I've had one or two like that, so far. Not nearly so improbable, being much simpler circuits, I suppose you could say primarily due to my inexperience. Take, for instance,
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The 2N5179s oscillated at some ungodly frequency, way beyond my 200MHz scope (unsurprisingly, as they are >1.3GHz transistors). Probably a combination of "bad" wiring and a not-too-great design (the next revision did away with the 3904 emitter follower drivers, which probably coupled too much end-to-end, exacerbating the problem).
Besides shitty performance that just didn't look right, I discovered this one by touching the circuit. Oftentimes, parasites respond to the most useful tool, the finger: it's dextrous, a thermometer (sometimes a voltmeter!), has a reasonably consistent capacitance and resistance.
The other prominent example was breadboarding discrete ECL gates. Great performance, it's wonderful to see 5-10ns edges coming from mere 2N3904s (on the breadboard, no less). Problem is, the stray capacitance and rail inductance sucks so damn much when breadboarding that any more than about five gates makes the thing oscillate. It only adds insult to injury that I was running out of space, so had to patch wires in to an adjecent breadboard to continue. Icky!
Tim
--
Deep Friar: a very philosophical monk.
Website: http://webpages.charter.net/dawill/tmoranwms
"John Larkin" wrote in message
news:dgp0b4d5p6dvborke4b2viiln0gvmf2rsu@4ax.com...
>
>
> We recently designed an 8-channel complex waveform generator. Each
> output stage is composed of a DAC, a lowpass filter, an output
> amplifier, a test relay, and an output connector. It\'s this one:
>
> http://www.highlandtechnology.com/DSS/V346DS.html
>
> You can see the gold output connectors, and the relays are hiding just
> behind the front panel.
>
> The harmonic distortion seemed a bit high, in the -40 dBc range at 32
> MHz and max level output. We were poking around with a spectrum
> analyzer and happened to do a 0-3 GHz sweep and lo, a big line at
> about 1 GHz. Something\'s oscillating!
>
> ...