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Entry  Wed Jun 10 03:19:58 2015, ericq, Update, LSC, AUX PDH error measured in CDS AUXspectra.png
    Reply  Fri Jun 12 17:09:58 2015, ericq, Update, LSC, Beatbox needs whitening gain beatGain.png
       Reply  Sat Jun 13 18:37:46 2015, ericq, Update, LSC, Beatbox needs whitening gain ADCdiagnosis.png
          Reply  Sat Jun 13 23:52:14 2015, ericq, Update, LSC, Beatbox needs whitening gain 
Message ID: 11356     Entry time: Sat Jun 13 18:37:46 2015     In reply to: 11355     Reply to this: 11357
Author: ericq 
Type: Update 
Category: LSC 
Subject: Beatbox needs whitening gain 

Here are the promised details!

I was worried about the lack of whitening gain, as I saw that the DC phase tracker Q output (which is the magnitude of the signal in the beatbox's I-Q plane) was no more than 200 ADC counts for X (~120mV), and 800 for Y (~500mV). I.e. this is the largest DC value that either the I or Q ADC channels can see, and the RMS fluctuations are on the order of mV, meaning we're wasting our entire ADC rangeno

However, I also had doubts about this since, even in the nominal state, the ASD of the ADC signals before dewhitening was higher than the expected ADC noise level. However, because of the non-linearity of the conversion of the BEAT_I and Q signals into the phase tracker output, evaluating the contribution of the beatbox output and ADC input voltage noises takes a few more steps. 

So, I hooked up a Marconi as the signal source for the beatbox's X channel , with no modulation (presumably the phase noise of the Marconi output is significantly lower than the sensitivity of the beatbox). For all of these measurements, the beat frequency was kept around 50MHz, with an amplitude of -30dBm on the control room analyzer, which is a typical X ALS operating point. 

At this point, the beatbox output noise was below the ADC noise (as measured by an SR785). Nevertheless, I found that the beat spectrum driven by the Marconi lined up to be very close to the ALS beat spectrum across a wide band, explaining much of the noise. 

At this point, I inserted SR560s in between the beatbox I and Q outputs, and the AA chassis leading to the ADC. A gain of 10 reduced the resultant phase tracker noise by that same factor at nearly all frequencies. A further increase in gain did not lead to the noise changing appreciably, probably because the real beatbox noise was now contributing, as is suggested by some common peaks in the direct beatbox output phase tracker spectra.

Going back to the real green beat signal with the SR560s still at G=10, I obtained the result shown in ELOG 11355. I will soon repeat this process with the Y ALS. 

As I mentioned in the previous ELOG, we may be further helped by more whitening gain than can be provided by the SR560s (and we obviously need a robust long term circuit for this gain). If it then turns out we are limited by beatbox noise to a degree we are not happy with, we could perhaps look into reintroducing some RF gain into the X beat. As Koji mentions in ELOG 8855, the beatbox operates best at an RF input of around -4dBm.

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