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You are stuck with the laws of physics and math. The phase shift below the cutoff frequency is totally controlled by the bandwidth and number of poles.
Your option is to select what you really want to do. If you really want to get rid of one spectral line, you can use a notch filter and have less phase shift.
Post what you are trying to accomplish and I will look into it.
The aade free filter program will plot phase shifts. You can do a cut and try with it. Use the lumped element calculations as they are the same as the active filter responses since both use the same pole and zero positions.
If you are comparing the phase shift of the filter output to another path, you can put additional phase shift in the other path. Use an all pass network with half as many poles as your LPF and with the same bandwidth. This should make the two phase shifts equal.
Thank you for your answerd.
what I am looking for is a LPF which can be even 2 poles at the most. I can also can exccept only a moderat attenuation at the stopband (even only 30-40db).
I see at some design that they put a lot of parallel cap. res or cap +res parallel to cap. THey olso use pi netwark on the feedback and they get realy strang filter, but I don't know how to desugn somthing like it.
If you can tell me some guid lines or a place were I can get a tools to design one I be appreciate it.
Can you send in the specifications of your filter. It would really help us understand your requirement? Or atleast suggest what application you are using the filter for? If you are talking below 1KHz, is it for any biomedical application? Or is it for any other application?
If you want the minimum phase shift in the low frequency ranges, you will need a filter which has a constant phase shift. Only thing which meets to this requirements is an active integrator circuit which produces a constant 90 degrees phase shift. And moreover, if you want a very slow phase shift, your poles of the filter should be atleast 10 times 1 KHz, because where the phase changes starts at 1K.
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