To me, understanding a circuit is about removing all the details and getting down to a few, separate building blocks, putting them together as my understanding improves. With the 4069 VCO this is really simple and this is partially why i like it as much as i do.


Allmost all VCOs can be split up to a few basic building blocks, nameley a CV-summer, a exponential converter, the core and optional waveshapers. Let's take René's original schematic


Looks a little scary, eh? Okay, let's break it down to the bits and pieces i've discussed so far:


The exponential converter is allready marked out so no need for me to do that. So, let's take a brief look at what all the different parts do.


The CV-Summer does exactly what the name says, it sums up the CVs that are to control the oscillator. Everything from keyboard CV, to panel pot CV and optional ADSRs, LFOs or whatever you can think of.


The common music-tuning scheme used in synthesizer, and the one we're using, is the equal tempered 12-TET, where frequencies are divided into octaves which in turn is divided into twelve notes. Two notes spaced one octave apart has some very similar properties, and mathematically you simply double the frequency for each octave. If we take a standard A at 440hz, the A one octave above should be exactly 880hz.

The VCO is supposed to follow the 1v/octave standard, let's take a look on how the transition from voltage to hertz would be if our lowest note (0v) corresponds to a 440hz A.

Input CV	Note	Output Hz
0v	A1	440
1v	A2	880
2v	A3	1760

Clearly, the linear input voltage has to be interpretated so that a exponential increase in frequency occurs. The transition from linear- to exponential is handled by a middle hand inbetween the CV-summer and the core which is this circuit that we call the "exponential converter", or "expo converter". Furthermore, it should be say that most VCOs has a core that is CCO - a current controlled oscillator, so the full extent of .