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Message ID: 358     Entry time: Mon Sep 13 23:37:52 2010
Author: tara 
Type: DailyProgress 
Category: FSS 
Subject: FSS debugging again 

The FSS servo is not working. When SLOWDC which controls the NPRO temperature is brought to near resonance,

it falls off the lock when we enable the loop. So it's a debugging day.

First, I aligned the beam on RFPD, making sure that the beam is on the center, and align the beam to the cavity.

The signal from transmitted beam oscillates a lot. The gain was too high

(This is surprising, we haven't changed any power but the gain is too high already.) 

the set up is changed as follow

common gain, 16  --> 7

Fast gain,   15 --> 14

RF power  7.0 --> 7.2

Phase Adj  4.5 --> 3.92

Then I checked the error signal from MIXER OUT channel on FSS card. The signal looks fine.

The DC level is ~ 17 mW, peak to peak level of the carrier is 160 mV, and 39 mV for sidebands.

[the setup is:

RF power 7.0 V
Phase adj 4.5 V +180 flip]

The transfer function between In2 and fast mon seems fine.

---------------------------------

Now the loop can be locked, but when FSS_FAST gets lower than -2 V. It runs away and lose lock.

This does not happen when it goes to plus sign. it can go up to 5 or 6 V before losing lock.

So we increased the RF power to maximum (10 V) which increases the error signal pk-pk to 394 mV.

FSS MON seems to stop railing, I'll adjust the gain setting again to minimize fluctuation in mixer out. 

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