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The circuit in post #1 doesn't implement a Bessel filter, it's roughly a critically damped (Q = 0.5) filter, very droopy as already stated. Looking primarily for a sharper corner and higher stop band attenuation, you would use Butterworth or Chebyshev filter of e.g. 4 to 8th order.
The really problematic point is "not change the phase", which can't be strictly achieved with an analog filter. There's another simple point here, instead of absolute phase, you'll better look for relative phase respectively the group delay. To achieve almost constant group delay within the pass band, you'll probably go for a Bessel filter, although the magnitude response is less sharp.
For a detailed discussion you should fix your requirements quantitatively.
See below a comparison of Bessel and Chebyshev 5th order filters:
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A point I forgot to mention. A probably much simpler way to implement an almost ideal digital-analog conversion with 100 kHz sampling rate is to use an oversampling audio DAC. It has a perfect built-in digital linear phase interpolation filter and only requires a simple RC filter for the oversampling rate (e.g. x64).
To start with a simple point, the specification doesn't particularly suggest a cut-off frequency of 40 kHz. You should better specify the maximum signal frequency and acceptable affection.Ok, this is the reason. The signals are generated by a DAC at 100kHz S/s, and they are synthesized, not sampled. Therefore there is no energy over Nyquist.
However I see after the filter the signal mirrored, i.e. if I output 23kHz I see energy at the mirrored frequency 100-23kHz. I want to eliminate this.
The filter that I described looks insufficient. I am trying to make it sharper by adding more stages after. I have to make sure I do not change the phase till 25-30kHz. Since signals are generated at 100kHz the best frequency for the cutoff is 40kHz.
The really problematic point is "not change the phase", which can't be strictly achieved with an analog filter. There's another simple point here, instead of absolute phase, you'll better look for relative phase respectively the group delay. To achieve almost constant group delay within the pass band, you'll probably go for a Bessel filter, although the magnitude response is less sharp.
For a detailed discussion you should fix your requirements quantitatively.
See below a comparison of Bessel and Chebyshev 5th order filters:
- - - Updated - - -
A point I forgot to mention. A probably much simpler way to implement an almost ideal digital-analog conversion with 100 kHz sampling rate is to use an oversampling audio DAC. It has a perfect built-in digital linear phase interpolation filter and only requires a simple RC filter for the oversampling rate (e.g. x64).