22 PulseModulation.pdf

PCM

1. Pulse Modulation Systems
Dr.G.Aarthi,
AssociateProfessor,Schoolof ElectronicsEngineering

2. Pulse Modulation
•Continuous-wave (CW) modulation (recap):
•A parameter of a sinusoidal carrier wave is variedcontinuously in
accordance with the message signal.
•Amplitude
•Frequency
•Phase
•Pulse Modulation:
•Analog pulse modulation: A periodic pulse train is used as a carrier.
The following parameters of the pulse are modified in accordance with
the message signal. Signal is transmitted at discrete intervals of time.
•Pulse amplitude
•Pulse width
•Pulse position
•Digital Pulse Modulation: Message signal represented in a form that
is discrete in both amplitude and time.
•The signal is transmitted as a sequence of coded pulses and No continuous wave in this form
of transmission

3. Analog Pulse Modulation
•PulseAmplitudeModulation
•Amplitudesofregularlyspacedpulsesvariedinproportiontothe
correspondingsampledvaluesofacontinuousmessagesignal.
•Pulse-amplitudemodulationissimilartonaturalsampling,wherethe
messagesignalismultipliedbyaperiodictrainofrectangularpulses.
•Innaturalsamplingthetopofeachmodulatedrectangularpulsevaries
withthemessagesignal.
PAM Signal

4. Other forms of Pulse Modulation
•Pulse Width Modulation (PWM)
•AlsoreferredtoasPulseDurationModulation(PDM)where
samplesofthemessagesignalareusedtovarythedurationof
theindividualpulsesinthecarrier.
•Pulse Position Modulation (PPM)
•Thepositionofapulserelativetoitsunmodulatedtimeof
occurrenceisvariedinaccordancewiththemessagesignal.

5. Other forms of Pulse Modulation

6. Pulse Amplitude Modulation
Low
Pass
Filter
6
Multiplier
Pulse
train
generator
Modulating
Signal
PAM
Signal
Thecarrierisintheformofnarrowpulseshaving
frequencyfs.Theuniformsamplingtakesplace
inmultipliertogeneratePAMsignal.Samplesare
placedTssecawayfromeachother.

7. Pulse Amplitude Modulation
•Depending upon the shape and polarity of the sampled pulses,
PAM is of two types,
•Natural PAM sampling occurs when top portion of the pulses are
subjected to follow the modulating wave.
7

8. Pulse Amplitude Modulation
•Flat topped PAMsampling is often used because of the ease of
generating the modulated wave. In this pulses have flat tops after
modulation.
8

9. PAM Detection
•The PAM signal can be detected by passing it through a low pass
filter.
9
9

10. PAM Advantages
•Through pulse amplitude modulation data transformation is done
efficiently.
•It is the quick way of data transfer
•It is the simplest form on base of which all digital or modern
modulation techniques work

11. PAM Disadvantages
•Amplitude variation occurs as the result ofwhich the receiver's peak
power varies with it.
•In the transmission of pulse amplitude modulation signals very large
bandwidth are required.

12. PAM Applications
•It is used inEthernet communication.
•It is used in many micro-controllers for generating control signals.
•It is used in Photo-biology.
•It is used as an electronic driver for LED lighting.

13. Pulse Width Modulation
•In PWM, the width of the modulated pulses varies in proportion with
the amplitude of modulating signal.

14. Pulse Width Modulation
•Thiscircuitissimplyahigh-gaincomparatorthatisswitchedonandoff
bythesawtoothwaveformderivedfromaverystable-frequency
oscillator.
•Themaximumoftheinputsignalshouldbelessthanthatofsawtooth
signal.
•PWMpulsesoccuratregularinterval,itsrisingedgecoincidingwiththe
fallingedgeofsawtoothsignal.

15. •Whensawtoothsignalisatitsminimum,whichisalwayslessthanthe
minimumofinputsignal,the+veinputofthecomparatorisathigher
potentialandhencecomparatoroutputispositive.
•Whenthesawtoothsignalriseswithafixedslopeandcrossesinputsignal
value,the–veinputofthecomparatorisatahigherpotentialandthe
comparatoroutputis–ve.
•Thedurationforwhichthecomparatorstaysathighisthusdependanton
inputsignalmagnitudeandthisdecidesonthewidthofthepulse
generated.

