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Message ID: 999     Entry time: Fri Sep 26 16:13:57 2008
Author: rana 
Type: Update 
Category: IOO 
Subject: MC_L / MC_F crossover 
We were trying to understand why the FAST_F signal had such large excursions (~1V ~ 5 MHz).

Some of this is due to the seismic noise and the resulting MC_F signals. Increasing the MCL
gain reduces it somewhat. But as you can see from the attached loop gain measurement, the
crossover is a healthy 90 Hz with the MCL digital gain = 1. But what's going on in the MC loop
in the 10-20 Hz band? That looks like bad news.

Then I noticed that changing the ISS gain slider puts a large step (~1V) into the FAST. My guess
is that the board has large DC offsets and also much of the switching supply noise. Not sure why
this would be worse than before though.

To prevent large noise in the FAST, I've changed the mcup script to set this gain to -5 dB. Our
intensity noise is now presumably 10-15 dB worse than the nominal good levels we had a year ago.
Needs investigation.
Attachment 1: mcx.png  34 kB  Uploaded Fri Sep 26 17:15:25 2008  | Hide | Hide all
mcx.png
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