Re: 1kHz 130dB distortion sinewave oscillator.

On 3/02/2025 5:40 am, Edward Rawde wrote:

"john larkin" <JL@gct.com> wrote in message news:tk6vpjp7tn5hct35jo5ue4846p9vt8ggt1@4ax.com...

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

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

 I'd probably ask you what you thought the best way of measuring it would be.
 

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

 I would too.
 

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There is a hobby of playing with LT Spice as a sort of video game,
never building or even needing the circuit. Nothing wrong with that I
guess, except that the pay isn't very good.

It can be a long term investment.

I wasn't expecting to get below -120dB in a simulation but the current circuit is approaching -140dB in a simulation.
I have definitely found it fun to do and also to improve it using suggestions from people here.
And I have learned a lot about how to use LTSpice which makes it much quicker and easier to simulate anything I might get paid for.
  From a sinewave oscillator point of view the original objective was to find out whether it's really necessary (at least at one fixed
frequency) to use lamps, thermistors or opto devices to get the lowest possible distortion.
That question seems to have been answered.

But it's the wrong question. The real questions are "can you stabilse the output of an oscillator without using a non-linear device", to which the answer is that you can't, and "which non-linear device gives you least distortion in the output" to which the answer is "it depends on exactly what you are trying to do".
--
Bill Sloman, Sydney