chris.whittle@LIGO.ORG - posted 15:48, Thursday 15 September 2016 - last comment - 19:52, Thursday 15 September 2016(29735)
IMC PDH daughter board functionality
Jeff K, Chris Whittle
Following on aLOG 28363 (initial proposal) and 29250 (installation), we measured the open loop gain of the IMC loop (without full CARM) with the AG4395A, both with and without the daughter board 200 kHz pole. The GPIB is still connected to the AG4395A by the Common Mode Board.
TFAG4395A_15-09-2016_140247.txt has the transfer function without the daughter board, TFAG4395A_15-09-2016_140553.txt with the daughter board. The attached plots were generated with daughter_tf_plots.py.
See the attached plots of open loop gain, loop suppression and closed loop gain. Takeaways:
- features around 800 kHz suppressed by 200 kHz pole
- we still have 48° of phase margin compared to our previous 63°
- unity gain frequency has moved from 75kHz to 64kHz
- gain margin has increased from 2.7 dB to 4.6 dB
- maximum gain peaking in the loop suppression has been reduced from 4 to 2.5
- closed loop gain is relatively unchanged below 40 kHz and should not affect CARM loop
Also note some variation in the OLG at about 200 kHz compared to the previous measurement.
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Comments related to this report
J. Kissel I've now measured the CARM Open Loop Gain Transfer Function in the two different states of the IMC PDH common mode board's new 200 kHz pole daughter board. As expected from above, the CARM loop is barely affected by the 200 kHz response change. (Also note that the daughter board can be turned on and off at will without affecting a 2W DC READOUT lock.) As such, I have left the 200 kHz pole enabled, and accepted H1:IMC-REFL_SERVO_FASTOPT as "On" in the down.snap of CS ECAT PLC2 beckhoff SDF system such that it sticks after reboots (there are no safe or OBSERVE.snaps for this SDF system). Attached are the results of the measurement. TFAG4395A_15-09-2016_191957.txt is the open loop gain with the daughter board ENABLED. TFAG4395A_15-09-2016_191352.txt is the open loop gain with the daughter board DISABLED. Summarizing the attached plots in words: the CARM loop has a UGF of 15 kHz, with a phase margin of 40 [deg] and very tolerable amount of gain peaking at about a factor of 2 at 20 kHz, all regardless of the configuration of this switchable pole in the IMC PDH common mode board.
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