# Why doesn't a lock-in measure the full voltage from a function generator? (3$omega$ setup)

I'm trying to build a setup for the 3$$\omega$$ technique, and the equipment is making my brain hurt. When I send a signal directly from the function generator to the lock-in, it only measures about 71% of the input However, the lock-in accurately reads the voltage from its own sine-out, and the amplitude of the output from the function generator was checked on an oscilloscope. I am baffled as to what is happening here.

It also reads 0 voltage on the third harmonic, which I'm not sure is supposed to happen either. If anyone has done 3$$\omega$$ experiments before and could advise me on how to get this set up correctly it would be much appreciated.

• Can you upload the oscilloscope measurement? – eranreches Jun 3 at 0:52
• What quadrature are you measuring? How did you set the phase? Does the $3\omega$ signal show up in a spectrum analyzer (or a Fourier transform of your oscilloscope data)? – Gilbert Jun 3 at 2:00
• $0.71\approx 1/\sqrt{2}$. I think this is significant. Could one of your machines be working with root-mean-square values, and another with full amplitude? – Cryo Jun 3 at 2:27
• @Cryo, good observation! I'm almost certain that this is it! (I can't upvote comments yet, but I can accept it if you post as an answer). Gilbert and Eranreches, I have no way of connecting the lab oscilloscope to a computer yet, but I'll work on it. – Wyatt Kuehster Jun 3 at 17:42
• Glad if it helps. Knowledge seared into memory by painful experience :-) – Cryo Jun 3 at 21:35