16. Pulse Width Demodulation
•The PWM signal received at the input of the detection circuit is
contaminated with noise. This signal is applied to pulse generator
circuit which regenerates the PWM signal.
•Thus, some of the noise is removed and the pulses are squared up.
PWM Detection Circuit

17. Pulse Width Demodulation
•The regenerated pulses are applied to a reference pulse generator. It
produces a train of constant amplitude, constant width pulses.
•These pulses are synchronized to the leading edges of the
regenerated PWM pulses but delayed by a fixed interval.
PWM Detection Circuit

18. Pulse Width Demodulation
•The regenerated PWM pulses are also applied to a ramp generator. At the
output of it, we get a constant slope ramp for the duration of the pulse. The
height of the ramp is thus proportional to the width of the PWM pulses.
•At the end of the pulse, a sample and hold amplifier retains the final ramp
voltage until it is reset at the end of the pulse.
•The constant amplitude pulses at the output of reference pulse generator
are then added to the ramp signal.
PWM Detection Circuit

19. Pulse Width Demodulation
•The output of the adder is then clipped off at a threshold level to
generate a PAM signal at the output of the clipper.
•A low pass filter is used to recover the original modulating signal back
from the PAM signal.
PWM Detection Circuit

20. Pulse Width Demodulation
Waveforms for PWM detection circuit

21. Pulse Width Modulation-Advantages
•Less effect of noise i.e., very good noise immunity.
•Synchronization between the transmitter and receiver is not essential
(Which is essential in PPM).
•It is possible to reconstruct the PWM signal from a noise,
contaminated PWM, as discussed in the detection circuit. Thus, it is
possible to separate out signal from noise (which is not possible in
PAM).

22. Pulse Width Modulation-Disadvantages
•Duetothevariablepulsewidth,thepulseshavevariablepower
contents.Hence,thetransmissionmustbepowerfulenoughto
handlethemaximumwidth,pulse,thoughtheaveragepower
transmittedcanbeaslowas50%ofthismaximumpower.
•Inordertoavoidanywaveformdistortion,thebandwidthrequired
forthePWMcommunicationislargeascomparedtobandwidthof
PAM.

23. Pulse Width Modulation-Applications
•PWM finds application in motor control, in delivery of power which is
precisely regulated by regulating the width of the pulse.

24. Pulse Position Modulation
•In PPM, the amplitude and width of the pulses is kept constant but the
position of each pulse is varied in accordance with the amplitudes of the
sampled values of the modulating signal.
•The position of the pulses is changed with respect to the position of
reference pulses.
•The PPM pulses can be derived from the PWM pulses

25. Pulse Position Modulation
•The vertical dotted lines drawn are treated as reference lines to measure
the shift in position of PPM pulses.
•The PPM pulses marked 1, 2 and 3 go away from their respective
reference lines. This is corresponding to increase in the modulating
signal amplitude.
•Then, as the modulating voltage decreases, the PPM pulses 4, 5, 6, 7
come progressively closer to their respective reference lines.

26. Generation of PPM Signal
•The PPM signal can be generated from PWM signal
•The PWM pulses obtained at the comparator output are applied to a
monostable multivibrator.
•The monostable is negative edge triggered.

27. Generation of PPM Signal
•Hence, corresponding to each trailing edge of the PWM signal, the
monostable output goes high.
•It remains high for a fixed time decided by its own RC components.
•Thus as the trailing edges of the PWM signal keep shifting in proportion
with the modulating signal x(t), the PPM pulses also keep shifting.

28. Demodulation of PPM Signal
•The noise corrupted PPM waveform is received by the PPM demodulator
circuit.
•The pulse generator develops a pulsed waveform at its output of fixed
duration and applies these pulses to the reset pin (R) of a SR flip-flop.
•A fixed period reference pulse is generated from the incoming PPM
waveform and the SR flip-flop is set by the reference pulses.
•Due to the set and reset signals applied to the flip-flop, we get a PWM
signal at its output.
•The PWM signal can be demodulated using the PWM demodulator.

29. Pulse Position Modulation-Advantages
•DuetoconstantamplitudeofPPMpulses,theinformationisnot
containedintheamplitude.Hence,thenoiseaddedtoPPMsignal
doesnotdistorttheinformation.Thus,ithasgoodnoiseimmunity.
•ItispossibletoreconstructPPMsignalfromthenoisecontaminated
PPMsignal.
•Duetoconstantamplitudeofpulses,thetransmittedpoweralways
remainsconstant.

30. Pulse Position Modulation-Disadvantages
•AsthepositionofthePPMpulsesisvariedwithrespecttoa
referencepulse,atransmitterhastosendsynchronizingpulsesto
operatethetimingcircuitsinthereceiver.
•Largebandwidthisrequiredtoensuretransmissionofundistorted
pulses.

31. Pulse Position Modulation-Applications
•Used in non coherent detection where a receiver does not need any
Phase lock loop for tracking the phase of the carrier.
•Used in radio frequency (RF) communication.
•Also used in contactless smart card, high frequency, RFID (radio
frequency ID) tags and etc.
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