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Sunday, October 18, 2009

Low Distortion Sine Wave Oscillator

One approach to generating sine waves is to filter a square wave. This leaves only the sine wave fundamental as the output. Since a square wave is easily amplitude stabilized by clipping, the sine wave output is also amplitude stabilized. For the solution of the problem, you can look at the figure below.

A lower distortion oscillator is needed. It can be used. Instead of driving the tuned circuit with a square wave, a symmetrically clipped sine wave is used. The clipped sine wave, of course, has less distortion than a square wave and yields a low distortion output when filtered. This circuit is not as tolerant of component values as tune sine wave oscillator. To insure oscillation, it is necessary that sufficient signal is applied to the zener for clipping to occur.

Clipping about 20% of the sine wave is usually a good value. The level of clipping must be high enough to insure oscillation over the entire tuning range. If the clipping is too small, it is possible for the circuit to cease oscillation due to tuning, component aging, or temperature changes. Higher clipping levels increase distortion. [Schematic’s diagram source: National Semiconductor. Inc]

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