[sdiy] how to calculate cutoff in an OTA filter?

[Guy McCusker]

Unless I have become confused again, the centre frequency of the SVF
bandpass (which is not the same as the 3dB point of the lowpass, but
is probably the right frequency to think about) is the same as the
unity gain frequency of the integrators. This assumes you have the
integrators set for the same gain, which you don't have to, and fun
can be had if you don't.

So for a typical OTA integrator, I think you would use your equation
but the gm would be calculated to take into account attenuation at the
input of the OTA.

Guy.

On Fri, Dec 11, 2020 at 5:06 PM Chris McDowell <declareupdate at gmail.com> wrote:
>
> The subject says it. How do we calculate the cutoff frequency in a filter with OTAs, or VCAs for that matter. Maybe this is more obvious than I think but I'm not making sense out of it.
>
> So for a state variable filter, if transconductance, gm = 1/R, then R = 1/gm, then why is the cutoff not just 1 / ( 2 * PI * (1/gm) * C) ?
>
> Cheers and thanks in advance,
> Chris
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