How can you tell if a system is oscillating?

From: Dr. David Kirkby (see_my_signature_for_my_real_address_at_hotmail.com)
Date: 09/15/04 

Date: 14 Sep 2004 17:18:56 -0700

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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