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Circuit Design: Pulse Amplitude Demodulation

Table of Contents:

  1. Circuit Design: Pulse Amplitude Demodulation
  2. PAM Modulator and LPF

 

The simple pulse modulation technique called Pulse Amplitude Modulation (PAM) proved to be more power efficient than the PWM and consumes constant power for individual pulses like PPM. In PAM the amplitude of the individual pulses are varied according to the amplitude of the modulating signals. The PAM modulator and demodulator circuits simple compared to other kind of modulation and demodulation techniques. There are two kinds of PAM one in which the pulses have the same polarity and the other in which the pulses can have both positive and negative polarity according to the amplitude of the modulating signal.

The PAM modulation technique is widely used in high speed digital communications like telephone modems, Ethernet etc. They are used to drive LED lights more efficiently than using PWM method. Unlike the PPM the transmitter and receiver synchronization is not required for the PAM. This article discusses about the demodulation of Pulse Amplitude Modulated signals with the help of simplest possible circuit.

DESCRIPTION:

The demodulator circuit explained in this article can extract the modulating signal from the Single Polarity PAM wave. The message signal used here is a pure sine wave generated using Wien Bridge Oscillator (WBO) and an FET based modulator circuit is used for the PAM modulation.  The demodulation of the PAM signals can be done easily using a Low-pass filter.

Pulse Amplitude Demodulation

{C}      1)      {C}Variable frequency sine wave generator

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.

The circuit diagram of the variable frequency sine wave oscillator is shown in the following:

sine wave oscillator

The frequency of the sine wave generated by the above circuit depends on the components R1, R2, C1 and C2 and the equation for the frequency is given below:

Formula for frequency

 

Sine Wave

{C}      2)      {C}SQUARE WAVE GENERATOR

The square wave is generated in this project using an op-amp based astable circuit. The op-amp based astable circuit is used to generate the square wave due to the less number of components required and less complex circuit. The ON-time and the OFF-time of the pulses can be made identical and the frequency can be easily adjusted without changing them. However in this project the circuit is designed to be producing a constant frequency.  The astable multivibrator circuit using op-amp 741 is given below.

Square Wave Generator

The time period of the pulses generated by the above circuit depends upon the values of the resistor R and the capacitor C. The following equation gives the approximate time period of the pulses generated by the op-amp astable multivibrator circuit.

T = 2.2 * R*C