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Analog Communication - FM Demodulators
In this chapter, let us discuss about the demodulators which demodulate the FM wave,
The following two methods demodulate FM wave.
= Frequency discrimination method
= Phase discrimination method
Frequency Discrimination Method
We know that the equation of FM wave is
s(t) = A, cos (emf anky f mie) tt)
Differentiate the above equation with respect to 't’.
ds (t)
dt
=—A, (2mf. 4 2k (0) sin (2 ft ' an; f m(at)
We can write, — sin @ as sin(@ — 180°)
= a = A, (2m fe4 2nitgm (0) sin 2 \ anky f m (eae 180")
> ast = A, (2nf.) [: + (Z) m(o)] sin
(anne +2nky [ m(t) dt— 180")
In the above equation, the amplitude term resembles the envelope of AM wave and the
angle term resembles the angle of FM wave. Here, our requirement is the modulating
signal m (t). Hence, we can recover it from the envelope of AM wave.
The following figure shows the block diagram of FM demodulator using frequency
discrimination method.
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Demodulated
output
Envelope
Differentiator Detector
This block diagram consists of the differentiator and the envelope detector. Differentiator
is used to convert the FM wave into a combination of AM wave and FM wave. This means,
it converts the frequency variations of FM wave into the corresponding voltage
(amplitude) variations of AM wave. We know the operation of the envelope detector, It
produces the demodulated output of AM wave, which is nothing but the modulating
signal.
Phase Discrimination Method
The following figure shows the block diagram of FM demodulator using phase
discrimination method.
Multiplier
Demodulated
output
Low Pass
Filter act)
Voltage
Controlled
Oscillator
This block diagram consists of the multiplier, the low pass filter, and the Voltage
Controlled Oscillator (VCO). VCO produces an output signal v(t), whose frequency is
proportional to the input signal voltage d(t). Initially, when the signal d(t) is zero,
adjust the VCO to produce an output signal v(t), having a carrier frequency and —90°
phase shift with respect to the carrier signal.
FM wave s(t) and the VCO output v(é) are applied as inputs of the multiplier. The
multiplier produces an output, having a high frequency component and a low frequency
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component. Low pass filter eliminates the high frequency component and produces only
the low frequency component as its output.
This low frequency component contains only the term-related phase difference. Hence,
we get the modulating signal mm (t) from this output of the low pass filter.
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