Post Project 53: Reference Sine Wave Generator

How do you build a generator using only a few components while obtaining the lowest possible distortion? Here follows the account of the practical experience from many experiments, which resulted in a statevariable oscillator with a distortion well below 0.001%.

By Hein van den Heuvel (Netherlands)

When measuring, calibrating and testing of audio (test) equipment the availability of a test oscillator, which is capable of generating a sine wave (of 1 kHz, for example) that is as pure and low distortion as possible, is practically indispensable. These days, an obvious solution would be to use the sound card of a PC as a sine wave generator. In conjunction with one of the many available (freeware) programs it is very easy to obtain a defined sine wave from the DAC in the sound card.

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If, however, we take a close look at the specifications of a good sound card (even including those with 24-bit resolution) then the specified values for distortion will be somewhat disappointing. Even a more expensive card or (even better) an external sound adapter which is connected via USB, generates signals with a THD somewhere between 0.01% (–80 dB) and 0.003% (–90 dB).

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