/dt {θ(t)} = θ’(t) Hz.
The instantaneous frequency deviation is the instantaneous change in carrier frequency and is equal to the rate at which instantaneous phase deviation takes place. Instantaneous frequency is defined as frequency of the carrier at a given instant of time and is given as ωi(t) =d/dt [ωc.t + θ(t)] = ωc + θ’(t) rad/sec. BDG(xx)

Instantaneous phase deviation θ (t) is proportional to modulating signal voltage, θ (t) = k em(t) rad. ( k is deviation sensitivity of phase.). Instantaneous frequency deviation θ’ (t) is proportional to modulating signal voltage, θ’ (t) = k1 em(t) rad. ( k1 is deviation sensitivity of frequency.)##2 BDG(xx)

Frequency modulation

Observations from the FM & PM waveforms –
1. Both FM & PM waveforms are identical except the phase shift. 2. For FM, the maximum frequency deviation takes place when modulating signal is at +ve and –ve peaks. 3. For PM, the maximum frequency deviation takes place near zero crossing of the modulating signal. 4. It is diffcult to know from modulated waveform whether the modulation is FM or PM. (##3) BDG(xx)

Bandwidth Requirement – for FM-
The BW requirement can be obtained depending on the modulation index (M.I). The M.I. can be classified as high(more than 10), medium (1 to 10) and low (less than 1). The low index systems are called narrowband FM in which frequency spectrum resembles AM. BW (fm) =2fm Hz. For high index modulation, BW = 2*δ.(Freq. dev.) BW can also be found out by Bessel table BWfm = 2.n.fm where n is the number of sidebands obtained from table. BDG(xx)

Rule gives approximate minimum BW of angle modulated signal as
Carson’s Rule – Rule gives approximate minimum BW of angle modulated signal as BW fm = 2{δ + fm(max)} Hz. From the above equation, it is found that the BW accommodates almost 98% of the total transmitted power BDG(xx)

BW for PM is expressed as BWpm = 2(mp+1)fm.
Bandwidth for PM – BW for PM is expressed as BWpm = 2(mp+1)fm. BDG(xx)

Block Diagram of NBFM Generator – Carrier is Ec cos ωc .t
Integrator Product Modulator Adder M(t) NBFM signal 90 degree Phase Shifter Carrier Generator BDG(xx)

Phasor Diagram of NBFM –
USB phasor at an angle of (ωm .t) and LSB phasor at an angle of (- ωm .t) Resultant Phasor LSB Phasor USB Phasor Phase Shift Carrier Phasor BDG(xx)

Carrier phasor is Ec cos ωc .t (always fixed).
USB phasor in clockwise direction (ωm) LSB phasor in counter clockwise direction (-ωm) Resultant of two sideband phasor is always perpendicular to carrier phasor. The net resultant phasor of NBFM due to Carrier and sidebands is as shown in the diagram. BDG(xx)

Wideband Frequency Modulation –
If the modulation index is higher than 10, then it is called wideband FM. Spectrum contains infinite numbers of sidebands and carrier as against two sidebands and carrier in NBFM. BW is = 2{δ+fm(max)} as against 2fm for NBFM. Used for broadcast and entertainment as against for mobile communication for NBFM. ##5. BDG(xx)

Advantages of Angle Modulation over AM-
1. As the amplitude of FM carrier is constant, the noise interference is minimum. 2. The amplitude of FM carrier is constant and is independent of depth of modulation. Hence transmitter power remains constant in FM whereas it varies in AM. 3. As against the limitation of depth of modulation in AM, in FM depth of modulation can be increased to any value, without causing any distortion. BDG(xx)

4. Because of guard bands provided in FM, adjacent channel interference is very less.
5. Since FM uses VHF and UHF bands of frequencies, the noise interference is minimum as compared to AM which uses MF and HF ranges. 6. Radius of propagation is limited as FM uses space waves with line of sight. So it is possible to operate many independent transmitters on the same frequency with minimum interference. BDG(xx)

