[Miyakawa, Nakano, Kokeyama, Zeidler, Kuwahara, Uchiyama, Miyo, Fujii]

From yesterday's result of the sensitivity with no fan which will be posted by Aso-kun soon, the sensitivity of high frequency seemed to be limited by ADC noise.

Today, we tried the same thing with whitening filter on REFL RF PD output to reduce the ADC noise effectively.

We used the KAGRA default universal whitening filter that has a zero at 1Hz and a pole at 10Hz. It has 3 stage whitening filters but only 1 stage was used today. To compensate analog whitening filter, we implemented anti whitening filter on digital stage at MICH filter.

1. The first attached graph shows the result of with/without whitening filer. I plotted two lines as

Red: without fan, without whitening filter (ADC limited)

Blue: without fan, with whitening (improved at high frequency)

Very high frequency above 2.5kHz is something wrong and should be calibrated again.

Probably this result is a bit better than yesterday's result since Nakano-kun kindly turned off fan at laser area today.

2. The second attached graph shows a summary of iKAGRA sensitivity.

Black: the first sensitivity obtained on one day before the 1st observation stated.

Gray: after 10 days commissioning on beginning of April

Red: After observation and some trial of noise hunting by stopping all fan and implemented a light whitening filter.

It was almost 1.5 order improvement in all the frequency by almost 1 month. Probably this will be the best sensitivity of iKAGRA with the current situation.