The strong zero VCO core is a digital oscillator core for the Furthrrrr-Generator which is inserted instead of the analog original one. This way precise and brilliant FM is made posible.
The new core is a PCB with an encapsulated chip which has a digital and thus very stable oscillator circuitry which is of importance for clean FM. The chip replaces the core of the carrier section - the old one just needs to be removed and the new one has to be inserted.
With a jumper on the Furthrrrr Generator's PCB you can select between linear and exponential FM.