Disadvantages of FM compared to AM-
1. BW requirement of FM is very high as compared to AM. 2. FM equipments are more complex and hence costly. Area covered by FM is limited, to line of sight area but AM coverage area is large. BDG(xx)

Comparison between FM and AM -
Parameter AM FM Origin AM method of audio transmission was first successfully carried out in the mid 1870s. FM radio was developed in the United states mainly by Edwin Armstrong in the 1930s. Modulating differences In AM, a radio wave known as the "carrier" or "carrier wave" is modulated in amplitude by the signal that is to be transmitted In FM, a radio wave known as the "carrier" or "carrier wave" is modulated in frequency by the signal that is to be transmitted. Importance It is used in both analog and digital communication and telemetry It is used in both analog and digital communication and telemetry Frequency Range AM radio ranges from 535 to 1705 KHz (OR) Up to 1200 Bits per second. FM radio ranges in a higher spectrum from 88 to 108 MHz. (OR) 1200 to 2400 bits per second. BDG(xx)

Comparison between FM and AM -
Parameter AM FM Bandwidth Requirements Twice the highest modulating frequency. In AM radio broadcasting, the modulating signal has bandwidth of 15kHz, and hence the bandwidth of an amplitude-modulated signal is 30kHz. Twice the sum of the modulating signal frequency and the frequency deviation. If the frequency deviation is 75kHz and the modulating signal frequency is 15kHz, the bandwidth required is 180kHz. Complexity Transmitter and receiver are simple but synchronization is needed in case of SSBSC AM carrier. Transmitter and receiver are more complex as variation of modulating signal has to be converted and detected from corresponding variation in frequencies.(i.e. voltage to frequency and frequency to voltage conversion has to be done). Noise AM is more susceptible to noise because noise affects amplitude, which is where information is "stored" in an AM signal. FM is less susceptible to noise because information in an FM signal is transmitted through varying the frequency, and not the amplitude. BDG(xx)

Comparison between FM and PM -
Sr No. FM PM 1 The max frequency deviation depends on amplitude of modulating signal and its frequency The max phase deviation depends on amplitude of modulating signal 2 Frequency of the carrier is modulated by modulating signal. Phase of the carrier is modulated by modulating signal. 3 Modulation index is increased as modulation frequency is reduced and vice versa. Modulation index remains same if modulating signal frequency is change. BDG(xx)

BDG(xx)

Carrier frequency can be generated by LC oscillator.
Modulators – Carrier frequency can be generated by LC oscillator. By varying the values of L or C of tank circuit, carrier frequency can be changed. Properties of BJT,FET and varactor diodes can be varied by changing the voltage across them. When these components are used with LC tank circuits, we are able to vary frequency of oscillator by changing the reactance of L or C. BDG(xx)

TWO types of FM Modulators –
1. Indirect FM – Modulation is obtained by phase modulation of the carrier An instantaneous phase of the carrier is directly proportional to the amplitude of the modulating signal. 2. Direct FM- The frequency of carrier is varied directly by modulating signal An instantaneous frequency variation is directly proportional to the amplitude of the modulating signal. BDG(xx)

There are two methods to derive FM by using FET and varactor.
1. Frequency modulation using Varactor Diode – There exists small junction capacitance in the reverse biased condition of all the diodes. The varactor diodes are designed to optimise this characteristic. As the reverse bias across varactor diode is varied, its junction capacitance changes. These changes are linear and wide (1 to 200pF) BDG(xx)

Frequency modulation using varactor diode –
All diodes show small junction capacitance in the reverse biased condition. ##6 BDG(xx)

Advantages of FM using Varactor Diode –
1. High frequency stability as crystal oscillator is isolated from modulator. Disadvantages – 1. To avoid distortion, the amplitude of modulating signal is to be kept small. 2. The varactor diode must have non linear characteristics of capacitance vs. voltage. Use – This method is used for low index narrow band FM generation. BDG(xx)

