NORMALLY, THE FIRST choice of anyone contemplating building or buying an audio oscillator is a Wein bridge type. These have the advantage of low distortion (usually) but their output amplitude often bounces all over the place as you sweep over each frequency range and is even worse when you switch ranges. It is possible to avoid these problems with careful design but the circuit will end up being more complicated (see our low distortion design in the February & March 1999 issues).

The second choice for an audio oscillator is typically a function generator but while these usually have good amplitude stability, their distortion content is usually fairly average. But now you have a third choice with this design which uses digitally generated sinewaves and employs a switched capacitor filter.

While thumbing through the Jaycar Electronics catalog some time ago, I came across an "IC bargain", an MF4CH-50 4th order switched capacitor Butterworth low-pass filter, for the trivial sum of $1.50. This set me thinking (I do that occasionally) about what level of distortion we would get if we fed a pseudo sinewave (digitally generated) into that sort of filter. Many moons later, this low cost audio oscillator is the outcome of those profound thoughts.

The oscillator is housed in a plastic zippy box measuring 157 x 95 x 50mm. It has three knobs on the front panel and these are the 4-position range switch, the frequency control and the sinewave output control. As well, there is a toggle power switch and three RCA sockets for the sinewave output and two square wave outputs. The circuit is battery operated but could be run from a plugpack if you wish; more on that later.

Performance Sinewave output........2Hz - 20kHz, 0-2V RMS Square-wave output........2Hz - 20kHz, 5V peak-to-peak Square-wave x100 output........200Hz - 2MHz, 5V peak-to-peak Sinewave distortion........less than 0.85% Current consumption........15mA from +5V, 6mA from -5V