Attached are power spectra of the sum signals of the 4 IR QPDs that are on the ETM Trans Mon tables (in-vac). Each Trans Mon has 2 QPDs (labeled A & B). Also included in the plot is a spectrum of QPD_B from the L1 Y-arm. As discussed in LLO log entry (hmm, LLO log is down), the L1 Y-arm QPDs show broad noise peaks at 30 Hz, 60 Hz, 90 Hz; these peaks show up in DARM and are believed to be caused by scattered light at the end station (the L1 X-arm QPDs do not show these noise peaks).
The good news is that the H1 TransMon QPDs do not show anything like the scattered light noise peaks seen in the L1 Y-arm, and in general the H1 QPD spectra are lower than the L1 spectra (all channels are normalized to unity for a single arm lock). The RMS of the X-arm transmitted power fluctuations is about 30/10,000 = 0.3%; about 1% pk-pk.
The bad news is that there is something wrong with QPD_A on the Y-arm. It shows an elevated 1/f noise above a few Hertz. This is presumably related to the QPD 'clipping' problem reported by Keita in entry 18204. Looks like some electronics problem to me. On the other hand, the X-arm QPD_A is anomolous in another way: its average value is about 30% lower than the others (see table below).
For reference, the average values of the QPD NSUM channels were:
| A | B | |
|---|---|---|
| X | 9100 | 12,000 |
| Y | 13,200 | 12,800 |
It seems like segment 2 is the bad guy (in that the excess noise comes from that segment), and it comes with about 900 counts of offset which indeed can quantitatively explain the "clipping" behavior.
Segment 4 is also strange in that the noise floor is 6 dB larger than the others.
Whitening interface is working fine in that the spectrum of all segments move up and down by the same amount at the same time for each step of the whitening slider from 0 to 45dB, and all combinations of whitening/dewhitening result in the same spectrum.
It seems like the input stage of the whitening box for Y QPDB SEG2 is broken.
Moving slider up and down, switching whitening filter on and off: All signals changes as they should.
Disconnecting the QPD cable between the chamber and the QPD transimpedance amplifer box: No change in the DC offset (QPD B SEG2_IN1 -900 to -1000 counts at the nominal whitening gain of 18dB, everything else within +-10 counts).
Disconnecting the cable between the QPD transimpedance box and the whitening box: SEG2_IN1 jumps to -32k, others no change.
Disconnecting the cable between the whitening box and the DAQ: Everything goes to zero-ish.
Since I don't want to mess with whitening I/O cables now, which have a history of acting up and taking a long time to diagnose and fix, I asked Richard to swap the whitening chassis the first thing in the morning tomorrow.
It turns out that the bad segment has been bad since about 2015/03/29 18:00 UTC, that's Sunday 11AM local time. Nothing on the alog.