This is a circuit that simple and easy to build. It uses common parts that can be purchased easily from any electronic shops. At the core of the circuit is a OP AMP where its input is connected to the headphone jack of the receiver. Some of the amplified signal is feedback through potentiometer VR1 to the non inverting input which helps to boosts the gain and hence called positive feedback. Some is passed to the inverting input through a LC circuit. This cancels the gain of the amplifier except at the resonant frequency - high impedance at resonance state. The amount of positive feedback is adjusted using the potentiometer VR1. This is the figure of the circuit;
L choke should be in the region of 5H but this is not critical. Try using the winding of an audio transformer to make this choke. The output of this filter is connected to the earphones. On Switch SW and on the radio. Vary the VR1 until it goes into oscillation and then turn it back a bit. Tune across the code station and you will hear a sharp peak response effectively cutting off other noise and interference. It is sharp at the peak at 20dB at the audio frequency of 800kHz - 900kHz.
0 comments:
Post a Comment