[sdiy] 4-pole filter concept

cheater cheater cheater00 at gmail.com
Sun May 3 16:18:29 CEST 2009

Interesting, make sure to keep us up to date.
I wonder how it'd work with audio-rate modulation, given all the buffering.

On 4/28/09, David G. Dixon <dixon at interchange.ubc.ca> wrote:
> I've been thinking about a different sort of four-pole filter for the last
> couple of weeks, and have finally done some simulations to confirm that it
> should work pretty well.  I was disappointed that the standard 4-pole
> configuration under self-oscillation gives sine waves at each pole which are
> -3dB from the previous one, and the first one always hits the rails.  My
> goal was to develop my own version of the Mankato filter (the schematic for
> which I've never seen, as it's apparently a closely guarded state secret),
> but with voltage-controlled resonance.  To this end, I've strung together
> four LPF blocks based on LM13700s, making use of the internal buffers.
> After much experimentation, I found that the LPF circuit published in the
> LM13700 datasheet was basically optimum in terms of giving large bandwidth
> and low THD.  Inbetween each block I've put another LM13700 VCA with
> automatic gain control (again, more or less as suggested in the datasheet).
> Stable oscillation is easily achieved.  And, with the judicious selection of
> resistor values, the AGC limits the amplitude of the resulting sine waves
> such that they stay more or less constant, even if the resonance (VCA gain)
> current is cranked up well beyond the oscillation point, although the THD
> does suffer slightly.  In other words, this should give very nice sine waves
> at +/-5V (or whatever level is desired).  Finally, unity-gain inverters at
> each pole ensure that all eight 45-degree quadrature waves are available, a
> la Mankato.
> I'm going to breadboard this tomorrow (if I have time) to confirm that it
> works like the simulations suggest (circuits virtually always do, I find).
> I'm wondering, has anyone else tried this?  Is there a similar circuit out
> there that I'm not aware of?  Am I in for major disappointment when I
> finally get this thing working?
> Dave Dixon
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