FET Reactance Modulator –
There are a number of devices whose reactance can be varied by the application of voltage. These include FET and BJT, varactor diode etc. If such a device is placed across the tank circuit of the L-C oscillator, then FM will be produced when the reactance of the device is varied by the modulating voltage. At the carrier frequency, the oscillator inductance is tuned by its own capacitance in parallel with the average reactance to the variable reactance device. BDG(xx)

Advantages of FET Reactance Modulator –
1. Due to FET characteristics, linear relationship between modulating voltage and transconductance can be achieved. 2 This method produces enough frequency deviation and hence no frequency multiplication is required. Disadvantages - Frequency stability is poor as lumped components are used. Use – This method is used for low modulation index application. BDG(xx)

Indirect FM – Phase modulation is used to achieve frequency modulation in the indirect method, It is necessary to integrate the modulating signal prior to applying it to the phase modulator, This transmitter is widely used in VHF and UHF radio telephone equipment. ##7 BDG(xx)

Use –Used for narrow band low index FM.
Advantage – 1. The crystal oscillator is isolated from modulator, so frequency stability is very good. Disadvantages – 1. Because of nonlinear capacitance Vs. voltage characteristics of varactor diode, there is a distortion in the modulated output waveform. 2. Amplitude of modulating signal should be kept small to avoid distortion. Use –Used for narrow band low index FM. BDG(xx)

Two types of transmitters – Indirect FM and Direct FM Transmitters.
Indirect FM Transmitters – Produces the FM whose phase deviation is directly proportional to modulating signal amplitude. Frequency of oscillator is not directly varied. Hence crystal oscillators can be used. Direct FM Transmitters – Frequency deviation is directly proportional to modulating signal. Carrier frequency is directly deviated. BDG(xx)

Need for Automatic Frequency Correction –
In FM transmitters, the frequency of the oscillator is directly varied. To obtain very stable frequency of oscillator, automatic frequency correction technique is employed. BDG(xx)

AFC loop is to maintain stable centre frequency.
FM Transmitters – Block Diagram –##8 The modulating signal is given to frequency modulator ( may be reactance modulator or VCO ) and oscillator. Let fc = F Mhz. Multiplied by 18 to generate the transmitted frequency F*18 Mhz. AFC loop is to maintain stable centre frequency. Multiplier output given to mixer is F*6. BDG(xx)

The crystal frequency oscillator - reference frequency is {(6
The crystal frequency oscillator - reference frequency is {(6*fc) – 2MHz.} The mixer generates 2 MHz difference frequency which is given to discriminator, which is tuned to 2 MHz. If there is a difference in the output frequency of mixer, discriminator generates d. c. correction voltage. If multiplier frequency is exactly 6*fc, then no correction is required and hence correction voltage must be zero. BDG(xx)

So d.c. correction voltage also have corresponding variation.
But with FM, there is a frequency deviation in 6*fc, which is proportional to modulating signal amplitude. So d.c. correction voltage also have corresponding variation. Therefore this d.c. voltage is passed through low pass filter to remove effect of frequency variation due to modulation. The filtered voltage is used for frequency correction. BDG(xx)

Phase Locked Loop direct FM transmitter –
This type is used to produce WBFM with high mod index. When both the input frequencies to phase comparator are same , they are locked and output is zero. The modulating signal is used to control the output frequency of VCO. The frequency of output FM of VCO is a function of modulating signal. BDG(xx)

If there is a deviation in the centre frequency of VCO, correction voltage is generated.
This d.c. voltage, passing through LPF, is added to modulating signal to correct the VCO output. Function of LPF is to remove rapid changes in correction voltage due to frequency variations in FM signal. BDG(xx)

Indirect FM Transmitter – Armstrong Method -
( Phase Mod. is employed to produce FM) Stability of the frequency is a major issue in FM. So direct methods of FM generation are not suitable for broadcasting . To overcome this drawback, indirect method to generate FM from PM is employed. (block dia.) To get the modulating signal of same frequency of carrier, AM signal is generated and shifted by 90degrees and added to fc signal vector. BDG(xx)

