Re: How can you tell if a system is oscillating?
From: Tim Shoppa (shoppa_at_trailing-edge.com)
Date: 09/15/04
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Date: 15 Sep 2004 02:58:51 -0700
see_my_signature_for_my_real_address@hotmail.com (Dr. David Kirkby) wrote in message news:<c99d2c79.0409141618.510b730b@posting.google.com>...
> I suspect it would, which makes me think it is just the
> amplified noise I am seeing, and no osciallations.
I think you're just seeing amplified noise, too. You've got so much
gain that I'd be surprised if you weren't getting limiting in there too,
and when the oscillator limits you're likely to see ringing at a much
higher frequency (>>70 MHz).
The "70 MHz wave" you see on your scope probably only extends a couple
of cycles before disappearing into the noise, right? That's excactly
what you'd expect from noise fed into a filter with a center of 70MHz
and a Q of 4 or so.
If you see pip(s) on the spectrum analyzer output, that could be
oscillation. Complex active systems don't necessarily oscillate at
only one mode, and especially when there's a lot of noise you'll be
driven from one mode to another, but I'd expect if there were any relevant
modes you'd see it as a pip on the noise bump. If the modes are so
spread out and mushed around that they aren't pips, I'd contend that
they are indistinguishable from noise so it doesn't matter. (Indeed,
thermal noise is physically a whole bunch of nearly uncorrelated oscillators).
Tim.
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