Noise

In my efforts to identify and eliminate the source of noise affecting my amplifiers in my last experiment I've learned some interesting bits.

Firstly, if you run cables anywhere there are lots of other cables, you should ALWAYS label both ends of each cable uniquely preferably also with some information about where the other end is. I've spent hours this week just chasing cables through overcrowded cable trays traversing three stories of tokamak and a long hallway only to find that the cable was run for something it wasn't being used for anymore. If the label on the end said where the other end was, it would have saved me a bit of blood, sweat, and a couple of hours. This also suggests you should pull out cables you aren't using anymore, but there's this nagging tendency to leave the cable there just in case you need it in the near future, then forget about it until someone else chases it down to find both ends disconnected.

Second, there are lots of noise sources in and around a tokamak. Something between my scintillators and the patch panel on the wall is causing a 500kHz 0-1V sinusoidal noise signal with an on/off cycle of ~80ms. Most annoyingly, this doesn't seem to affect one of my signal cables, when all three of them are run in the same cable tray along (almost) the same path, so there's something special about two of them... what is it?

It would be just keen if the pit had a patch panel of fiber optics so I could just export an optical signal and do all the amplification and digitization outside, but that'll have to be a future upgrade. For now we have only coax and twisted pair wires, so I have to track down this darn noise source.

A third bit, almost a corollary to the second, is that cables HAVE to be terminated properly otherwise you run the risk of screwing up your signal with reflections. I toggled back and forth the 1MΩ/50Ω termination selector on my digitizer and it made a world of difference. The 1MΩ terminated signal had all sorts of bad news oscillations, and the 50Ω (matched to my 50Ω RG-58 coax) terminated signal held up a nice hard edged square wave. Turns out Rebeiz wasn't fooling around when he said terminations are critical in HF circuits.

So you learn something new every day, but I still haven't found the source of my noise so I'll be chasing it in the meantime.