Zjs log: Difference between revisions

* '''Added 50 ohm impedance matching resistors to the photodiode inputs.''' I had been hesitant to do this on account of it shrinking our input signal but decided to try it on account of noise; perhaps the low impedance would be less susceptible from noise coming from the photodiodes or noise from some ground loop through the photodiodes. Indeed the error signal shrank somewhat, but was still plenty big enough to do feedback.

* '''Added a new passive fast feedback branch that goes directly to the laser diode.''' I talked to Larry yesterday about our issues trying to feedback through the laser driver. He said that that's problematic for same two reasons we had been suspecting: ground/noise issues and the fact the laser driver mod input has very high gain, so you need very low noise. He suggested a fast feedback branch directly to the diode and sketched out a little circuit. Turns out we had a lot of little discarded fast feedback boxes lying around so i cannibalized one of those used the layout suggested by Larry. His circuit is nice and simple and passive, so we also don't have to worry about powering it. I stuck it inline immediately before the laser. As for an error signal pick-off, i added one inside the lockbox, picking off inside the fast feedback branch, immediately after the first op-amp buffer. This makes it so the phase is correct, and not inverted relative to the slow feedback branch. I didn't disconnect the existing fast feedback but left it in place. We could try sending that directly to he diode as well.