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From: Bill Sloman <bill.sloman@ieee.org>
Newsgroups: sci.electronics.design
Subject: Re: 1kHz 130dB distortion sinewave oscillator.
Date: Mon, 3 Feb 2025 12:29:33 +1100
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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.
>>
>> This circuit was simulated in LTSpice 24.1.1 with all component updates as of 30th January 2025.
>>
>> 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.
>>
>> 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.
>>
>> 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