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Entry  Mon Oct 3 13:11:22 2022, Yehonathan, Update, BHD, Some comparison of LO phase lock schemes MICH_sens_vs_LO_phase.pdfLO_phase_sens_vs_LO_phase_RF.pdfLO_phase_sens_vs_LO_phase_double_audio.pdf
    Reply  Tue Nov 15 17:08:59 2022, Paco, Update, BHD, Request for estimates 
       Reply  Wed Nov 23 12:58:33 2022, Yehonathan, Update, BHD, Some more calculations LO_phase_sens_vs_LO_phase_RF.pdfLO_phase_sens_vs_LO_phase_RF_fixed_demod.pdfLO_phase_sens_vs_MICH_Offset.pdf
          Reply  Wed Nov 23 17:28:39 2022, Yehonathan, Update, BHD, Some more calculations Fields_at_BHDBS.pdf
Message ID: 17170     Entry time: Mon Oct 3 13:11:22 2022     Reply to this: 17268
Author: Yehonathan 
Type: Update 
Category: BHD 
Subject: Some comparison of LO phase lock schemes 

I pushed a notebook and a Finesse model for comparing different LO phase locking schemes. Notebook is on https://git.ligo.org/40m/bhd/-/blob/master/controls/compare_LO_phase_locking_schemes.ipynb,

Here's a description of the Finesse modeling:

I use a 40m kat model https://git.ligo.org/40m/bhd/-/blob/master/finesse/C1_w_initial_BHD_with_BHD55.kat derived from the usual 40m kat file. There I added and EOMs (in the spaces between the BS and ITMs and in front of LO2) to simulate audio dithering. A PD was added at a 5% pickoff from one of the BHD ports to simulate the RFPD recently installed on the ITMY table.

First I find the nominal LO phase by shaking MICH and maximizing the BHD response as a function of the LO phase (attachment 1).

Then, I run another simulation where I shake the LO phase at some arbitrary frequency and measure the response at different demodulation schemes at the RFPD and at the BHD readout.

The optimal responses are found by using the 'max' keyword instead of specifying the demodulation phase. This uses the demodulation phase that maximizes the signal. For example to extract the signal in the 2 RF sideband scheme I use:

pd3 BHD55_2RF_SB $f1 max $f2 max $fs max nPickoffPDs

I plot these responses as a function of LO phase relative to the nominal phase divided by 90 degrees (attachment 2). The schemes are:

1. 2 RF sidebands where 11MHz and 55MHz on the LO and AS ports are used.

2. Single RF sideband (11/55 MHz) together with the LO carrier. As expected, this scheme is useful only when trying to detect the amplitude quadrature.

3. Audio dithering MICH and using it together with one of the LO RF sidebands. The actuation strength is chosen by taking the BS actuation TF 1e-11 m/cts*(50/f)**2 and using 10000 cts giving an amplitude of 3nm for the ITMs.

For LO actuation I can use 13 times more actuation strentgh becasue its coild drivers' output current is 13 more then the old ones.

4. Double audio dithering of LO2+MICH detecting it directly at the BHD readout (attachment 3).

Without noise considerations, it seems like double audio dithering is by far the best option and audio+RF is the next best thing.

The next thing to do is to make some noise models in order to make the comparison more concrete.

This noise model will include Input noises, residual MICH motion, and laser noise. Displacement noise will not be included since it is the thing we want to be detected.

Attachment 1: MICH_sens_vs_LO_phase.pdf  12 kB  Uploaded Mon Oct 3 19:16:55 2022  | Hide | Hide all
Attachment 2: LO_phase_sens_vs_LO_phase_RF.pdf  20 kB  Uploaded Mon Oct 3 19:17:05 2022  | Hide | Hide all
Attachment 3: LO_phase_sens_vs_LO_phase_double_audio.pdf  16 kB  Uploaded Mon Oct 3 19:17:13 2022  | Hide | Hide all
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