Reports until 11:44, Tuesday 12 July 2022
H1 ISC
marc.pirello@LIGO.ORG - posted 11:44, Tuesday 12 July 2022 - last comment - 12:09, Tuesday 12 July 2022(63900)
IMC VCO Replaced with new VCO

Daniel S., Craig C., Marc P.

Per WP10544 the IMC VCO S1200566 located in next to the PSL at rack ISC-R1 was replaced with the new style VCO S2101408.  This new VCO does not rely on a mixer to maintain equivalent phase noise.

The old VCO output power was 17.8dBm. 

OUT1 is directly connected to cable ISC_R1_RF11 which leads to the 3rd patch panel, far left connection.  This cable ultimately leads to the PSL AOM input which had a 10dB attenuator attached.

OUT2 had a 3dB attached to cable ISC_R1_RF9 which leads to the 2nd patch panel, center connection and ultimately a frequency counter.

The new VCO output power is 13dBm.

OUT1 due to the change in power, we reduced the attenuator at the AOM from 10dB to 5dB.  The new VCO power to the AOM should be equivalent to the old VCO.

OUT2 the frequency counter can handle more than 13dB power, so we removed the 3dB attenuator.

Verified with Daniel and Jenny that everything locks after swapping the chassis and that power ranges are equivalent.

Comments related to this report
craig.cahillane@LIGO.ORG - 11:56, Tuesday 12 July 2022 (63904)PSL
Link 

Before and after IMC OLG measurements.

Blue and orange are from February 2022, right after the PSL upgrade was finished and the IMC required for the first time.

Green was from today just after the IMC VCO was replaced, but before the new 5dB attenuator between the VCO and AOM was installed.

Red is from the situation now, with the new 5dB attenuator.

There was no gain change from the attenuator, but Marc and I reckon this is expected since the signal comes from the frequency, not the amplitude, being sent to the AOM.  Probably there will be less noise due the the lower attenuation.

The measurements from today are stored at:
https://git.ligo.org/aligo_commissioning/psl_measurements/-/tree/main/data/imc_olg
with the 12-07-2022 date on the .txts.
Non-image files attached to this comment
daniel.sigg@LIGO.ORG - 12:09, Tuesday 12 July 2022 (63909)
Link

An optical gain change of the reference cavity (due to the AOM RF power) changes the FSS gain, not the IMC servo gain. This can be seen clearly from the phase development near 100kHz in the IMC servo response.

I also noticed that the 1dB attenuator at the output of the AOM RF amplifier is only rated for 1W, whereas the output power is nominally a little higher than that.