PULSE MODULATION
Introduction:
Pulse Modulation
Carrier is a train of pulses
Example: Pulse Amplitude Modulation (PAM), Pulse width modulation (PWM) ,
Pulse Position Modulation (PPM)
Types of Pulse Modulation:
The immediate result of sampling is a pulse-amplitude modulation (PAM) signal
PAM is an analog scheme in which the amplitude of the pulse is proportional to the
amplitude of the signal at the instant of sampling
Another analog pulse-forming technique is known as pulse-duration modulation
(PDM). This is also known as pulse-width modulation (PWM)
Pulse-position modulation is closely related to PDM
Pulse Amplitude Modulation:
In PAM, amplitude of pulses is varied in accordance with instantaneous value of
modulating signal.
PAM Generation:
The carrier is in the form of narrow pulses having frequency fc. The uniform
sampling takes place in multiplier to generate PAM signal. Samples are placed Ts sec
away from each other.
Fig.12. 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) and
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.13. PAM Demodulator
Pulse Width Modulation:
In this type, 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.
Pulse Position Modulation:
In this type, 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 & PWM Modulator:
Fig.14. PWM & PPM 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.15. PWM & PPM Modulation waveforms
PWM Demodulator:
Fig.16. 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 B-
C 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.17. 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.
