Change the values simultaneously of each of the 10k resistors in the RC portions of the low pass filter circuit to adjust the Resonance frequency. Adjust the potentiometer to change the amount of resonance. The 10k resistors can be replaced with a voltage controlled resistor either using OTAs, FETs, or LEDs with photo resistors as long as they are all tuned to produce similar resistances based on voltage inputs.
Resonance works by feeding the output of the filter back into the input, but only resonance occurs if the phases are matched. Each of the 4 poles in this circuit shifts the signal phase by 45 degrees. After moving through 4 poles, you have a 180 degree phase shift. Feeding this back into the first pole would not produce any resonance at the cutoff frequency Q, but running that output through an inverting amplifier (another 180 degree phase shift) makes the filtered output signal in phase with the unfiltered input signal. Mixing those together at the filter input stage produces that resonance at the the Q frequency.
Enjoy :)
