I have a simple voltage regulator. 10K resistor feeds a 15V zener in the usual fashion. That voltage then controls the gate of a mosfet and the source of the mosfet is my output voltage. It works out to be about 12V. I've used this circuit many times and its worked fine.
This time, I ended up with what looked like a lot of noise in my circuit, till I traced it back to the mosfet. On closer inspection it looked like some kind of random(ish) oscillation. The level was about 500mV pk-pk. Fine I said, might be a mosfet thing, so lets put a cap across the zener.
So, now I have a 1uF cap across the zener. And initially that seemed to work. Until, I looked more closely. I still had a little bit of what looked like noise.
But then, I looked closer.
Across the zener and cap I get an odd little waveform. Its about 5-15mV peak to peak, and its got nice neat straight ramps upwards (the kind you get when charging a cap), and sudden drops downwards. A sawtooth, yes.. but a random one. The ramps terminate randomly. So do the drops.
I'm fairly sure this is the zener. For one thing if I supply power at <15V, then there is no oscillation. Reach that threshold and it starts to oscillate. Increase the supplied voltage and the ramps climb more steeply. (Typical period 10-100 us).
What it looks like is a zener that wants to suddenly conduct.. then stops.. then conducts.. over a random(ish) 5-15mV interval.
Has anyone seen this before? Is it some kind of thermal cycling effect? How do I make it go away (for sure)?