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Reports until 11:08, Thursday 26 May 2016
H1 ISC
haocun.yu@LIGO.ORG - posted 11:08, Thursday 26 May 2016 - last comment - 18:17, Thursday 26 May 2016(27402)
90MHz check at AS port
Sheila, Keita, Haocun

We took several more measurements on 90MHz signals at AS port yesterday.


In summary, the noise at 90MHz is really large compared to signals. Even when the MC was unclocked, the spectrum from RF IN had a peak of -88dBm at 91MHz, and the signals were around -71dBm both at RF in and RF MON when the IFO was locked.
There are large signals at 54MHz in RF IN (-45dBm), and -68dBm in RF MON. We put in an 91MHz Bandpass Filter (Lark Engineering MC91.5-H2.5-3BA), but this did not help with the noises at 91MHz. (RF IN: -78dBm; RF MON: -70dBm @91MHz)
Comments related to this report
keita.kawabe@LIGO.ORG - 18:17, Thursday 26 May 2016 (27414)

To rephrase the above and add some:

Forget about RF MON for 90MHz for now as it's dominated by the 90 MHz LO leakage/pickup or something inside the demod chassis. Even when the RF input on the demod chassis is terminated we see -71 dBm in RF MON at 90 MHz.

RF level from the WFS at RF IN on the demod is about -71 dBm when IFO is locked (and -88 dBm when MC is unlocked, which is 17 dB lower than locked and is therefore negligible). This sounds small, but we have a 90MHz notch on the WFS head itself.

54MHz signal is much larger than 90MHz in WFS 90MHz output (-45dBm for 54MHz VS -71 dBm for 90MHz on RF IN of the demod). It wasn't clear if this was doing anything bad, but inserting 90MHz BP between the triplexer and the demod didn't change the demod output significantly. It's not clear to me if BP properly  blocks 54MHz when triplexer assumes 50 Ohm resistive load and BP adds complex load, though.

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