In a discussion with Craig sometime back, it was brought up what happens when I lower the gains of the FSS loops. So today I did a test which lowers the Common and Fast Gain values on the FSS boxes by 3 dB in each step and sees what happens to the beatnote.

Measurement

• The measurement was done solely by bnvsFSSgains.py script present in the Data folder.
• Before each measurement, the gain values are stepped and the script waits for 5 s before measurement is attempted.
• Measurement is done through the usual mokuPhaseMeterTimeSeries.py script which is used for all beatnote measurements of the lab. Recently I was able to make it more robust with some discussion with liquid instruments application engineer.
• All default settings were used except for duration which was kept to 10s to keep reasonable time series data file size.
• SN101 detector's RF out coupled through a 10 dB directional coupler was used to make the measurement (as usual).
• I ensured by watching the error signal on an oscilloscope that none of the FSS loops went into oscillation during the measurement. Its is hard to say for lower gain settings though if that happened or not.

Inference

• The beatnote spectrum noise does not start going up until after the gains have been reduced significantly by around 12 dB at each stage.
• Also, weirdly the beatnote spectrum was actually lower for 2 particular settings of gain which were midway. There the beatnote spectrum is only 2 times as much as estimated!
• I'll repeat this measurement again to make sure this optimal gain setting region was not a coincidence due to some other parameter out of my control.
• But since we do not see a clear scaling of beatnote noise with lowering of FSS gains, I think it is safe to infer that we are actually not limited by residual NPRO noise as has been the thesis for quite some time.

Data