Ushiba, Tanaka, Takano
Abstract
We started to investigate the source of the strange coupling of ALS signal and IR REFL. We checked the slow DAQ of GrPDH of each arm with the shutter closed. We found that GrPDHX signal is noisier than GrPDHY above ~20 Hz, with many peaks and white noise floor,
and nonstationary peaks above 7 kHz were seen for both PDH signals.
Detail
For years we have suffered from the mysterious coupling between ALS signal and IR signal (ref: 11411). It might be due to a GND loop around REFL/POP/POS area and could be coupled to DARM sensitivity. To solve this issue once for all we started to investigate the source of this coupling.
First we suspected that some electronics for GrPDH might oscillate at some high frequency. To check the behaviour in high frequencies, we monitored the spectra of GrPDH slow DAQ, not at 16 kHz but at 64 kHz. Figure1 shows the spectra of GrPDHX and GrPDHY in terms of the input equivalent noise of the common mode servo. The spectra labelled noX are the measurements taken in different time and REF are the spectra with the input of the servo open electrically. It is certain that above 20 Hz the ADC noise is below the signal from the RFPD.
From these measurements we found these characteristics;
- For GrPDHX the floor level is almost flat above 20 Hz, 6e-7 V/√Hz, whereas the floor level of GrPDHY has some weird frequency dependence. At 100 Hz GrPDHX is about two times larger than GrPDHY.
- For GrPDHX many peaks are seen, at 60, 120, 180, 240, 300, 480 (power line), 68, 136, 234, 260 (mysterious) Hz.
- For both spectra, many peaks are found above 7 kHz. The center frequency of these peaks also changes in time (measurement no2 was taken 5 min after no1 measurement).
Anyway, it seems that the condition of GrPDHX is worse than GrPDHY.
Next
We will check the spectra of the related electronics one by one in high frequencies. Especially, we are interested in
- The original RF signal from RFPDs
- The demodulated signal above 32 kHz to check whether RFPDs are oscilating or not
