This variable notch filter circuit for enabling an adjustable audio frequency cancellation as desired by a specific application.
A Notch Filter can also be named as Band Stop filter or Band Reject Filter.
Most of these filters cancel/attenuate signals in a particular frequency band referred to as stop band frequency range and allow the signals over and below this band.
In electronic music circuits there exists requirement for an all pass notch filter obtaining a movable notch frequency.
The variable notch filter circuit explained here is designed to move the reject frequency over a 10 kHz range through the entire full range of sound frequencies, the location of the notch being determined by the voltage placed on the control input.
IC1 and IC2 both are all pass filters acquiring a flat frequency response effectively above the acoustic range, but possessing a phase variation amongst input and output signals of 0.5/CR.
This phase variance gets to be 180 degrees, to ensure if the output and input are combined, signal termination takes place i.e. the variable notch filter circuit is currently functioning as an all pass notch filter, enabling via all frequencies apart from at 0.5/CR.
The two transistor circuits Q1, 2 and Q3, 4 work as voltage managed resistors which permit the notch frequency to be shifted once the control voltage is altered.