CW filter

How to add a CW filter to the receiver or transceiver? Older factory equipment (mostly on VHF) they do not have a built-in intermediate frequency CW filter, respectively, they do not even allow it to be built in. Thus, the CW bandwidth is determined by the width of the SSB filter, which, however, is too large. Nevertheless, it is possible to supplement such devices with a CW filter.

NF CW filter

We can also achieve relatively good results with the NF filter. There are different constructions, but none compares to this in simplicity. It uses the characteristics of a parallel LC circuit, which we set to the desired frequency and connect to the volume potentiometer. With the components shown in the example, it is approx. 800 Hz.

C1 is the decoupling capacitor behind the detector. If switch S1 is open, The LC circuit is not connected and the signal goes through the volume potentiometer to the NF amplifier without change. If switch S2 is closed, the signal reaches the parallel LC circuit. From it we take the signal for the NF amplifier. Due to the properties of the parallel LC circuit, the highest voltage is obtained at its resonance frequency, at other frequencies the output voltage will be lower, making it appear as a normal filter.

CW filter simulation

The properties of such a filter are largely influenced by different impedances. It is already clear from looking at the diagram, that the potentiometer P1 reduces the Q of the circuit. Another non-negligible impedance is the impedance of the detector. I tried to simulate the behavior of the filter at different impedances and the results vary, but they are good anyway.

Frequency response of the filter with resistance P1 47k and detector impedance 5k

Frequency characteristic of the filter with resistance P1 22k and detector impedance 1k