Re: How can you tell if a system is oscillating?
From: Chuck Olson (chuckolson01_at_REMOVETHIScomcast.net)
Date: 09/15/04
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Date: Wed, 15 Sep 2004 06:23:55 GMT
Usually if it no longer obeys ohm's law, it's oscillating.
"Dr. David Kirkby" <see_my_signature_for_my_real_address@hotmail.com> wrote
in message news:c99d2c79.0409141618.510b730b@posting.google.com...
> 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
> detects light. The actual detector device has a gain of 30, and there
> are a couple of other stages of amplfifiction on the PCB.
>
> 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.
>
> If I look at the output of the system on an oscilloscope, there is a
> very noisy sine wave, with a period of about 15 ns (about 70 MHz). I
> don't have the unit in front of me, but assuming the peak-to-peak
> amplitude of the sine wave is 1 V, then there is about 300 mV p-pk of
> noise, so the sine wave looks very noisy indeed.
>
> If I look at the sytem on a spectrum analyser, I see a 'signal' that
> is some 30 MHz wide, centred on 70 MHz.
>
> There are two possibilities here.
>
> 1) The detector device generates wide-band noise (which I know it
> does), which gets filtered, so no is narrow band. Then this is
> amplified, so the output consists of noise passed through a filter,
> which then takes on the shape of the filter.
>
> This is what I think is happening, but there is another possibility
> too.
>
> 2) The system, with a very high gain (over 100 dB) is acting as an
> oscillator. Since the gain is much higher between 5 and 85 MHz due to
> the filter, it will osciallate somewhere between there, where the loop
> gain is > 1. Whether or not this would result in a clean oscillation,
> rather than something that is noisy on a spectrum analyser, I don't
> know. I suspect it would, which makes me think it is just the
> amplified noise I am seeing, and no osciallations.
>
> If I put into the optical detector a light source which is RF
> modulated, the output RF modulation can be seen on the spectrum
> analyser if the bandwidth scanned is small (20 kHz or less), but can't
> be seen on the oscilloscope at all. It is probably burried beneth the
> noise.
>
> But how can I determine for sure if the system is oscillating or not?
> I don't think it is, but the question arrises how does one prove this
> one way or the other?
>
> Dr. David Kirkby BSc MSc PhD CEng MIEE
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