- No extraordinary features were found in the bullseye signals at HF/VHF frequencies.

[Summary]

During the commissioning call in the past Friday, there was a suggestion to check the HF/VHF (~MHz region) behavior of the bullseye sensor in order to make sure that the sensor is detecting what it should detect. In response to it, I have measured high frequency part of the signals coming out from the bullseye segments using an RF analyzer (an HP 4395A). I didn't find extraordinary features. I am concluding that the HF/VHF contents are not an issue.

[Some details]

The measurement was done by inserting a 9-pin Dsub breakout board right in front of the whitening board (34154) during the maintenance period yesterday. This measurement was performed after Peter and Jason had finished their flow sensor task (34811). The PSL laser was running in a nominal condition. The IMC was locked with a low power (~2 W). The interferometer was kept unlocked.

Each segment was read out before they go into the whitening board. I hooked up an HP 4395A to the breakout board using a pair of clips and a BNC cable. Unfortunately, this cheap setup was so good that it picked up lots of RF lines. Nonetheless, I didn't see an outrageous line or suspicious feature in the bullseye signals. Most of the lines are as small as -60 dBm and many are less than that except for the one at 160 MHz which I think is a pick up from the measurement cable (as high as -50 dBm). See the attached for the spectrum. I also attach the raw data just in case.

Since all four segments showed almost identical spectrum, I saved the data only for the center segment. For comparison, I also saved the background spectrum with the clips left open. Note that the background spectrum wasn't stationary. After the measurement, I removed the breakout board and completely cleaned up the measurement setup.