We took relative intensity noise (RIN) spectra of the ISS error point and the monitor PD (attm1).
In-loop RIN is the sensor PD and "Out of the loop RIN" is the monitor PD.
The ISS gain slider was at 8dB in this measurement.
It looks normal.
An open loop transfer function of the ISS loop was measured (attm2). The UGF was 22kHz with the phase margin of ~22deg.
We should increase the UGF up to ~60kHz
When we increase the gain up to 14dB, the CS saturation warning comes up in the EPICS screen.
We confirmed this by monitoring the CS drive signal with an oscilloscope.
It is the output of an AD602J, which has +/-3V output range.
By increasing the gain of AD602J, we saw that the output signal hits the rail.
There seems to be a lot of high frequency (100kHz - a few MHz) noise, out of the control band.
We also observed that AD602J itself oscillates at about 10MHz (don't remember the exact number) when the gain is increased.
(We saw this even when the loop is off. There is no such an oscillation in the input to the AD602J).
When we took wide band spectra of the CS drive signal, we saw many large harmonics of ~180kHz. We believe these peaks are limiting
our ISS gain now (causing the CS saturation). The harmonics persisted even when we disconnected the PDs. So it is not coming from the light.
We saw the same harmonics in the power lines. They may be the switching noise of the Sorensens.
We took spectra of those harmonics, but the netgpibdata.py somehow did not save the data from AG4395A correctly. I have to debug this.
Stefan removed DC offsets from the AD829s (many of them are used in the ISS board) by turning the pots for offset adjustment.
This eliminated the problem of getting a large DC CS feedback (observable in C1:PSL-ISS_CSDRIVE_MEAN) when the gain is increased.
During the investigation, I noticed that increasing the PMC gain too much (~22dB) caused an oscillation of the PMC loop and consequently made
the ISS saturate. When the ISS is behaving bad, we should check the PMC gain.
Currently, the ISS is running OK with the gain = 8dB. I modified the mcup script to set the ISS gain to 8dB when the MC is locked.
Wait for Peter's answer about spare ISS boards.
Power line filtering.
Find the cause of AD602J oscillation (Well this is the one mounted upright. So just mounting it normally might solve the problem :-).
The first column is distance from the front face of the laser in cm.
The second column is beam diameter in the horizontal direction in microns.
The third column is the beam diameter in the vertical direction in microns.
% FP Cavity HOM Frequencies Estimate
% Alberto Stochino, October 2008
R1 = 7280; % Mirror1 radius of curvature
R2 = 57.57; % Mirror2 radius of curvature
L = 38.458; % Length of the FP Cavity
n = 1; % X Order of the Mode
m = 0; % Y Order of the Mode
c = 299792458; % Speed of Light
svn: In directory 'c1/lsc/help'
svn: Can't copy 'c1/lsc/help/.svn/tmp/text-base/C1LSC_RFadjust.txt.svn-base' to 'c1/lsc/help/.svn/tmp/C1LSC_RFadjust.txt.tmp.tmp': No such file or directory
noise in m/s = -------------------
10 * 802(V/(m/s))
FSR = (3897627 +/- 5 ) Hz
L = (38.45833 +/- 0.00005) m
g2x = 0.31197 +/- 0.00004
g2y = 0.32283 +/- 0.00004
R-ETM_x = (55.8957 +/- 0.0045) m
R-ETM_y = (56.7937 +/- 0.0038) m
FSR = ( 3879252 +/- 30 ) Hz
L = (38.6462 +/- 0.0003) m
g2x = 0.31188 +/- 0.00004
g2y = 0.32601 +/- 0.00004
R-ETM_x = (56.1620 +/- 0.0013) m
R-ETM_y = (57.3395 +/- 0.0011) m