[Dick, Kiwamu]

 We took a look at the IMC demod phase shifter [1] which had been found to be out of Beckoff's control the other day last week [2]. The conclusion is that it magically recovered by itself and became under Beckoff's control again. Anyway we set the delay by the hardware switches and disabled the digital switching as this is relatively safer than setting it digitally. There should be no impact on the IMC locking.


The optimum delay setting
   The delay is set to be 14.4375 nsec (= 1/16 + 1/8 + 1/4 + 2 + 4 + 8 nsec)
 where the I signal is maximized. This corresponds to a step setting of 231=(1+2+4+32+64+128) in the digital system. Note that the IMC modulation frequency is at 24078360 Hz.

The Symptom
 The symptom we saw last week was that the delay time didn't change when we changed the digital values in the corner Beckoff. I don't know what had happened to the electronics and digital system. Somehow we were able to change the delay from the Beckoff today.

The right half of the device is not functioning
 This is not important. A sad thing we found is that the right half of the delay line device doesn't seem working for some reason. We are not going to fix this because we haven't been using it and also we are not going to use it. Dick found that the loss of the circuit heavily depends on the amount of the delay and sometime it can reduce the ~15 dBm input signal down to -20 dBm or even less at the output port. We are guessing that there is some kind of loose contact in the circuit.


[1] DCC S1103425
[2] LHO alog 6265 "IMC locked, relocking well, visibility is good, but not TRANS beam, as it is clipped by it's bellows."