I started the evening shift with calibration of the VCO + Frequency divider used for the ALS PLL (S1200570 and S1000748).  The objective was to measure the response of the VCO output to its Tune input, which is used for locking the arm cavity and thus provides a good calibration point for the arm cavity signal.

I set up the DC calibration measurement with a signal generator (output displayed on a scope to be sure), and a frequency counter.  I took 6 samples between -1V and 1V, which resulted in an excellent linear relationship: f = 39.6MHz + 139 kHz/V * V_tune.  For example, at V_tune = 1V, I measured f = 39.739 MHz.  This is not the expected result of 50kHz/V given the nominal 100kHz/V from the VCO and the division by 2 which takes us from 80MHz to 40MHz.

From there I moved on to measure the AC response.  I did this by rigging up a frequency to voltage converter with some bubble gum and duct tape I found nearby.  The output of the converter was 725mV / V_tune.  With the help of an SR785 and a lot of patience I turned this into a response measurment between 0.5Hz and 10kHz.  I saved the curves on the SR785 floppy, which is a lot like throwing them into a black hole, so I also wrote down a few points: {-30.7dB, 178dg} at high frequency, {-27.7dB, 138dg} at 40Hz, {-16.8dB 112dg} at 8Hz, {-5.8dB, 136dg} at 1.6Hz and {-3.26dB, 136dg} at 5.62Hz.  These are well fit by the measured DC response and the expected 1.6Hz pole and 40Hz zero.

Lastly, in preparation for using this calibration for the arm cavity, I cabled up an SR560 to the IR_PWR_MON channel.  The SR560 is AC coupled with a gain of 10, and the tabletop interface gain is set to 10.  From the output of the SR560 to counts of IR_PWR_MON, I measured 32060 counts/V.  Just to be sure, I checked the gain of the SR560 and appears to be very close to the stated value of 10, so we should expect 320.6 counts/mV refered to the SR560 input, which is attached to V_tune.  Assuming the above VCO calibration is correct, this converts to 0.434Hz/count as seen at IR_PWR_MON.  The cavity should lock should convert length to frequency as L_cavity / f_laser = 3995m / 5.635e14Hz = 7.09 pm/Hz, so we will be left with 3.07pm/count as our length calibration.   (To relate this to the number found with the OSEMS in 3363, we should remove the gain of the SR560, the TTIF box, and another factor of 10 for the FMON signal filtering.  This would give 3nm/count instead of the 1nm/count found with the BOSEMS, so somewhere we're missing a factor of 3, not unlike the 139kHz/V vs. 50kHz/V discrepence... to be investigated tomorrow in the light of day.)

On a side note: I found the PLL unlocked and no beatnote evident.  On closer inspection there was no light coming from the fiber, so I went to the optics lab and found the laser in Standby.  I have no idea why it would be left in this state, since this clearly makes cavity work impossible.  That should have been enough to make me leave it as I found it, but instead I turned it back on and relocked the reference cavity.  From the trend data, it looks like it unlocked about 15 hours ago (at 16:20 UTC, see figure).