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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 12:37:10 +1100
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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:
>>
>>> 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)
> 
> I'd probably ask you what you thought the best way of measuring it would be.
> 
>>
>> I'd expect it to be worse than the sim, even ignoring noise.
> 
> I would too.
> 
>>
>> 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