The resultant vector output is phase modulated.
Since AM and carrier vectors are having same frequency(fc), the out put is FM. Thus phase modulation produces FM. The phase modulated signal can be defined as e(pm) = Ec sin (ωc t + m cos ωm t). ## BDG(xx)

FOSTER SEELEY DISCRIMINATOR – (Phase discriminator)-
The Foster Seeley Discriminator is a common type of FM detector circuit used mainly within radio sets constructed using discrete components. The Foster Seeley detector (or the Foster Seeley discriminator) has many similarities to the ratio detector. The circuit topology looks very similar, having a transformer and a pair of diodes, but there is no third winding and instead a choke is used. BDG(xx)

Cc,C1 & C2 offers short circuit for IF center frequency.
Right side of L3 is at ground potential and IF(Vin) is fed directly (in phase)across L3.(VL3) 180 degree phase out by T1 – La & Lb equal division. At resonant frequency of tank circuit(IF) secondary current (Is) is in phase with Vs and 180degree out of phase with VL3. BDG(xx)

This gives a signal that is 90 degrees out of phase.
Like the ratio detector, the Foster-Seeley circuit operates using a phase difference between signals. To obtain the different phased signals a connection is made to the primary side of the transformer using a capacitor, and this is taken to the center tap of the transformer. This gives a signal that is 90 degrees out of phase. BDG(xx)

Voltage induced in secondary is 90 degree out of phase with Vin(VL3)
Due to loose coupling , primary of T1 acts as inductor and Ip is 90 degree out of phase with Vin. Voltage induced in secondary is 90 degree out of phase with Vin(VL3) VLa and Vlb are 180 degree out of phase with each other and 90 degree out of phase with VL3. Voltage across VD1 is vector sum of VL3 and VLa and VD2 is vector sum of VL3 and VLb. BDG(xx)

When IF > resonance, secondary tank circuit impedance becomes inductive and secondary current lags voltage by theta which is proportional to frequency deviation. When IF < resonance, secondary current leads secondary voltage by theta which is proportional to frequency deviation. F.S.D. is tunned by injecting a frequency equal to the IF center frequency and tunning Co for zero volts output. BDG(xx)

In recent years the Ratio detector has been less widely used.
Ratio detector or discriminator is widely used for FM demodulation within radio sets using discrete components. It was capable of providing a good level of performance. In recent years the Ratio detector has been less widely used. The main reason for this is that it requires the use of wire wound inductors and these are expensive to manufacture. BDG(xx)

Ratio Detector Circuit –
BDG(xx)

Ratio FM detector basics -
Other types of FM demodulator have overtaken them, mainly as a result of the fact that the other FM demodulator configurations lend themselves more easily to being incorporated into integrated circuits. Ratio FM detector basics - When circuits employing discrete components were more widely used, the Ratio and Foster-Seeley detectors were widely used. Of these the ratio detector was the most popular as it offers a better level of amplitude modulation rejection. BDG(xx)

This enables it to provide a greater level of noise immunity as most noise is amplitude noise, and it also enables the circuit to operate satisfactorily with lower levels of limiting in the preceding IF stages of the receiver. BDG(xx)

The operation of the ratio detector centres around a frequency sensitive phase shift network with a transformer and the diodes that are effectively in series with one another. When a steady carrier is applied to the circuit the diodes act to produce a steady voltage across the resistors R1 and R2, and the capacitor C3 charges up as a result. The transformer enables the circuit to detect changes in the frequency of the incoming signal. It has three windings. BDG(xx)

The primary and secondary windings are tuned and lightly coupled.
The primary and secondary act in the normal way to produce a signal at the output. The third winding is un-tuned and the coupling between the primary and the third winding is very tight, and this means that the phasing between signals in these two windings is the same. The primary and secondary windings are tuned and lightly coupled. BDG(xx)

