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Message ID: 160     Entry time: Mon Jun 14 10:16:54 2010
Author: tarac 
Type: Misc 
Category: Other 
Subject: weird TF from PMC servo 

Since the VCO has been borrowed, I decide to work on RefCav path again. To optimize the RefCav alignment, PMC will be locked to the laser while the laser frequency is modulated by a function generator. This is where a problem comes in. The PMC servo cannot catch up with the laser. Even though I lower the modulating frequency and amplitude to 4 Hz, 1 Vpp ,which roughly corresponds to ~1MHz shift in laser frequency, the PMC servo cannot stay stable for longer than a minute. so I measure the transfer function of the PMC servo, and it does not look right, see figure1. The real magnitude which goes to the PZT has higher value, since the signal comes out of the out mon channel passes through a voltage divider.

I also measured TFs of FSS servo, both Fast and PC paths look ok. I measure TF by using swept sine measurement , the source is split by a T connector, one goes to chA of SRS785, another one goes to TF1 test (where demodulated signal goes.) The output is taken from out mon (for PMC servo), fast mon (for FSS' fast path), PC mon (for FSS's PC path), and connected to chB of SRS785.

I made sure that I switch to the right path in the medm control screen when I measure the TFs

Attachment 1: PMC_bode.png  27 kB  | Hide | Hide all
PMC_bode.png
Attachment 2: FSS_fast_bode.png  13 kB  | Hide | Hide all
FSS_fast_bode.png
Attachment 3: FSS_PC_bode.png  15 kB  | Hide | Hide all
FSS_PC_bode.png
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