SVFs from Super Synthesis is a dual voltage-controlled State Variable Filter in Eurorack format. Each filter has 1 signal input, 2 control voltage inputs with one attenuverter, and 3 signal outputs: Lowpass, Highpass, and Bandpass.
When the resonance control is turned fully clockwise, the filter will self-oscillate, yielding a sine wave on the three outputs, 90 degrees out of phase with each other.
SVFs' signal inputs are DC coupled, so the filters can process both audio and control voltages. Great for making slewed and resonant envelopes and sequences.
Check out the documentation here (right click to download)
Demo videos from an earlier panel revision, but the circuit is the same!