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Circuit Design: Pulse Width Demodulation

Table of Contents:

  1. Circuit Design: Pulse Width Demodulation
  2. PPM Generator and LPF

 

A burst power when used other than the continuous power can save the total power supplied to an inertial load while achieving the same performance from the device. The performance can be varied by varying the width of the pulses. This is the technique called Pulse Width Modulation (PWM) which is in use since a long time for controlling motor speed and other similar inertial machineries.

The PWM technique is use in devices like DC motors, Loudspeakers, Class -D Amplifiers, SMPS etc. They are also used in communication field as-well. The modulation techniques like AM, FM are widely used RF communication whereas the PWM is modulation technique is mostly used in Optical Fiber Communication (OFC).

The PWM in a communication link greatly saves the transmitter power. The immunity of the PWM transmission against the inter-symbol interference is another advantage. This article discusses the technique of demodulating a PWM wave.

DESCRIPTION:

There are different methods for extracting the message signal from a PWM wave. One method which is very simple is discussed here. It is very easy to convert the PWM wave to a PPM wave, and for the PPM wave the demodulation circuitry is extremely simple. The idea is hence to convert the PWM wave to the PPM wave and then demodulate it. The following block diagram represents the implementation of a PWM de-modulator.

{C}      1)      {C}PWM GENERATION

The PWM required for this project is generated using the conventional method of comparing the message signal with a ramp waveform. The message signal used here is a pure sine waveform generated using the Wien Bridge Oscillator (WBO). The ramp signal is generated with the help of a RC charging circuit and a comparator IC. Another comparator IC which is fed with ramp signal as one of its input and the message signal as other is used to produce the PWM wave. The block diagram of the PWM generation circuit is given below:

PWM Demodulation

A sine wave generator circuit is used in this project which is based on the Wien Bridge Oscillator (WBO) circuit. The Wien Bridge oscillator circuit can produce distortion less sinusoidal sweep at its output. The circuit is designed in such a way that both the amplitude and frequency of the oscillator can be adjusted using potentiometers. Here the sine wave generator is adjusted to produce a waveform of frequency 1 KHz.

The Ramp generator used in this circuit is designed with an op-amp and an RC charging circuit. The RC charging circuit is connected to the output of the op-amp and the voltage across the capacitor is connected to one of the input of the op-amp. To another input of the op-amp the variable pin of a potential divider is connected to which divides the voltage from the output of the op-amp.

The ramp waveform is applied to one of the input of another comparator circuit and the output of the comparator circuit will be a PWM waveform.

Pulse Position Modulated Wave

 

The circuit diagram is given below:

Comparator Circuit

The image of PWM circuit wired in the breadboard is given below:

Pulse Width Modulation

 

 

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