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From: "Edward Rawde" <invalid@invalid.invalid>
Newsgroups: sci.electronics.design
Subject: Re: Sinewave oscillator without gain control.
Date: Wed, 29 Jan 2025 18:25:29 -0500
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"Bill Sloman" <bill.sloman@ieee.org> wrote in message news:vnck1i$295d5$1@dont-email.me...
> On 24/01/2025 5:02 pm, Edward Rawde wrote:
>> "Bill Sloman" <bill.sloman@ieee.org> wrote in message news:vmv6ac$22ufe$1@dont-email.me...
>>> On 24/01/2025 11:14 am, Edward Rawde wrote:
>>>> Before you can make a 1kHz sinewave oscillator with constant output level and better than 120dB harmonic distortion you'll need
>>>> an
>>>> oscillator which does better than that when it passes through the required output level.
>>>
>>> This is obviously true. Why do you think you need to tell us about it?
>>>
>>
>> You do make me laugh sometimes Bill.
>> Your circuit doesn't come anywhere close to 120dB but it does depend on which simulator you ask.
>
> I never said it did. I was looking for comments on the way the current mirrors might screw up the waveform. I didn't get any.
>
>> Adding a FET gain stabilizer to the circuit I posted seems to have negligible effect on the distortion.
>
> Then you haven't looked at the current waveform going through the the FET.
>
>> So I'd concentrate on the oscillator distortion level rather than the gain controller contribution if I wanted to attempt better
>> than 120dB.
>
> And you'd probably better dump LTSpice 24.1 - it has been claimed that it has a serious bug. That might explain why your 
> simulations of my circuit run a thousand times slower for you than they do for me and a couple of other people.

If U3 and U8 (LT1115) in your circuit are replaced with LT1678 then on my computer with LTSpice 24.1.1 and all component updates, 
the simulation completes almost as fast as I can start it, at about 55 ms/s.
A sample near 10s is not quite 60dB down at 3kHz and 5kHz and 70dB down at 7kHz, approaching 80dB down everywhere else.
The parasitic 23MHz signal has gone.

So the reason for the slow simulation speed in 24.1.1 is something to do with LT1115.

The circuit I simulated in 24.1.1 with LT1678 is below.

Version 4.1
SHEET 1 3608 920
WIRE -1680 -2400 -1984 -2400
WIRE -1360 -2400 -1680 -2400
WIRE -1200 -2400 -1360 -2400
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WIRE -736 -2400 -1104 -2400
WIRE -320 -2400 -736 -2400
WIRE 208 -2400 -320 -2400
WIRE 336 -2400 208 -2400
WIRE 576 -2400 336 -2400
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WIRE 208 -2384 208 -2400
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WIRE 3376 -2272 -1616 -2272
WIRE 880 -2192 -1776 -2192
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WIRE -176 -2160 -320 -2160
WIRE 208 -2144 208 -2320
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WIRE 16 -1520 -32 -1520
WIRE 64 -1520 16 -1520
========== REMAINDER OF ARTICLE TRUNCATED ==========