Three Monkeys – Oscilloscope Style

With the start of my graduate studies, the common oscilloscope has suddenly become an everyday tool for me in a version that it has never been before. The novelty of this rather complex device has allowed me extraordinary opportunities to “turn myself into a monkey”, as we in Germany are wont to say.

Monkey One: See no signal

Two days ago, I was trying to adjust a back-reflection mirror. I.e., at the end of five meter beampath, with a fibre, two lens systems and any number of polarizers and mirrors in between, a perpendicular mirror should reflect the laser beam all the way, precisely. We had a photodiode set up that would detect this happening, and normally it’s simple. But no matter what I did, I couldn’t see a signal. I checked again that the laser was being reflected back properly (done by holding an IR card near the incoming beam to see if a reflection showed up sideways – ideally it should be right on the incoming beam). I even went so far as to check at the photodiode itself – the beam is weak there, and I had to turn of all the lights in our lab and the neighbors’, until I could with a lot of effort see a slight pinprick right on the diode. What was wrong? The oscilloscope has two channels, and I had the wrong one switched on.

Monkey Two: Hear no signal

Yesterday, photodiode again, but a different one, much easier. I could easily see the beam going into the diode. But nothing’s happening; flatline. The epic reason: I had the GND button toggled. This connects the signal to ground and is normally used to find the green line again if you’ve turned so many wheels you can’t see it anymore. Somehow, I must have touched it on putting in the cable.

Monkey Three: Speak no signal, either

Similar situation: Get a beam to hit a photodiode (this is a recurring theme in our work, yes). The beam was definitely going in there, and the channel was not grounded. If I turned the diode (very rough method), I could sometimes see a little flicker like it was about to hit the diode, but I could never get a decent hit resulting in a high, steady signal. Hmmm. There’s a button beside GND, called DC/AC (not the band). If you put it to AC, it’s optimized for watching fast-changing signals. Suddenly putting a steady signal into an AC channel results in a short increase which drops back to zero as the signal stops changing. Guess what happened? Yes, I was in AC mode. The strong, constant signal from the photodiode had been there the entire time. Arghlll.

All of these things have something in common: It’s difficult to see. You must know to check for the problem – and now I do. I remain with the old adage “it’s only dumb the second time”.