Re: Oscillator Distortion

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.