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From: john larkin <jl@glen--canyon.com>
Newsgroups: sci.electronics.design,sci.electronics.repair
Subject: Re: Oscillator Distortion
Date: Tue, 19 Nov 2024 12:20:47 -0800
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On Tue, 19 Nov 2024 18:49:33 -0000 (UTC), Phil Hobbs
<pcdhSpamMeSenseless@electrooptical.net> wrote:

>Liz Tuddenham <liz@poppyrecords.invalid.invalid> wrote:
>> Phil Hobbs <pcdhSpamMeSenseless@electrooptical.net> wrote:
>> 
>> [...] 
>>> Any oscillator with a nonlinear or bilinear gain control element that
>>> has to respond during a cycle has to deal with the distortion caused by
>>> that element.  OTAs, JFET variable resistors, PIN diode attenuators, 
>>> Vactrols, light bulbs, and so on, all have that problem. 
>> 
>> Light bulbs and thermistors can have a controlling DC superimposed on a
>> miniscule signal current, so that the distortion caused by the latter is
>> negligible.  
>> 
>> Another alternative is an indirectly-heated thermistor with a very small
>> signal current in a large thermistor which is primarily heated by a
>> separate resistive element.  It would be slow to respond, but at 1 Kc/s
>> and -90 dB distortion, a long response time is essential to avoid
>> distortion from the amplitude-settling transient.
>> 
>> 
>
>Depending on omega*tau_th, sure.  The HP 200 exhibits increasing
>second-order distortion at lower frequencies. 
>
>Down at -90 dBc, depending on the signal level you might have to worry
>about deviations from Ohm’s law in an oxide thermistor.  (Metals are pretty
>linear, but the carrier density in an oxide is going to be much much
>lower.)
>
>Eventually it’s bound to be a tradeoff between distortion and noise. 
>
>Cheers 
>
>Phil Hobbs 

Use the opamp dual-integrator sort of oscillator with a loop gain of
1.01, and give the (always nonlinear) variable-gain element 2%
influence.

In the old HP Wein bridge oscillators, the light bulb had a huge
influence on gain.