J. Kissel, T. Vo

We received the daughter boards (D1001631) which take place of the Binary I/O connection for control of the Oplev Whitening chassis (D1100013), specifically the internal ISC Whitening board (D1001530). For each of the 4 channels, the whitening board has 7 independently switchable states:

(1) 24 dB (x16)  Gain
(2) 12 dB (x4)   Gain
(3) 6 dB  (x2)   Gain
(4) 3 dB  (x1.4) Gain
(5) [10:1] Whitening
(6) [10:1] Whitening
(7) [10:1] Whitening

where my notation for zeros and poles is [z:p]. So, Thomas and I installed the daughter on H2 SUS ITMY's optical lever, and explored the raw voltage spectra of the QPD segments with each of the settings on (well, most of them). I attach the results.

To fill out the legend:

RED: This was the state that the optical levers had been in, with no control (i.e. the BIO input spigot was left open).
BLUE: Daughter board installed, but with all switched flipped to OFF. This is the same as red -- good. That means the chassis doesn't *need* the daughter board to be functional, and the board sits at its default state (no stages engaged) with nothing plugged in. Fair enough. 
GREEN: 12 dB Gain stage turned ON
BROWN: 4 dB Gain stage turned ON
MAGNENTA: One [10:1] whitening stage turned ON
CYAN: Two [10:1] whitening stages turned ON
BLACK: All Three [10:1] whitening stages turned ON

Note that we tried switching on the 24 dB gain stage (alone), as well as having both 6 dB gain + One [10:1] whitening stage. Both of these configurations saturated the ADC.

Assuming the amplitude of the signal represents a reasonably quiet time, we decided to leave the chassis in the CYAN state, with two [10:1] whitening stages and no gain, to give some head room for noisy day time activity, or when the ISI is not functioning as well as it is now. Because we now have whitening in the chain, these filters have been compensated for in the $(IFO):SUS-$(OPTIC)_L3_OPLEV_SEG# banks (in addition to the -1 discussed in earlier logs).

One thing to note: the ADC noise floor (visible in the unwhitened spectra, above ~40 Hz) is 5 uV/rtHz.

Calibration Details:
--------------------
Here's a schematic of the QPD signal chain: 

                                                                                          +--- $(IFO):SUS-$(OPTIC)_L3_OPLEV_SEG1_IN1
      +-------+                                                                           |
Up ^  | 1 | 3 |      +-------------------+      +-----------------------+      +-----+    +--- $(IFO):SUS-$(OPTIC)_L3_OPLEV_SEG2_IN1
   |  |---+---| ---- | Whitening Chassis | ---- | Anti-aliasing Chassis | ---- | ADC | ---| 
   |  | 4 | 2 |      +-------------------+   ^  +-----------------------+      +-----+    +--- $(IFO):SUS-$(OPTIC)_L3_OPLEV_SEG3_IN1
      +---+---+                              |                                            |
                                             |                                            +--- $(IFO):SUS-$(OPTIC)_L3_OPLEV_SEG4_IN1
                                        equivalent
                                   voltage measured here

(staring at the face of QPD, as though you were an incident laser beam). 

We used DTT to measure the above listed channels, which were then calibrated using the 16-bit ADC calibration of
40 / 2^16 [V/ct] = 6.1e-4 [V/ct]
hence, as I point out in the diagram, we're effectively measuring the output of the whitening chassis since the AA chassis has a gain of 1 and no frequency response.