Re: The low distortion oscillator problem

There have been quite a few postings about 1kHz low distortion sine wave
oscillators.
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The problem is that if you want a get stable output from a sine wave
oscillator you have to add a non-linear element to control the gain
around the oscillating circuit.
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In the original example - the Hewlett Packard sine wave oscillator which
got the company going - the non-linear element was the filament in an
incandescent lamp whose resistance increased as it got hotter when the
circuit put more current through it. It had enough thermal mass that the
resistance didn't change much over a single cycle of the sine wave.
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The popular option today is a FET where you can modulate the channel
resistance by changing the gate-to-channel voltage. The channel
resistance isn't completely independent of the current through the
channel - it tends to increase a bit with current, independent of the
polarity of the current. There's also some ripple on the control voltage
applied to the FET gate.
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It can still work very well.
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I like precision four quadrant multipliers. You can set one up to add a
controlled amplitude copy of the output to vary the gain around the
oscillating loop - which is handy at start-up - or subtract it from the
output. This means that you can trim the oscillating loop so that the
multiplier normally only contributes the minimal correction required to
compensate for component drift and temperature excursions.
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I've set up an LTSpice simulation which illustrates the point, but it
used an AD734 as it's analog multiplier, which was horribly expensive at
the time and is $A72.99 now.
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In theory you could use good quality DAC to generate the correction
waveform. It's going to have more distortion than a good quality analog
oscillator, but if you can keep the correction waveform small enough the
extra distortion introduced will be less than the distortion coming from
the basic oscillator.
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If you got fancy, you could use the DAC to generate a distorted waveform
which precisely compensated for the distortions introduced by the analog
part of the oscillator. You'd have to throw in a precision A/D converter
to find out what they were, which would make for a very complicated
circuit which would be a pain to set up, and not all that cheap.