FM Receiver

FM Receiver is a compact tunable lying in the range of 88 to 108 MHz with audio amplifier. The FM Receiver described here uses very economical and high quality receiver IC. This receiver covers FM range 88MHz to 108MHz. The IC integrates the following functions :

• Input mixer with AGC
• IF amplifier and local oscillator
• FM Detector
• AF Preamplifier

The detailed circuit diagram is shown in figure.
About the Circuit : The circuit is divided into two parts as FM section and Audio Amplifier section.
FM Section :  

• The FM section consists of an FM tuner, an FM-IF amplifier and the detector. The FM frequency is selected by FM trimmer C18/C19. The IF tuned circuit provides high selectivity to the receiver. Antenna is directly coupled through capacitor C14 to pin1 of IC2. IC2 contains superheterodyne receiver. The FM section receives the incoming signal via a wideband antenna circuit and pin1. From there, the signal is amplified and then applied to a mixer via a parallel- tuned circuit. In the mixer, the signal is mixed with an oscillator signal, which is also controlled by a parallel-tuned circuit. The oscillator is combined with an automatic frequency control circuit- AFC - which only needs an external buffer capacitor.
• The output of the mixer is applied to an external filter and from there to the first (internal) IF amplifier. From there, it is again filtered externally and then applied to the second IF amplifier. Finally, the signal is demodulated in an FM discriminator. The resulting audio frequency signal is output via pin13. The amplified RF signal is applied to a mixer together with the output of an appropriate oscillator. The mixer is followed by IF filters and an IF amplifier. The output of the IF amplifier controls the automatic gain control - AGC - circuit. The AGC holds the output of the IF amplifier and mixer substantially constant in spite of variations in the RF signal.
• The output of the IF amplifier is demodulated by a suitable detector and the consequent audio signal is applied to pin13.

AF Section :
The AF section consists of IC1, which is low power audio amplifier IC specially designed to deliver 1 to 8W output power at 4W load at 12V input supply. The output of this amplifier exhibits low distortion and low noise. The input is fed at pin3 of IC1 through variable pot P1 to control the input signals to the amplifier. The output is taken from pin7 of the same amplifier.
PCB Fabrication :
All the inductors are hand-made. The coils are wound closely so that there is no gap between the turns. The end of the coils L1/L2/L3 should be scratched off with the help of a blade or an iron scrapper so that the enameled cover is removed. After winding these coils, you may find it difficult to wind the IFT but it becomes easy after gaining some experience. The construction details of the inductors and IFT are given in TABLE.