Phase control in 4th order butterworth MFB LPF

[xitronics]

I intend to design a 4th order Butterworth MFB low pass filter with 20khz cutoff.

I want to include a pot for varying the phase response for the cut-off frequency marginally (+- 15deg). Which passive component should I keep variable so as to have minimal impact on the gain & cutoff frequency..?

I have posted the schematic for direct reference.

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Regards,
Xitronics.

 

[jcx]

in a minimum phase system amplitude and phase are codeterminant - specify one and the other is completely determined

So you can't change phase and have Butterworth response by changing any component of the filter

you can add a series all-pass circuit that is non-minmum phase and can adjust the phase shift without changing the amplitude response