I wanted to add more audio filtering to my 40 meter CW transceiver https://circuitsaladdotcom.files.wordpress.com/2015/08/qrpxcvr2.jpg but didn’t want to put much effort into it, so I used a MAX7401, 8 pole switch cap filter IC. This IC requires just a few discretes and has an extremely low passband ripple and group delay, as it is a Bessel configuration. I have used this IC before in some of my guitar effects pedals. It has worked very well in my other designs. The knee of the filter is adjustable via a capacitor(C4). To integrate into my receiver, I simply connected the input and output of the filter across the input capacitor(C27 removed) connections to the final audio amp. It may be desirable to put a single RC low pass filter stage in series with the output of the switch cap filter to remove clk artifacts on the output.(clk is 100X greater than the roll off frequency). Performance is very good with clear tone and no ringing.
Below, I have a video demo of the XCVR utilizing the filter. This version of the XCVR is tuned via a POT connected to the microcontroller A2D converter instead of a rotary encoder. It also has a push button to shift in 5Khz increments. When the button is depressed for longer, the frequency in KHz is sounded in morse code. Only the KHz is sounded, so for 7.100 Mhz for example, only 100 is sounded in morse code. In the video you can also see my cool 3D printed Code Key. It uses rare earth magnets instead of a spring for key action. It’s a really delightful bug.
Picture of filter Daughter Board connected to XCVR
Video of XCVR using the Filter