Unit 7

Analog Pulse Modulation

7.1 Analog pulse modulation: PAM, PWM, PPM and demodulation

PAM

In PAM, amplitude of pulses is varied in accordance with instantaneous value of modulating signal.

Fig.1: PAM

PAM Generation:

The carrier which is in the form of narrow pulses has frequency fc. The uniform sampling takes place in multiplier to generate PAM signal. Samples are placed Ts sec away from each other.

Fig.2: PAM Modulator

• The circuit is simple emitter follower. 
• In the absence of the clock signal, the output follows input.
• The modulating signal is applied as the input signal.
• Another input to the base of the transistor is the clock signal.
• The frequency of the clock signal is made equal to the desired carrier pulse train frequency. 
• The amplitude of the clock signal is chosen the high level is at ground level(0v)
• Low level at some negative voltage sufficient to bring the transistor in cutoff region. 
• When clock is high, circuit operates as emitter follower and the output follows in the input modulating signal. 
• When clock signal is low, transistor is cutoff and output is zero.
• Thus the output is the desired PAM signal.

PAM Demodulator: 

The PAM demodulator circuit which is just an envelope detector followed by a  second order op-amp low pass filter (to have good filtering characteristics) is as shown below

Fig.3: PAM Demodulator

Pulse Width Modulation 

In this type of modulation, the amplitude is maintained constant but the width of each pulse is varied in accordance with instantaneous value of the analog signal.  In PWM information is contained in width variation. This is similar to FM.  In pulse width modulation (PWM), the width of each pulse is made directly proportional to the amplitude of the information signal.

Fig.4: Output waveform

PPM

In this type of modulation, the sampled waveform has fixed amplitude and width whereas the position of each pulse is varied as per instantaneous value of the analog signal.  PPM signal is further modification of a PWM signal.

PPM and PWM modulator

Fig.5: PPM and PWM modulator

• The PPM signal can be generated from PWM signal. 
• The PWM pulses obtained at the comparator output are applied to a mono stable multi vibrator which is negative edge triggered.
• Hence for each trailing edge of PWM signal, the monostable output goes high. It remains high for a fixed time decided by its RC components.

• Thus as the trailing edges of the PWM signal keeps shifting in proportion with the modulating signal, the PPM pulses also keep shifting.
• Therefore all the PPM pulses have the same amplitude and width. The information is conveyed via changing position of pulses.

Fig.6: PWM and PPM Modulation waveforms.

PWM Demodulator

Fig.7: PWM Demodulator

• Transistor T1 works as an inverter. 
• During time interval A-B when the PWM signal is high the input to transistor T2 is low. 
• Therefore, during this time interval T2 is cut-off and capacitor C is charged through  an R-C combination. 
• During time interval B-C when PWM signal is low, the input to transistor T2 is high, and it gets saturated. 
• The capacitor C discharges rapidly through T2.
• The collector voltage of T2 during BC is low.
• Thus, the waveform at the collector of T2is similar to saw-tooth waveform whose envelope is the modulating signal. 
• Passing it through 2nd order op-amp Low Pass Filter, gives demodulated signal.

PPM Demodulator

Fig.8: PPM Demodulator

• The gaps between the pulses of a PPM signal contain the information regarding the modulating signal. 
• During gap A-B between the pulses the transistor is cut-off and the capacitor C gets charged through R-C combination.
• During the pulse duration B-C the capacitor discharges through transistor and the collector voltage becomes low.
• Thus, waveform across collector is saw-tooth waveform whose envelope is the modulating signal. 
• Passing it through 2nd order op-amp Low Pass Filter, gives demodulated signal.

Key Takeaways:

1. In PAM, amplitude of pulses is varied in accordance with instantaneous value of modulating signal.
2. In PWM, the amplitude is maintained constant but the width of each pulse is varied in accordance with instantaneous value of the analog signal.  In PWM information is contained in width variation. This is similar to FM.  In pulse width modulation (PWM), the width of each pulse is made directly proportional to the amplitude of the information signal.
3. In PPM, the sampled waveform has fixed amplitude and width whereas the position of each pulse is varied as per instantaneous value of the analog signal.  PPM signal is further modification of a PWM signal.

7.2 Comparative study of various analog pulse modulation

1) PAM

2 ) PPM and PWM

References:

1. “Electronic communication system”, by Kennedy, TMH
2. “Communication system”, by haykin wiley
3. “Communication system”, by bruce carison, TMH
4. “Modern digital and analog communication”, B.P. LATHI, Oxford