Re: 1kHz 130dB distortion sinewave oscillator.

On 3/02/2025 3:22 am, john larkin wrote:

On Thu, 30 Jan 2025 13:47:31 -0500, "Edward Rawde"
<invalid@invalid.invalid> wrote:
 

Approaching 130dB now. Any suggestions for improvement?
It wouldn't be hard to add another four rectifier phases but then I'd have more components than Bill.
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This circuit was simulated in LTSpice 24.1.1 with all component updates as of 30th January 2025.
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Take a sample near 20s and FFT on current zoom extent with Blackman-Harris window.
It's approaching 130dB at 2kHz and approaching 140dB everywhere else.
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I do not know why C13/R28 and similar are needed but without them the simulation speed goes down to us/s
It appears that LT1115 doesn't like being simulated with very little load on its output.
Whether or not that's true in reality I've no idea and it may not be the case in earlier versions of LTSpice.
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Simulation speed in 24.1.1 appears to be about twice as fast as earlier versions for this circuit.

 If you actually build one, how would you measure the distortion?
 (I can think of one way).

So could pretty much everybody who is interested in the problem. Buy a good audio A/D converter, digitise a couple seconds of output and Fourier transformer the that data to pull out the harmonics.
The fact that you couldn't be bothered to post that explicity is telling.

I'd expect it to be worse than the sim, even ignoring noise.

Simulations have to simulate a simplified version of reality. Sometimes quantisation error can make simulation perform worse than reality - it happened with out ultra-sound scanner back in 1977 - but it isn't common.

There is a hobby of playing with LT Spice as a sort of video game,
never building or even needing the circuit.

You'd probably wouldn't bother if you had never built a circuit.

Nothing wrong with that I guess, except that the pay isn't very good.

It's more of a long term investment.
--
Bill Sloman, Sydney