Analog Communication FM Demodulators - Analog Communication

What are FM Demodulators? What are the methods to demodulate FM wave?

In this chapter, let us learn about the demodulators which demodulate the FM wave. The following are the two methods that demodulate FM wave.

  • Frequency discrimination method
  • Phase discrimination method

Frequency Discrimination Method

Frequency Discrimination Method

In the above equation, the amplitude term resembles the envelope of AM wave and the angle term resembles the angle of FM wave. So, in order to get the modulating signal m(t), as per the reqirement we can recover it from the envelope of AM wave.

The following figure shows the block diagram of FM demodulator using frequency discrimination method.

FM demodulator using frequency discrimination method.

As you see the above block diagram consists of the differentiator and the envelope detector. Differentiator is used to convert the FM wave into a combination of both AM wave and FM wave. This means, it converts the frequency variations of FM wave into the corresponding voltage (amplitude) variations of AM wave. As per the operation of the envelope detector, it produces the demodulated output of AM wave, which is called the modulating signal.

Phase Discrimination Method

The following figure shows the block diagram of FM demodulator using phase discrimination method.

Phase Discrimination Method

This block diagram consists of the multiplier, the low pass filter, and the Voltage Controlled Oscillator (VCO). VCO generates an output signal v(t), when the frequency is proportional to the input signal voltage d(t). Initially, when the signal becomes d(t) zero, we need to adjust the VCO in order to produce an output signal v(t), that has a carrier frequency and -900 phase shift with respect to the carrier signal.

FM wave s(t) and the VCO output v(t) which is the resultant of frequency proportional to the input signal voltage are applied as inputs of the multiplier. With the high and low frequency components the multiplier produces the resultant output. Low pass filter recognises and eliminates the high frequency component thus producing only the low frequency component as its output.

This low frequency component consists of only the term-related phase difference. Hence, we get the required modulating signal m(t) from this output of the low pass filter.

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