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Entry  Thu Jul 16 03:01:57 2015, ericq, Update, LSC, Old beatbox hooked back up beat_comparison.pngaLIGO_vs_beatbox.xml.zip
    Reply  Thu Jul 23 03:09:05 2015, ericq, Update, LSC, aLIGO demod board lives! beatbox_vs_aLIGO.png
       Reply  Thu Jul 30 14:42:03 2015, ericq, Update, LSC, aLIGO demod board lives! partialALSbudget.pngdemodDriveLevels.pngALScoherences.pngpartialALSbudget.zip
          Reply  Thu Jul 30 15:58:00 2015, ericq, Update, LSC, Beat note Alignment fluctuation effects measured RF_RIN.pngRF_RINspec.pngRFampCoh.xml.zip
             Reply  Tue Aug 4 03:02:30 2015, ericq, Update, LSC, Beat note Alignment fluctuation effects measured coherences.pnglinX.png
                Reply  Tue Aug 11 02:40:29 2015, ericq, Update, LSC, 50m delay lines - Rough calibrations 
Message ID: 11419     Entry time: Thu Jul 16 03:01:57 2015     Reply to this: 11438
Author: ericq 
Type: Update 
Category: LSC 
Subject: Old beatbox hooked back up 

I was having issues trying to get reasonable noise performance out of the aLIGO demod board as an ALS DFD. Terminating the inputs to the LSC whitening inputs did not show much 60Hz noise, and an RMS in the single Hz range. 

A 60Hz line of hundreds of uV was visible in the power spectrum of the single ended BNC and double-ended DB25 outputs of the board no matter how I drove or terminated.

So, I tried out hooking up the ALS beatbox. It turns out to work better for the time being; not only is the 60Hz line in the analog outputs about ten times smaller, the broadband noise floor in the resultant beat spectrum when driven by a 55MHz LO on the LSC rack is a fair bit lower too. I wonder if this is due to not driving the aLIGO board LO at the +10dBm it expects. With the amplifiers and beat note amplitudes we have, we'd only be able to supply around 0 dBm anyways. 

Here's a comparison of the aLIGO board (black) and ALS beatbox (dark green) driven with the 55MHz LO, both going through the LSC whitening filters for a resultant magnitude of 3kCounts in the I-Q plane. The RMS sensing noise is about 30 times lower for the beatbox. (Note, this is with the old delay cables. When we switch to the 50m cables, we'll win further frequency noise sensitivity through the better degrees->Hz calibration.) I'm very interested to see what the green beat spectrum looks like with this setup. 

Not only is the 60Hz line smaller, there is simply less junk in the beatbox signal. I did not expect this to be the case. 

There were some indications of funky status of the aLIGO board: channels 3 and 4 are totally nonfunctioning, so who knows what's going on in there. I've pulled it out, to take a gander if I can figure out how to make it suitiable for our purposes. 

Attachment 1: beat_comparison.png  435 kB  | Hide | Hide all
beat_comparison.png
Attachment 2: aLIGO_vs_beatbox.xml.zip  996 kB
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