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Entry  Fri Aug 28 23:33:38 2020, gautam, Update, BHD, Some more hardware changes 
    Reply  Sat Aug 29 22:46:29 2020, gautam, Update, BHD, New homodyne-phase control electronics zetaDrive.pdftrekTFs.pdf
       Reply  Wed Sep 2 00:49:47 2020, gautam, Update, BHD, Some notes about homodyne phase heterodyneMICH.pdfunwrappedPhase.pdfunwrappedPhase_zoom.pdfphaseNoisePSD.pdf
          Reply  Thu Sep 3 15:55:04 2020, gautam, Update, BHD, Phase drift between LO and IFO after fiber replacement LOphaseDrift.pdfphaseDrift_tempCorr.png
Message ID: 15549     Entry time: Sat Aug 29 22:46:29 2020     In reply to: 15545     Reply to this: 15553
Author: gautam 
Type: Update 
Category: BHD 
Subject: New homodyne-phase control electronics 


The electronics chain used to drive the three elements of the PI PZT on which a mirror is mounted with the intention of controlling the LO phase has been changed, to now use the Trek Mode603 power amplifier instead of the OMC high voltage driver. Attachment #1 shows the new configuration.


The text of Attachment #1 contains most of the details. The main requirement was to map the DAC output voltage range, to something appropriate for the Trek amplifier. The latter applies a 50V/V gain to the signal received on its input pin, and also provides a voltage monitor output which I hooked up to an ADC channel in c1ioo. The gain of the interfacing electronics was chosen to map the full output range of the DAC (-5 to +5 V for a single-ended receiving config in which one pin is always grounded) to 0-2.5 V at the input of the Trek amplifier, so that the effective high voltage drive range is 0-125 V. I don't know what the damage threshold is for the PI PZT, maybe we can go higher. The only recommendation given in the Trek manual is to not exceed +/-12 V on the input jack, so I have configured D2000396 to have a supply voltage of 11.5 V, so that in the event of electronics failure, we still don't exceed this number.

On the electronics bench, I tested the drive chain, and also measured the transfer function, see Attachment #2. Seems reasonable (the Trek amplifier was driving a 3uF capacitive load used to protect the SR785 measurement device from any high voltage, hence the roll-off). The gain of D2000396 was changed from 1/8 to 1/4 after I realized that the DAC full range is only +/- 5 V when the receiving device is single-ended at both input and output. Maybe the next iteration of this curcuit should have differential sending, to preserve the range.


To test the chain, I used the single bounce beam from the ITM, and interfered it with the LO. Clear fringing due to the seismic motion of the ITM (and also LO phase noise) is visible. In this configuration, I drove the PZT mirror in the LO path at a higher frequency, hoping to see the phase modulation in the DCPD output. However, I saw no signal, even when driving the PZT with 50% of the full DAC range. The voltage monitor ADC channel is reporting that the voltage is faithfully being sent to the PZTs, and I measured the capacitance of the PZTs (looked okay), so not sure what is going on here. Needs more investigation.

Update Aug 30 5pm: Turns out the problem here was a flaky elbow connector I used to pipe the high voltage to the PI PZT, it had some kind of flaky contact in it which meant the HV wasn't actually making it to the PZT. I rectified this and was immediately able to see the signal. Played around with the dark fringe Michelson for a while, trying to lock the homodyne phase by generating a dither line, but had no success with a simple loop shape. Probably needs more tuning of the servo shape (some boosts, notches etc) and also the dither/demod settings themselves (frequency, amplitude, post mixer LPF etc). At least the setup can now be worked on interferometrically.

Attachment 1: zetaDrive.pdf  68 kB  Uploaded Sat Aug 29 23:48:21 2020  | Hide | Hide all
Attachment 2: trekTFs.pdf  74 kB  Uploaded Sat Aug 29 23:58:12 2020  | Hide | Hide all
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