AF, what problem are you trying to solve with this filter? It's a bit unusual to put a filter like this after an oscillator, so if you describe what you are trying to do we may be in a better place to advise, or suggest an alternative approach.

I'm trying to design the voltage control for a theremin. I need to turn a change in oscillation frequency into a DC signal control voltage. I intend to follow this with simple rectifier.

 

If you do this via nodel analysis you have a 6'th order circuit, which is
algebraically challenging. Consider using signal flow analysis since
the T(s) falls out quite easily when applying the method. Then you
will need to factor the T(s), there are tools out there that can take
an algebraic equation and generate the factored form. Off hand I do
not remember who/where the tools are available. MATLAB maybe ....?

Totally agree with MrAI, the Ls need to have their finite Q accounted for.

Have you stated what your actual goals are, eg. G, Zin, Zout, BW for the filter ?
Do you care about its phase response ? Do you want a passive only solution ?
Or would active be acceptable ? Latter gets rid of L's, and easily can be tailored
for G. Does design need to handle wide Temperature variation ? Noise of filter
of concern ?


Regards, Dana.

Something like this: Zin=2.2k, Zout=2.2k, BW=100kHz, CF=1.2MHz

Phase response doesn't matter. Passive is better than active. Noise isn't of concern. Temperature isn't expected to change in environment, room temperature.

Hand calculations will probably not be possible for me because I rarely design filters and am not good with algebra anymore.

 

I've never designed a theremin so my understanding may be way off base.

What sort of accuracy and resolution do you need? i.e. do you need to know that the frequency is within ??Hz of some specified value? Or do you only need to be able to detect frequency changes - if so how big?

If you haven't done so already, you may want to google frequency discriminators - this is the general name of circuits that convert frequency to voltage. If this sounds relevant, look up AFC circuits (automatic frequency control) for FM radios, these were analogue circuits used to make the local oscillator track the received carrier frequency.

 

Turns out it was because the power supply wasn't regulated.