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.

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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.