[ Rana, Jenne]
We remeasured the Yend PDH box.
When we first started, the green couldn't hold lock to the arm - it kept flickering between modes. Changing the gain of the PDH box (from 7.5 to 6.0) helped.
We measured a calibration, from our injection point to our measurement point.
The concept was that we'd take the mixer output, and put that into an SR560, and put the swept sine injection into the other input port of the '560, and use A-B. So, for this calibration, we left A unplugged, and just had the RF out of the 4395 going to input B of the '560. The 600 Ohm output of the '560 went to the error point input on the PDH box (during normal operation the mixer output is connected directly to the error point input). The SR560 was set to gain of 1, no filtering. I don't recall if we were using high range or low noise, but we tried both and didn't really see a difference between them.
We had the 4395 take that calibration out, and then we measured the closed loop gain up to 1 MHz. (Same measurement setup as above, but we connected the mixer out to the input of the SR560 to close the loop, and made sure we were locked on a TEM00 green mode.) Rana used an ipython notebook to infer the open loop gain from our measurement. Our conclusion is that we don't have nearly enough gain margin in our loop. We found the PDH box gain knob at 7.5, and we turned it down to 6.0, but the loop is still pretty borderline. We used the high impedance active probe to measure the error point monitor, since we aren't sure that that point can drive a 50 Ohm load.
We also measured the error point spectra and the control point spectra. Unfortunately, the saved data from the analyzer (no matter what is on the screen) comes out in spectrum, not spectral density. So, we need to check our conversion, but right now to get from Watts power to Volts, we do sqrt(50 ohm * data). We then need to get to spectral density, and right now we're just dividing by the square root of the bandwith that is reported in the .par file. This last step is the one we want to especially check, by perhaps putting some known amount of noise (from an SR785?) into the 4395, and checking that our calibration math returns the expected noise spectrum.
What still needs to be done is to calibrate this into Hz/rtHz. To do this, we were thinking that we should look at the error point on a 'scope while the cavity is flashing.
Anyhow, here is the uncalibrated error point spectrum. Purple is a measurement up to 30kHz, with 30Hz bandwidth. Blue is a measurement up to 300kHz with 300Hz bandwidth. The gain peaking schmutz above 10kHz sucks, and we'd like to get rid of it. We also see the same peak at ~150kHz that Q saw earlier today. We were using the high impedance probe here too.
We have the data for the control point (all the data files are in /users/jenne/ALS/PDHloops/Yend_18Aug2014), but we haven't plotted it yet.
Things that need doing:
* (JCD) Think about this box's purpose in life. What kind of gain do we need? Do we need more / less than we're currently getting? NPRO freq noise is 1/f and is 10kHz/rtHz at 1Hz (this is from a plot of an iLIGO NPRO from Rana's thesis, but it's probably similar). Talk to Kiwamu; the noise budget in the paper seems to indicate that we had some kind of boost on or something. Also, if we need much more gain than we already have, we'll definitely need a different box, maybe the PDH2 box that they have over in WBridge.
* (EQ, priority 1) Measure and calibrate error point noise down to lower freq for both arms. What could we win by putting in a boost? If the residual noise is high, maybe the laser isn't good at following arm, so beatnote isn't good length info for the arm, and we can't succeed.
* (EQ, priority 2) Measure TF of PDH box, and a separate measurement of the Pomona box that is between the mixer and the error point - is that eating a bunch of phase? It's already an LC circuit which is good, but do we really want a 120kHz lowpass when our modulation frequency is roughly 200kHz? Ask ChrisW - he worked on one of these with Dmass.
* (EQ, priority 2ish) Measure TF of Xend PDH loop (unless you already have one, up to ~1MHz).
* (JCD) Make DCC tree leaf for PDH box #17. Take photos of box.