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Active filter design : 2nd order sharp roll-off
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nick_d:
@grouchobyte that is a cool circuit. Kudos for the effort to design and simulate. At first I could not figure out why you had low passed the top branch. But then I realized that in my suggested way, adding the original signal to the high passed signal, the high passed signal will be significantly phase shifted as it starts to roll on, initially causing cancellation until the phase shift reduces throughout the pass band (it would be 180deg at DC, 90deg at cutoff and 0deg at infinite frequency?). This must be what OP complained of, whereas your low pass starts to roll off and add phase shift just as the high pass rolls on, to get reinforcement?
cheers, Nick
grouchobyte:

--- Quote from: nick_d on February 07, 2019, 07:52:52 am ---@grouchobyte that is a cool circuit. Kudos for the effort to design and simulate. At first I could not figure out why you had low passed the top branch. But then I realized that in my suggested way, adding the original signal to the high passed signal, the high passed signal will be significantly phase shifted as it starts to roll on, initially causing cancellation until the phase shift reduces throughout the pass band (it would be 180deg at DC, 90deg at cutoff and 0deg at infinite frequency?). This must be what OP complained of, whereas your low pass starts to roll off and add phase shift just as the high pass rolls on, to get reinforcement?
cheers, Nick

--- End quote ---

Nick

Your assessment is correct in terms of the overlapping technique of the LP and HP sections. I guess 45 years doing analog design for HP, Tek and a dozen other giants has its benefits. Your suggestions actually led me to the current solution. The whole exercise including drawing it up in LTspice took me about 20 minutes. The hard part was reading through the OP’s posts and trying to extract the requirements. I am not certain it meets his needs 100%, but it is close and leaves a little for him to tweak and learn. (I am pretty sure we may be asked to assist in getting it dialed in) I have a PhD client that reminds me of the OP in the way he defines his requirements ( no offense to the OP) so it’s all in a days work for me. Cheers!

grouchobyte (Bob)
Rutherberg:
@grouchobyte cool circuit indeed, thanks a lot for the effort !
It's starting to look like what I need. The limit of attenuation to approx. -20dB is good and the knee at the cut-off frequency is sharp enough.

Now the idea would be to make the slope more progressive (see attached, the red curve) but keeping the sharp knee at cut-off frequency, if only it is possible of course.

I messed around for hours with your simulation but I can't get the results I need.
The closer I can get is with the second circuit attached, but the knee at cut-off is not sharp enough
T3sl4co1l:
You're asking for fractions of a dB tweak, I don't see for what purpose that would be productive.

Tim
ogden:

--- Quote from: T3sl4co1l on February 08, 2019, 08:32:42 am ---You're asking for fractions of a dB tweak, I don't see for what purpose that would be productive.

--- End quote ---

It won't be achievable w/o precision capacitors/resistors (or trimpots) anyway. I would suggest to run Monte Carlo simulation for selected tolerance (spread) of resistors and capacitors to see possible range of the error. As an illustration LC filter simulation (10% inductors and 5% capacitors):

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