filiberto.clara@LIGO.ORG - posted 11:17, Tuesday 09 April 2019 - last comment - 19:34, Wednesday 24 June 2020(48338)
OMC DCPD (PI) Reackback Channels
WP 8158
AA chassis S/N S1102788
Channels 13-16 were modified with faster OpAmps for U2 and U3. New opAmp installed ADA4075-2ARZ. Same mods as were done with the CM SQZ readback channels, alog 47459.
Verified the following capacitors for the Twin T notch were all removed as mentioned in alog 28010 (CH15 & 16): C1, C3, C4, C5, C6, C7, C9, C10, and C11.
F. Clara, J. Kissel, D. Sigg
F. Clara, J. Kissel, D. Sigg While the team had the OMC DCPD's AA chassis out in the shop, I gathered transfer functions each channel, (a) to confirm their functionality after the ECR change (E1900105) (b) to understand the DC gain of the chassis better, given the recent confusion about AA chassis gains (see LHO aLOG 48272 and IIET Ticket 12642) It turns out, in the heat of battle, I measured the wrong channels for the low-frequency (i.e. gravitational wave channel) versions of the OMC DCPDs -- CH13 and 14, I measured 4 channels over, CH09 and 10. But I still learned a lot. Here're my conclusions: (1) The low-frequency channels I measured (CH09 and CH10 of the AA chassis) behave as expected, with a DC gain of 1.0 +/- 0.04%, and a butterworth lowpass with a notch at *roughly* 65 kHz (measured minimum magnitude's frequency within 2% of 2^16 Hz), and suppression *at* 2^16 Hz for both channels is -90 dB = 3e-5 V/V. (2) The PI channels for the OMC DCPD (CH15 and 16 of the AA chassis) have had all anti-aliasing removed, and the through-signal has no modification of signal from DC to 10 kHz, and above 10 kHz, at MOST a 0.25% / 1 deg change up to 100 kHz. (3) As far as measuring the low-frequency "DC" gain of the AA channels, electrical grounding of measurement equipment is very important. If one does *not* properly reference all equipment to a common ground, i.e. leaves the ground floating, then the data will errantly suggest that the DC gain of the AA chassis is 0.99, the frequency of the notch will shift, and the depth of the notch will also shift. Check out the first attachment 2019-04-09_H1OMC_AAChassisTFs.pdf. The first two pages shows the "final answer" for CHs 09, 10, 15, and 16 for the OMC AA chassis, as reported with correctly grounded test equipment. The legend label "CHSGND" stands for "properly referencing the measurement equipment to the AA CHaSsis GrouND." "FLTGND" stands for "the ground of the measurement equipment is FLoaTing with respect to the chassis GrouND." The third page shows what happens what you measure the coil driver test box (D1000931) with proper grounding (as was done in today's measurement) vs. the ground floating (i.e. as shown in all of the diagrams in -v1 of D1900027-v1). One can clearly a reported gain of 1.983 V_{se}/V_{diff} when the ground properly referenced, and a reported gain of 2.002 V_{se}/V_{diff} when floating. The fourth page shows what happens when you measure the anti-aliasing channels with proper grounding (as was done in today's measurement) vs. with the ground floating. Here, one can clearly see that with the floating ground, the AA notch has far less Q and the notch frequency is much more variable. Unfortunately with today's measurement, we had the rare treat of being able to pull the chassis out of the rack, pull off the lid, and directly access test points and grounding pins the filter board (D070081). Rarely do we get this luxury, as we're often stuck in the rack, at an end station, having to use the clumsy SCSI breakout board. The second attachment (AAChassisSetup_20190409.pdf) shows the proposed method of performing this clumsy technique while paying much better attention to referencing a common electrical ground. The third attachment (2019-04-09_AAChassis_Meas_Pics.pdf) shows some pictures of what it was like today, comparing properly grounded configurations and floating configurations that correspond to I believe the lack of attention to (3) is what has lead LHO to measure a low-frequency gain of 0.99 back in ER7/ER8 (i.e. in 2015), and we've been using the model from that data set for both AA and AI filters for *anything* that has one since then. That model is ^/trunk/Runs/ER8/H1/Scripts/AAAI/20150813_H1ER7_AA_upto10kHzFitLTI.mat which we used for O1 and O2, and has now been unceremoniously dumped into a new .mat file in a python compatible format, but in doing so lost its origin story, ^/trunk/Common/pyDARM/H1aa.mat which is now used for O3. I've followed the rabbit hole of that model, which lead me to the last time we systematically measured the PCAL AA chassis, in May 2015 (see LHO aLOG 18658), during Pre ER7 times, which was processed by the script ^/trunk/Runs/PreER7/H1/Measurements/ElectronicsMeasurements/process_H1PCALEX_AA_Measurements_20150527.m which reveals a plot that I re-attach because I couldn't find the aLOG in which I said I'd write about it. Check out the fourth attachment, 2015-05-26_H1PCALEY_AAChassis_S1203519.pdf. It has a DC gain of 1.0 +/- 0.02%, which is *not* the case for the model we've been using for the AA chassis since ER7. That model, came from Kiwamu's processing script, ^/trunk/Runs/ER8/H1/Scripts/AAAI/gen_H1ER7_AA_upto10kHzFitLTI.m which took data from ^/trunk/Runs/PreER7/H1/Measurements/ElectronicsMeasurements which just like above, uses the Coil Driver Test Box to create a differential signal, whose transfer function, ^/trunk/Runs/PreER7/H1/Measurements/ElectronicsMeasurements/TESTBOX_CH1In_Pins1-6Out_2015-05-19_092805.txt you can clearly see has a gain of 2.002 V_{se}/V_{diff} instead of the properly grounded 1.983 V_{se}/V_{diff}. No bueno!! Well -- hopefully now this lesson has been learned -- we'll update our calibration group documentation in such a way that hopefully this mistake will never happen again. AND we'll save a new python-friendly model, likely based on the data that Joe / LLO originally put together in ^/trunk/Common/Documents/T1500165/ or we'll go around and measure every AA and AI chassis properly. The message: AA and AI filters have a DC gain of 1.0 +/- 0.0(something)%, so we should stop treating them as otherwise.