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From: "Edward Rawde" <invalid@invalid.invalid>
Newsgroups: sci.electronics.design
Subject: Re: A variation on my current mirror low distortion sine wave oscillator - 10dB less distortion and much the same number of components
Date: Thu, 13 Feb 2025 09:45:14 -0500
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"Bill Sloman" <bill.sloman@ieee.org> wrote in message news:voh7a5$26aqj$1@dont-email.me...
> On 10/02/2025 5:18 pm, Bill Sloman wrote:
>> Basically same idea, but two separate controllable asymmetric current mirrors, rather than one, and no current steering. The 
>> half-wave rectifier still seems to be the source of the distortion in the stabilised output.
>>
>> C25 and C26 take out as much of it as I can. Increasing them - from 15nF to 33nF makes the distortion worse. Splitting the 
>> resistors into three rather than two and adding two more capacitors might help, but what this circuit needs is more insight, 
>> rather than more components.
>
> Splitting the resistors did help, and the optimum capacitor value at C25, C26, C27 and C28 turned out to be 4.7nF. The second and 
> fifth harmonics were just 80dB below the fundamental and the third 91dB down. Not dramatically good, but respectable.
>
> Other changes were less succesful - the current nmirror approach does suffer from the need to split the waveform in order to 
> generate the asmplitude correction waveform and minimising the 2usec wide switching spikes that show up at cross-over is what it 
> takes to get it to work tolerably well
>
> I've swapped out the LT1115 for the LT1678 - that doesn't seem to suffer from parasitic oscillations in LTSpice 24, so it should 
> simulate tolerably fast.
>
After fixing line wraps I had to move U1 down into position.
I then noticed an issue with C10 so I converted to ANSI in Notepad++ and saved the file.
Simulation then failed without giving any clue what was wrong.
But instead of spending hours tracing the problem I removed .ENDS from the BAS70 model.
Simulation now runs fine at about 44 ms/s in LTSPice 24.1.2
FFT is approaching 60dB
Simulated circuit included below.
I can get 80dB by adding an LC tuned circuit to a simple phase shift oscillator of the type which turns up here:
https://www.google.com/search?q=sine+wave+oscillator&udm=2
No gain control yet but for unknown reasons it does run at constant (unpredictable) amplitude with very critical emitter resistor 
adjustment.

I'm thinking of trying the sample/hold method posted by JM but with real components.
So I need to turn a FET on (not sure for how long yet) at the peaks of the sine wave.

Version 4.1
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