VCO Core : Triangle Core vs Saw Core

Sound Theory -

VCO Core : Triangle Core vs Saw Core

VCO Core

VCO Circuits do not produce all of their output waveforms independently, they are derived from the VCO core which is the portion of a VCO that generates the initial waveform from which all of the other waveforms that the VCO outputs are derived. 

There are two basic VCO core types

They both use a charging current and an integrating capacitor to produce a linear change in voltage until a reset voltage is reached. 

Sawtooth core - The sawtooth is the simpler circuit and more common.

It functions by converting the control voltage to a current, then using that current to charge a capacitor referred to as the integrating capacitor.

As the capacitor charges the voltage increases at a rate proportional to the control voltage.

The charging current determines the rate at which the capacitor charges which in turn determines the output frequency.

Triangle Core - The Triangle core is different in that,

Instead of discharging the capacitor, it reverses the charging current.

The voltage changes linearly in the opposite direction until the reset voltage is reached in the opposite direction reversing the charging current again. 

With wave shaping circuits it is easy to produce a pure sine wave. 

Triangle core was used in the famous Curtis 3340 chip that is responsible for the lush sounds of a variety of polyphonic synthesizers. 

The two basic components of the core are

- A circuit that charges the capacitor at a rate determined by the input control voltage, and

- A timing mechanism that resets the waveform back to its starting point by discharging the capacitor. 

The timing mechanism is usually a comparator that compares the cycling waveform voltage with a reference voltage, when the waveform exceeds the reference voltage the comparator triggers the circuit to reset the waveform to the beginning of the cycle. 

When the comparator is replaced with a digital timer or counter that triggers the reset, the oscillator changes to a DCO digitally controlled oscillator. 

Sawtooth Core VCO schematic

* A large part of this article is inspired and some material is taken from the Make Noise O-Coast user manual as well as some resources on the internet through