Re: How can you tell if a system is oscillating?
From: Rich Grise (null_at_example.net)
Date: 09/15/04
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Date: Wed, 15 Sep 2004 17:00:40 GMT
On Tuesday 14 September 2004 05:18 pm, Dr. David Kirkby did deign to grace
us with the following:
> I have an electronic system consisting of 3 main parts, and whilst I
> don't think it is, there is a possibility the sytem is oscillating, as
> clearly a signal can be seen on an oscilloscope or spectrum analyser,
> with no input whatever.
>
> The system consists of:
>
> 1) A special photodetector called an avalanche photodiode (APD), which
> 2) The output of this detector module is fed into an RF amplifer (500
> MHz bandwidth)
> 3) The output of this amplifer is fed in to a band-pass filter, with a
> centre frequency of 70 MHz, and a bandwidth of +/- 15 MHz. (i.e. 3 dB
> points of 55 and 85 MHz).
> 4) There are further stages of amplification, and variable
> attenuation. There are over 100 dB of RF amplification.
I agree with Tim Wescott about the gain threshold - that's very easy to
test, with a variable attenuator. Probe various parts of the circuit,
to see what the signal looks like, because
1. how noisy is the APD, and what's its output?
2. 100 dB gain is a _lot_
3. If there is some stray positive feedback, but not enough to oscillate,
you'd have even more gain, so just before it oscillates it will be
amplifying the system noise by as much as it can. But the spectrum
you've described sounds just like well-filtered noise, albeit it
does indicate some nonlinearity in the circuit, because it seems to
be modulating, and not just adding, but that could be more an artifact
of the filter's response.
What happens if you replace the sensor with a short? (or if there's
supposed to be an offset or bias current, you should simulate that,
but as quietly as possible.)
Good Luck!
Rich
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