This little VCO is made arround a NE566 VCO chip, that is used here as current controlled oscillator. People complain about its small frequency sweep in the standard application (only 1:10) and therefore this chip is rarely used in synthezisers. But pin 6 is actually mis(?)usable as a current input, so with exponential control a much larger sweep range is archeived. I provide the circuit here as a starting point for own investigations, and not for a recipe on how to do. The circuit in this state lacks most of the features one would expect in a VCO module, such as PWM, and still needs level shifting and buffering outputs.
Below is a variation on the circuit, but I did not try it yet, but I included it for your inspiration. I hope to make this into a highfrequency VCO to control delaylines.