The FM Demodulator and AF Amplifier

Since the receiver is a triple superhet for FM we have to mix the signal down to 455 kHz first. A high degree of filtering on the 8 MHz IF is required first to suppress image reception. In order do avoid unwanted mixing product an integrated double symetric mixer is used for the mixing.

Circuit Diagram of the last mixer for FM

For the demodulation itself a integrated circuit NE614 is used. It features a high gain limiting amplifier, the demodulation circuit and a radio signal strengh indicator with a dynamic range of more than 80dB.

Circuit Diagram of the FM demodulator

What's left to do now is to build a squelch circuit for the FM receiver. Since we already have a voltage that is proportional to the input signal strength this is the easy part. We just have to use a comparator to decide weather there is a input signal or not. The output signal of the comparator is feet back into the demodulator which has a digital mute input.

Circuit Diagram of the squelch circuit