I started to look at times when the fast shutter fired to check if there are times when more energy passed the shutter than desired (in part because of the broken beam dump in HAM6). I wrote a script that looked at about 40 days of minute trend of the AS WFS (which are behind the shutter) and picked out times when they were high. Attached are plots of the three events where the most energy seems to be getting past the shutter in those 40 days. In all of these three cases, the power on the AS_C PD doesn't seem to have exceeded the threshold for the shutter to fire until there had already been several seconds of large power fluctuations at the AS port. The shutter triggers as expected when the power goes over the threshold.
- Beckhoff channels are reporting events happening about 0.1 seconds before the front end (39329) For this reason I've shifted the time of all the channels labeled beckhoff in the attached plots by +0.125 seconds.
- I do not understand the behavior of the channel labeled Shutter G Trigger. The best explanation I can think of for what I see is that this is somehow the shutter is triggered on AS WFS DC. The channel labeled SHUTTER G Trigger Volts is the beckhoff channel which Koji and I believe should be a readback of the outputs labeled PD+ and PD- on sheet 2 of D1102312. (For reference, see the diagram of the fast shutter wiring in E1300819 QPD amp is D1001974 shutter controller is D1102312 and shutter driver is D1400078)
- The voltage read back by the Trigger Volts channel spikes and goes to zero after the trigger fires, similar to the AS WFS channels which are behind the shutter. AS_C should not be blocked by the shutter firing, and goes to zero more slowly after the lockloss transient has passed. I went to the floor and sent a DC voltage into the shutter controller, as you would expect the readback of the trigger volts channel is not impacted by the shutter triggering.
- DC power:
- This trigger comes from the sum on D1001974 I believe this should have a gain of 0.2485V/mA on the QPD (a factor of 10 smaller than what is on the schematic), because there is the transimpendance of 1kOhm, then a gain of 0.2485 before the summation. The normal photocurrent from the diode then is about 1mA. (I checked with a calibrator that putting 1 V into the shutter driver input results in 1V read back in beckhoff)
- The calibration of the AS_C channel into mA is 1V/mA (TI gain)*2 (differential driver in TI amp chassis)*10^(36/20)whitening gain*1638.4 cnts/V = 2/07e5 cnts/mA. By undoing the front end calibration into Watts into HAM6 (29078) we can estimate that AS_C has about 0.125 mA photo current.
- We expect about 10 times more power on the AS WFS than AS_C, since we have 800 ppm for OM1 (plus 50% splitter) and 1% transmission for OM3 (split between 2 WFS).
- Two caveats:
- There is a jumper in the transimpedance chassis which can be used to turn on the 0.4z40p whitening stage, (this is after the trigger spigot) we are currently compensating this in the front end but if it is not on that could explain why AS_C sometimes overshoots and goes to zero.
- I checked the cables, and everything seems to be as it should be.
This afternoon Gerard and Koji helped me confirm that the fast shutter is cabled correctly by shining a flashlight on AS_A, AS_B, and AS_C. Indeed AS_C is connected to the fast shutter trigger.
Fil also helped me interpret the PCB layout for the transimpedance amp, I peaked inside the one for AS_C and confirmed that when looking from the front of the chassis (side where the LED is on the board) the red jumpers are on the left side, between pins 1 and 2, which means that the whitening is used. This means that the whitening is correctly compensated in the front end calibration of AS_C.
After talking with Daniel, my confusion was caused by the AS_C readback saturating in the whitening stage of the transimpendance amp. Since the sum output bypasses this whitening stage, it does not saturate and the signal readback by the beckhoff (Trigger Volts) reflects the realistic power on the PD.
So it looks like it is not really feasible to check how much energy is getting past the fast shutter using one of our HAM6 PDs. The AS WFS saturate the ADC during the lockloss transient, so they aren't very useful, and AS_C saturates the whitening stage.