This is design for filter circuit that is for HF CW are often based upon VFO tuning which can be a problem, especially when using the higher frequency bands. This circuit is narrow band pass filter. This circuit is based on LM201. This is the figure of the circuit.
Supply voltage for the circuit is +15v and -15 volts and is supplied to all OP-Amps. The OP-Amp with the 10p compensation capacitor is the filter which is tuned by the 2N4447 FET acting as a variable resistor. The remaining OP-Amps form comparators (no compensation capacitors) an integrator (the middle one) and a differentiator (bottom right). These adjust the frequency of the filter so that there is 180 degree phase difference between the IN and OUT signals to the filter. This only occurs when the filter is tuned to the incoming audio frequency. With the values given the filter will track an incoming tone from 330 Hz to 3 KHz.
Supply voltage for the circuit is +15v and -15 volts and is supplied to all OP-Amps. The OP-Amp with the 10p compensation capacitor is the filter which is tuned by the 2N4447 FET acting as a variable resistor. The remaining OP-Amps form comparators (no compensation capacitors) an integrator (the middle one) and a differentiator (bottom right). These adjust the frequency of the filter so that there is 180 degree phase difference between the IN and OUT signals to the filter. This only occurs when the filter is tuned to the incoming audio frequency. With the values given the filter will track an incoming tone from 330 Hz to 3 KHz.
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