Reports until 00:51, Thursday 18 December 2014
H1 ISC
daniel.hoak@LIGO.ORG - posted 00:51, Thursday 18 December 2014 - last comment - 10:50, Friday 19 December 2014(15706)
output gain stage on ASC-AS_C changed to accommodate shutter threshold

Dan, Fil, Thomas

We made a quick modification to the ASC-AS_C transimpedance board today, so that the HAM6 fast shutter threshold can be set for 1W of light into the chamber.  The modification was a swap of R23, from 1.24k ohm to 422 ohm.  This changes the gain on the SUM_OUT channel on the back of the chassis, that's used for the fast shutter logic.

ASC-AS_C gets 2.5% of the light into HAM6, so we'd like to set the threshold at 25mW.  The input to the shutter controller rails at 2V.  The QPD transimpedance is 1000 ohms, the quantum efficiency is probably about 80%, and the new resistor changes the final gain stage to 422/4.99k = 0.0845.  So, the correct shutter threshold is 0.025*1000*0.8*0.0845 = 1.7 volts.

This modification only affects the signal path to the shutter controller; it doesn't change the signal path that is acquired by the ASC front end.  So the ASC-AS_C sum that you see on the MEDM screen hasn't changed.  (The input to the HAM6 shutter controller is recorded by the channel H1:SYS-MOTION_C_SHUTTER_G_TRIGGER_VOLTS.)

Comments related to this report
rich.abbott@LIGO.ORG - 10:50, Friday 19 December 2014 (15742)
Link 

Something seems odd here.  The QPD amp single ended sum output (D1001974) is designed to accommodate the full design dynamic range of the QPD (10mA per quadrant)by use of the R3 = 1.24k and to produce 10V at the sum output during this full scale optical input.  The shutter controller (D1102312) photodiode input is a unity gain receiver, although for some bizarre reason, this design only operates on +5V for all the opamps.  The correct way to fix this dynamic range problem should have been by lowering R13 from 10k to 2k thus preserving best SNR on the QPD sum output.  Also, I see no mention of serial numbers here.  Hopefully the eTraveler's are being updated as this is the only way to track such changes at the board and chassis level.