This means that there is a phase difference of 90 degrees between the signals in these windings at the centre frequency. If the signal moves away from the centre frequency the phase difference will change. In turn the phase difference between the secondary and third windings also varies. When this occurs the voltage will subtract from one side of the secondary and add to the other causing an imbalance across the resistors R1 and R2. BDG(xx)

As a result this causes a current to flow in the third winding and the modulation to appear at the output. The capacitors C1 and C2 filter any remaining RF signal which may appear across the resistors. The capacitor C4 and R3 also act as filters ensuring no RF reaches the audio section of the receiver. BDG(xx)

Ratio detector advantages & disadvantages -
As with any circuit there are a number of advantages and disadvantages to be considered when choosing between several options. BDG(xx)

Simple to construct using discrete components
Advantages – Simple to construct using discrete components Offers good level of performance and reasonable linearity. Disadvantages – High cost of transformer Typically lends itself to use in only circuits using discrete components and not integrated circuits. BDG(xx)

Pre-emphasis Pre-emphasis refers to boosting the relative amplitudes of the modulating voltage for higher audio frequencies from 2 to approximately 15 KHz. De-emphasis De-emphasis means attenuating those frequencies by the amount by which they are boosted. BDG(xx)

The purpose is to improve the signal-to-noise ratio for FM reception.
However pre-emphasis is done at the transmitter and the de-emphasis is done in the receiver. The purpose is to improve the signal-to-noise ratio for FM reception. A time constant of 75µs is specified in the RC or L/Z network for pre-emphasis and de-emphasis. BDG(xx)

Pre-emphasis circuit At the transmitter, the modulating signal is passed through a simple network which amplifies the high frequency components more than the low- frequency components. The simplest form of such a circuit is a simple high pass filter of the type shown in fig. Specification dictate a time constant of 75 microseconds (µs) where t = RC. Any combination of resistor and capacitor (or resistor and inductor) giving this time constant will be satisfactory. BDG(xx)

BDG(xx)

The pre-emphasis curve is shown in Fig.
Such a circuit has a cut off frequency fco of Hz. This means that frequencies higher than 2122 Hz will he linearly enhanced. The output amplitude increases with frequency at a rate of 6 dB per octave. The pre-emphasis curve is shown in Fig. This pre-emphasis circuit increases the energy content of the higher-frequency signals so that they will tend to become stronger than the high frequency noise components. This improves the signal to noise ratio and increases intelligibility and fidelity. BDG(xx)

BDG(xx)

This upper break frequency is computed with the expression.
The pre-emphasis circuit also has an upper break frequency fu where the signal enhancement flattens out. This upper break frequency is computed with the expression. fu = R1 +(R2/2πR1^2 *C) It is usually set at some very high value beyond the audio range. An fu of greater than 30KHz is typical. BDG(xx)

De-emphasis Circuit- BDG(xx)

BDG(xx)

To return the frequency response to its normal level, a de-emphasis circuit is used at the receiver.
This is a simple low-pass filter with a constant of 75 πs. See figure (c). It features a cut off of 2122 Hz and causes signals above this frequency to be attenuated at the rate of 6bB per octave. The response curve is shown in Fig (d). As a result, the pre-emphasis at the transmitter is exactly offset by the de-emphasis circuit in the receiver, providing a normal frequency response. BDG(xx)

The combined effect of pre-emphasis and de- emphasis is to increase the high-frequency components during transmission so that they will be stronger and not masked by noise. BDG(xx)

AMPLITUDE MODULATION.

Similar presentations

Presentation on theme: "AMPLITUDE MODULATION."— Presentation transcript:

Similar presentations

About project

Feedback

Log in

Auth with social network:

AMPLITUDE MODULATION.

Similar presentations

Presentation on theme: "AMPLITUDE MODULATION."— Presentation transcript:

Similar presentations

About project

Feedback