The ASM-VCO Core Module

Voltage controlled oscillators (VCOs) are the heart of any synthesiser. The quality of the VCOs ultimately determines the performance of the synthesiser.

The two principal requirements of a synthesiser VCO are:-

  1. stability - stability means that if the control voltage applied to the VCO remains constant, then the frequency of the VCO should also remain constant and not drift, and
  2. Tracking means that the VCO must follow the prescribed logarithmic 1 Volt/Octave characteristic as closely as possible. In particular, where several VCOs are used they should all have similar characteristics.

These parameters are particularly important in a chording instrument where a number of VCOs are used simultaneously. In a synthesiser using only one VCO slight drift or deviation from the 1 Volt/octave characteristic might not be noticed since the ear is not particularly good at judging absolute frequency, unless a person has `perfect pitch'. In any chording instrument however, even slight mistuning is immediately apparent due to the formation of beat notes.

The ASM-VCO Core is a sawtooth-based design substantially borrowed from Electronotes and offers the builder a good quality VCO core for their own oscillator designs. Simply add input components and any wave shaping output circuitry as required.

For an absolute minimal configuration simply apply a control voltage to the 1V/Octave input and connect the Sawtooth Output to your system, apply +/-12V to +/-15V and you are up and running. See example configurations below.

Build Notes Bill of Materials


Terry Mikulic
"New Design Ideas for Voltage-Controlled Oscillators"
Electronotes, Vol. 8, No. 62, Feb. 1976, pp 13-15

Bernie Hutchins
"The ENS-76 Home-Built Synthesizer System - Part 7, VCO Options"
Vol. 9, No. 75, March 1977, pp 3-10

Copyright 2000. All rights reserved.     Revised: January 14, 2021