The last time I posted here about a VCO, I forgot to mention that it's not very good in most applications as it was only linear FM and the waveforms were very... not right, I'll say. My latest design is somewhat complex, but can be simplified as it has eight different waveforms, a ton of different kinds of modulation (including an accurate 1 volt per octave FM input), and a very modular design made up of simple parts. Let's get started!
As this is an electronic device, it needs power.
The FM Inputs:
For the inputs, you have manual frequency, exponential frequency, 1v/oct with built-in portamento (glide), and linear FM. This can be used on most VCO circuits, not just this one.
The part in the box is the built-in portamento (aka glide) section. It is made up of two buffers with an RC lowpass filter in between. Lowering the cutoff via the rheostat (or pot, whatever you use) smooths out the stepped voltage signal, thus making the voltage 'glide' to the next voltage value.
The core of this new VCO is a simple Schmitt-trigger ramp (inverted saw) oscillator with linear FM capable of a wide range of frequencies.
Then you have the problem of getting the ramp output to other parts of the oscillator or even other parts of the synth. With too much output load (that is, too much circuitry after the ramp is extracted from the cap), the oscillator will lose stability. That is why both the square (trigger) output and ramp output are fed into Class-A transistor amplifiers - to act as buffers. They also make the signal very loud, so the resistors attenuate the amplified signal to a more manageable level.
Waveshaping the Basic Waveforms:
Like I said, this oscillator has 8 waveforms, one of which (ramp) has already been covered. Most good synthesizer oscillators have at least four waveforms: saw, triangle, sine, and square. This next section will show you how to get those waveforms.
The square wave section also uses a comparator, but instead of grounding the reference side of the comparator, there is a linear pulse width modulation section that allows manual and voltage control over the pulse width.
Waveshaping the Complex Waveforms:
By this point, we have the basic waveforms. But, every good oscillator has those, so why not add some flair with more unique waveforms?
The bottom section is composed of two parts: the spike generator and the morpher. The spike generator is a simple RC highpass filter which could be voltage controlled if you wanted with either a transistor, vactrol, or optocoupler. The morph section splits the voltage into a normal diode and a zener diode. The voltage cutoffs of each of these make for an interesting waveform out of each diode. The pot acts as a crossfader between the two. This can also be voltage-controlled with a VC crossfader.
Putting It All Together:
Put all those parts together, and you have the Xanu Oscillator:
I hope you enjoyed reading this, and if you have any questions, feel free to comment below. Oh and happy new year!