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Entry  Thu Jul 17 01:24:35 2014, rana, Update, IOO, MC / EOM Stability Mystery Solved! MC_OLG.pdf
    Reply  Fri Jul 18 14:59:07 2014, Evan, Update, IOO, MC servo TFs mcServoTFSettings.pngMCtfs.pdf
       Reply  Tue Jul 22 08:36:08 2014, Evan, Update, IOO, MC servo TFs MCtfExpectations.pdf
Message ID: 10225     Entry time: Thu Jul 17 01:24:35 2014     Reply to this: 10235
Author: rana 
Type: Update 
Category: IOO 
Subject: MC / EOM Stability Mystery Solved! 

MC loop is near unstable. Common gain reduced. Needs more loop tuning.

We've often seen that the MC gets into a state where it keeps losing lock and the EOM drive shows a large RMS. We've usually been looking at the noise spectrum to diagnose this.

Tonight we finally just measured the OLG. The attached plot shows the loop gain measured with the 4395 on the MC servo board

Although the phase margin is a healthy 45 degrees, its close to instability at 1 MHz. For this plot, I reduced the gain by 3 dB and now the margin is ~7 dB. So usually its pretty close to unstable and at least its always making a noise peak.

That whole TF above a few hundred kHz is weird. We should tune out whatever makes it so flat and also remove the resonance that makes the 1 MHz peak; maybe its from some post mixer low pass?

Anyone interested in helping in the investigation ought to measure the TF of the MC demod board, the MC servo board, and the FSS box.

Silver lining: if we fix this loop shape, we might be able to have a much more stable IMC and IFO.

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