VC Phaser using the LM13600

[Michael B. Irwin]

Hello,
The phase versus frequency function for a typical single RC all-pass
filter is given by

 phase(degrees) = -2*arctan(2*pi*f*RC) for a lagging type filter and

phase(degrees)= -180-2*arctan(2*pi*f*RC) for a leading type filter.

(This is the one op amp type filter with 1 capacitor, 2 fixed R's and 1
variable R). The function looks like an "S" elongated in the horizontal
direction. The rate of phase change is greatest at the turnover
frequency. Varying R (or the control current of the OTA) does not change
the overall shape of the curve, it just varies the turnover frequency.
When the phase-shifted signal is summed with the dry signal a notch will
be produced at the turnover frequency (for 2 all-pass filters in series
giving a cumulative phase shift of 180 degrees at the turnover
frequency). Putting more  filters in series produces additional
cancellations where in each case the cumulative phase shift (at each
notch frequency) is an odd  multiple of 180 degrees (for a summing-type
output stage).