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.
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:
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:
2) 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.
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*CSign up here with your email
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