cascading filter after a 2 poles sallenkey

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FvM said:
But Butterworth doesn't meet the linear phase requirement in post #18. A 40 kHz Bessel filter would, but it needs f0=60 kHz Q=0.5.
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I don't see this criteria there, however it makes sense, KlausST mentioned phase shift earlier maybe, but Butterworth doesn't seem too bad I think. In the pass band the group delay is not so dramatical if the output signal's bandwidth is much smaller than 40kHz, and what is placed down now on the PCB I found it as a trade-off.
 

FvM said:
But Butterworth doesn't meet the linear phase requirement in post #18.
Click to expand...

Yes - the same applies to Chebyshev (as in my example) - however, a special equalizer stage could improve the phase response considerably.
 

The OP claimed the filter should "not change the phase till 25-30kHz", see the discussion in my previous post #21. If we take this serious, Butterworth won't be acceptable.
 

Hmm, I should be more careful with reading, thank you by the way. My last recommendation is a 6th order "Linear Phase with Equiripple Error of 0.05°" characteristic. It has a stage with Q=0.55 and a normalised Fc=0.9807, not too far from the ready filter stage....maybe. The group delay is very flat up to Fc. However I have never designed this one.
See page 42 for the characteristic and page 53 for the design table. https://www.analog.com/media/en/training-seminars/design-handbooks/Basic-Linear-Design/Chapter8.pdf
 

frankrose said:
My last recommendation is a 6th order "Linear Phase with Equiripple Error of 0.05°" characteristic. It has a stage with Q=0.55 and a normalised Fc=0.9807, not too far from the ready filter stage....
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Frank - did I overlook some information? How do you know that the existing 2nd-oder stage has a Q in the range of 0.55 ?
 

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