Introduction To Ham Radio Circuit Audio Filter

One of the earliest mode of radio communications is the use of Morse Code on a continuous wave carrier(CW) which is interrupted to generate a DIT and DAH in forming an alphabet. Though it looks simple, this method is often make less reliable as the surrounding man-made noise and atmospheric noise caused interference to the receiver station. This project will help to filter out the interference signal and ensure that the signal received from the Morse code station stand out.




Circuit Description





The schematic diagram of the filter circuit is 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 postive 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.

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.

Figure below shows the filter's selectivity response curve. It is sharp at the peak at 20dB at the audio frequency of 800kHz - 900kHz.