Dual VCO

A dual tri-core OTA VCO

It is a triangle core design using a special type of op-amp called an operational transconductance amplifier (OTA). The VCOs range from approx. 19Hz up to 9500Hz, they are quite simple designs with reasonably low drift and can be tuned to 1V/oct (meaning for every 1V change on the CV input the frequency will change by 1 octave, useful if using a keyboard), but they are not exactly rock steady either. There is a trade-off between component quality and design complexity versus drift and accuracy. Anyhoo, the VCOs output triangle, square and pulse waveforms. The pulse width can be set with a pot or controlled by a CV (control voltage). Both VCOs have CV inputs to control pitch, one has a soft sync input, the other has an FM input. Try injecting all kinds of CVs and audio rate signals into these to see what effects you can get.

http://nonlinearcircuits.blogspot.com/2014/08/dual-ota-vco.html