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.