[sdiy] Reverse-exponential VCAs

David G. Dixon dixon at interchange.ubc.ca
Fri Jun 4 04:32:12 CEST 2010

> As I understand this sort of 4 stage setup, each stage phase shifts 90
> degrees and the feedback loop is running a 360 phase shift, which is to
> say in phase with the input.

Actually, each stage shifts the signal by 45 degrees, so the fourth stage is
180 degrees out of phase with the input.  It is the inverse of the
fourth-stage output that is recycled back to the first stage.  Of course,
when integrators are used for the "C" part of each RC network, every other
stage output is inverted, and is therefore shifted -135 degrees.

> The per stage VCA you have could allow independent
> control over each of these shifts, I am not sure what all the implications
> are but I have a feeling this may give you control over harmonic content
> that is not available in any similar filter. I suggest that you breadboard
> this design with separate VC inputs for each stage and let us know how it
> behaves with matched and independent control voltages. I think you may be
> on to something really cool.

Every 4P cascaded filter has a separate VCA (or OTA) on each stage, serving
as the voltage-controlled resistor in the RC network.  In that sense, there
is nothing novel about my circuit.  The same control voltage (or current) is
always sent to each of these VCAs (or OTAs).

According to my simulation, sending different control voltages to each VCA
is not very exciting, and it reduces the effect of resonance feedback.  I
don't believe that it affects the phase shift very much.

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