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Digital Communication Lab Experiments

Analyze Pulse Width Modulation (PWM).


Analyze Pulse Width Modulation (PWM)

Apparatus Required:

  1. Pulse width modulation kit
  2. Cathode Ray Oscilloscope (CRO)
  3. Power supply
  4. Patch cords


In PPM system, the position of the pulses relative to zero reference level is varied in accordance with the instantaneous level of modulating signal as shown in figure 4.1. Amplitude and width of the pulses are kept constant. In this system, the position of each pulse is related to the position of the recurrent reference pulse. The N message bits are encoded by transmitting a single pulse in one of 2N possible time shifts. This is repeated every T seconds, such that the transmitted bit rate is N/T bits per second. It is primarily used for radio communication, control of aircrafts, cars etc.



  1. Make the circuit as shown in figure 4.2.
  2. Connect the power supply in proper polarity to the kit and & switch it on.
  3. Set the pulse generator output to be 2Vpp at 8 Khz.
  4. The input and output waveforms are measured using CRO

Observation:The different output waveforms shown in figures 4.3 to 4.4 are observed with the help of CRO and are traced with the help of graph paper.


Conclusion:The study of pulse width modulation has been carried out successfully


  1. What is PWM?
  2. An audio signal consists of frequencies in the range of 100Hz to 5.5KHz.What is the minimum frequency at which it should be sampled in order to transmit it through pulse modulation?
  3. What do you infer from the frequency spectrum of a PWM signal?
  4. Describe the spectral representation of PWM.


  1. Check the connections before giving the supply.
  2. Observations should be done carefully.