The Big Rat is a signal source based on an accurate physical model of an RC network and a driving relaxation oscillator.
It is a module which started out as a recreation of the looping ASR envelopes in the Jen Synthetone SX-2000 monosynth. The project soon grew arms and legs however, and free-running LFO and VCO capabilities were added, together with bipolar operation. The four main parameters are voltage-controllable, and when used as a VCO, will track a 1V/Oct pitch input.