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Circuit Design: FM Demodulation

Table of Contents:

  1. Circuit Design: FM Demodulation
  2. Clamper and Amplifier
  3. FM Demodulation Circuit

Frequency modulation (FM) is a technique in which the frequency of a transmitted waveform is varied according to the variations in the message wave. The FM is a very popular technique since they are widely used by the FM radio stations. The main reason behind using the FM modulations by the radio stations is the quality of the signal that can be recreated in the FM receiver. The signal to noise ratio is comparatively very high at the output of FM demodulator circuits. However the FM modulator and demodulator circuits are complex compared to other modulation and demodulation techniques.

The FM demodulator is done with the help of a circuit called Phase Locked Loop (PLL). A PLL should have basic functional blocks like Voltage Controlled Oscillator (VCO), Phase comparator, Low Pass Filter (LPF) and Source follower. In the demodulator circuit the VCO generates a frequency which matches the original carrier frequency and compares the phase of that with received FM wave using the Phase comparator. The output of the Phase comparator is filtered out using the LPF and is current amplified using a Source follower. The output of the source follower matches the original message signal.

To implement all the above mentioned circuitry is a difficult task, but there are ICs available which has all these circuit blocks embedded in it. In this article the FM demodulation circuit is made with the help of a PLL IC called HEF4046.

SUMMARY:

The HEF4046B is a 16 pin DIP IC which works on 3 V to 15 V DC. It has built-in phase-locked loop (PLL) circuit that consists of a linear Voltage Controlled Oscillator (VCO) and two different Phase comparators. The Low Pass Filter (LPF) has to be connected externally to the IC. This IC can recreate the FM signals with good quality using only a few external components. The functional diagram of the IC is given below:

The C1 is the capacitor and R1 and R2 are the resistors on which the VCO depends on its output frequency. The value of the C1, R1 and R2 should be selected in such a way that the VCO produces pulses which matches the original un-modulated carrier frequency of the FM wave. The resistors R3 and C2 form the external Low Pass Filter (LPF), the cut of frequency of which depends on the value of R3 and C2. The value of the R3 and C2 should be selected in such a way that the RC constant should match the range of the frequencies at which the message signals can be expected.

In this project the FM modulation of a pure sine wave is done with the help of a 555 IC wired as a VCO. The pure sine wave is generated using a Wien Bridge Oscillator (WBO) which is then clamped to the positive voltage side using a positive clamper circuit. This signal is then applied to the 555 VCO circuit to generate the corresponding FM wave.

 The block diagram of the entire set up for FM generation is shown in the following diagram:

Frequency Modulation

The WBO circuit is designed to generate pure sine wave of 1 KHz with peak-to-peak amplitude around the supply voltage of 5V. The WBO circuit and the image of the waveform generated is shown in the following figure:

Variable frequency sine wave generator

Input Message Signal Wave