Subject: Priniciples of Communication Engineering

Difficulty : Medium

Marks : 10

Pulse Width Modulation (PWM) :

As its name suggests this method of modulating the carrier signal alters the width of the digital pulses in accordance with the modulating signal. The width of the pulse is proportional to the instantaneous amplitude of the modulating signal. For a positive modulating signal the width of the pulse increases proportionally, and for a negative modulating signal the width of the pulse decreases proportionally.

Generation:

The simplest analog form of generating fixed frequency PWM is by comparison with a linear slope waveform such as a sawtooth. As seen in the figure below the output signal goes high when the sine wave is higher than the sawtooth. This is implemented using a comparator whose outputvoltage goes to a logic HIGH when ne input is greater than the other.

Demodulation:

1. One method is that the PWM signal must first be converted to a pulse-amplitude modulation (PAM) signal and then passed through a low-pass filter. The PWM signal is applied to an integrator and hold circuit. When the positive edge of pulse appears, the integrator generates ramp output whose magnitude is proportional to the pulse width. After the negative edge, the hold circuit maintains the peak ramp voltage for a given period and then forces the output voltage to zero. The waveform is the sum of a sequence of constant-amplitude and constant-width pulse generated by demodulator. This signal is then applied to the input of clipping circuit, which cuts off the portion of signal below the threshold voltage and outputs the reminder. Therefore the output of clipping circuit is a PAM signal whose amplitude is proportional to the width of PWM signal. Finally, the PAM signal passes through a simple low-pass filter and the original audio signal is obtained.

1. The other technique for demodulating a PWM signal is consist of a product detector and a lowpass filter. The PWM and the carrier signals are connected to the inputs of a product detector, and then a sequence of pulses having the width inversely proportional to the width of PWM pulse presents at output. When the Va signal passes through the low-pass filter, a demodulated signal is obtained.

Waveforms: