Switched Capacitor Phase Shifters

[Sean Costello]

At 02:53 PM 11/5/96 PST, you wrote:
>I've been looking for a simple phase shifter network.
>
>I have a PAIA Shepard function generator kit laying around with two of the
six channel VCA cards.
>I thought I'd build a barber pole phaser but needed a simple phase shifter
I could
>reproduce eight times.
>
>I think I found it while doing a patent search on switched capacitor filters.
>
>The classic phase shift element uses one op-amp, three resistors, one cap and a
>variable resister per stage. The various designs I've looked at use FET's
or Vactec
>LDR's for the variable resistor. The patent search turned up a circuit using an
>electronic switch (like a CD4066) with pulse width modulation for the variable
>resister.
>
>Boy is that simple, four per package and you only need one 41khz sawtooth
for the
>whole array. So, one audio quad op-amp, a CD4066, one LM339 section per four
>poles. Makes a four feature phase shifter section. Cheap and low parts count.

Check out US Pat. No. 4,306,480, by Frank Eventoff and Serge A. Tcherepnin.
This describes a VC phase shift stage using 1/4 of a LM3900.  These stages
can apparently be cascaded, resulting in a 4-stage phase shifter using one
chip (that costs about $.58), plus caps, resistors, etc.  No LDR's, no
FET's, no OTA's.  Can't tell you how it works, or how it sounds.  Anyone out
there tried it? 

The same circuit, with a slight modification, can be used to implement a VC
full-wave rectifier.  I think this is the circuit used in the Serge Wave
Multiplier.

Later,

Sean