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From: Cursitor Doom <cd999666@notformail.com>
Newsgroups: sci.electronics.design,sci.electronics.repair
Subject: Re: Oscillator Distortion
Date: Sun, 20 Oct 2024 11:35:19 -0000 (UTC)
Organization: A noiseless patient Spider
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On Sun, 20 Oct 2024 11:24:06 +0200, albert wrote:

> In article <veguu6$ofj1$3@dont-email.me>,
> Cursitor Doom  <cd999666@notformail.com> wrote:
>>Gentlemen,
>>
>>Last week I got an old (1968) pulse generator out of mothballs and
>>managed to get it fully functional again. However, before replacing the
>>case, I (true to form) dropped it on the bench and something on the PCB
>>must have shorted out against the metal tools it fell on, because it no
>>longer works properly.
>>I've found an issue with the principal oscillator. It's generating
>>distorted sine waves. It's a wien bridge type using BJTs as the gain
>>element and fine tungsten filaments as thermistors, so should produce
>>near perfect sine waves before they're chopped and shaped by subsequent
>>circuitry, but since the fall, it's not.
>>
>>Here's the oscillator output:
>>https://disk.yandex.com/i/eKAe95xMsiIvNA
>>
>>I found some weird periodic spikes on the power supply rails in the
>>oscillator stage. They are actually present on the rail, not just picked
>>up by the ground lead of the scope out of the ether, as I used a short
>>ground clip in this instance. I'm not sure if these could cause the
>>distortion or not.
>>https://disk.yandex.com/i/eKAe95xMsiIvNA
>>
>>I'm out of ideas. What could cause such distortion if the PS rail isn't
>>responsible?
> 
> Not entirely unrelated to this subject.
> Image a Wien bridge oscillator, stabilized with a lightbulb. This could
> be described with differential equations, including the heating and
> cooling of the tungsten wire.
> 
> I suspect a relation between the cooling time constant and the periods
> of an oscillator, and the distortion.
> A rule could be if you need N periods to have the distance to the the
> stable wave halved, you have circa 1/N distortion.
> 
> Is this known territory?
>>
>>Your pal,
>>
>>CD.
> 
> Groetjes Albert
>>
>>

You people who explain everything they see in terms of higher mathematics 
normally leave me baffled. But I believe I've deduced what you're trying 
to say and it's not an issue in this case. Thanks for the suggestion 
anyway.