Re: Homebrew CW audio filter for uBITX - thumping/distortion on leading edges

Rafael Pinto [PU1OWL]


Very sharp bandpass filters tend to ring. Also as you made 3 identical sections the phase margin gets really low, allowing the filter to act as an oscillator.

The thump sound might also be op amp instability because of the really sharp response.

Your filter is centered at 700hz, but what is your expected bandwidth? Maybe you can try with one section only and check whether the ringing and thumping improves. If that is the case then I suggest redesigning the filter as a 6th order Butterworth biquad cascade.

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