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Reports until 02:01, Monday 25 July 2016
H1 ISC (ISC)
carl.blair@LIGO.ORG - posted 02:01, Monday 25 July 2016 - last comment - 15:44, Friday 19 August 2016(28611)
PI dark region

I have been taking spectra from the SR785 (WP6005) whenever I get a chance over the last week to see if there is any evidence of three mode interactions in the 60kHz to 70kHz region that we will not be sensitive to with the aliased OMC DCPD HF channels that we normally analyze.
There is a consistent peak at 62935Hz, this peak is present with no optical power in the arm cavities.
There are several other more transient peaks, one of the times several had large amplitudes is shown in the first figure.
The largest peak is at 63776Hz  The maximum amplitude seen was 5E-6  This is about the same as the 18040Hz mode when it is 2 orders of magnitude above thermal noise and 2 orders of magnitude under unlocking the cavity.
The second largest is at 70160Hz and the third largest at 62336.

There is no evidence of peaks in DARM at this time at the expected aliased frequencies 1760Hz,  3200Hz or 4624Hz and the peaks that appear in the HF channels that do not appear in the normal DCPD channels do not coincide in frequency, see the second image.

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terra.hardwick@LIGO.ORG - 21:50, Monday 25 July 2016 (28631)

We disconnected this SR785 around 11am local time today. This closed work permit #6005.

jeffrey.kissel@LIGO.ORG - 15:44, Friday 19 August 2016 (29204)
J. Kissel, T. Hardwick

We've taken the liberty of rifling through Carl's home directory in hopes to find the raw data from this entry to re-plot for clarity. We found it!

The newly attached plot now highlights the PI modes that Carl mentions in his aLOG, and also shows the anticipated ADC noise. Thus, any modes below 6.3e-6 [V/rtHz] should not be resolved by the ADC, and therefore will show up aliased into digitized signals in the detection band. 

Terra notes that the mode Carl mentions at 70160 Hz is the largest of several peaks at 69.84, 69.95, and 70.03 kHz (not highlighted), which is likely a mode cluster.

Other details:
The raw data lives here (determined by Terra knowing that Carl keep his GPIB data in his home folder, then lining up the data on the figure with the filename): 
/ligo/home/carl.blair/gpib/netgpibdata/dataSPSR785_24-07-2016_212424.txt

This data (as described in the referenced work permit 6005), is the raw analog output of the TMSY's red QPD's whitening chassis.

This data also happens to cover the frequency region surrounding the 65536 [Hz] native sampling frequency of the General Standards ADC, and the corresponding notch in all Anit-Aliasing (AA) chassis. One can see, delightfully, that there is very little noise or lines in this frequency band on this channel that might also otherwise be aliased down to low frequency. 

We should perform a similar spectral analysis of the OMC DCPD whitening chassis output voltage to check if their AA chassis is also sufficiently notched so as to not contribute noise in to the DARM sensitivity.
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