Subject : Principle of Communication Engineering

Topic : Pulse-Modulation and demodulation

Difficulty : High

Pulse Position Modulation (PPM)

• 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. With increase in the modulating voltage the PPM pulses shift further with respect to reference
• The vertical treated dotted lines are reference lines to measure the shift in position of PPM pulses. The PPM pulses go away from their respective reference lines

• This is corrsponding to increase in the modulating signal amplitude. Then as the modulating voltage decreaes the PPM pulses come progressively closer to their respective reference lines.

  

  Fig.1 Waveform of Generation of PPM from PWM

Generation of PPM Signal

• The PPM Signal can be generated from PWM signal. The PWM pules obtained at the comparator output are applied to a mono stable multivibrator

• Hence corresponding to each trailing edge of PWM signal, the mono stable output goes high. It remains high for a fixed time decided by its own RC comparator.

• Thus as the trailing edges of the PWM signal keep shifting in proportion with the modulating signal x(t), the PWM pulses also keep shifting

• All the PPM pulses have the same with and amplitude. The information is conveyed via changing the portion of pulses.

  

Advantages of PPM

• Due to constant amplitude of PPM pules, the information is not obtained in the amplitude. Hence the noise added to the PPM signal does not distort the information. Thus it has good noise immunity.

• It is possible to reconstruct PPM signal from the noise contaminated PPM signal. Tis is also possible in PWM but not possible in PAM.

• Due to constant amplitude of pulses, the transmitted power always remains constant. It does not change as it is used to in PWM.

Disadvantages of PPM

• As the position of PPM pulses is varied with reference to a reference pulse, a transmitter has to send synchronizing pulses to operate the timing circuits in the receiver. Without them the demodulation won’t be possible to achieve.

• Large bandwidth is required to ensure transmission of undistorted pulses.