Method:

• I injected excitation at FP1Test and readout at Mixer Out (FP3Test).
• All TF were taken in three stages encompassing the whole frequency range. The same configuration files were used.
• First, I took TF measurement when the loop was closed and the variable gain is 6 dB.
• Then I opened the loop by removing mixed down PD signal from FP2Test and shorting it. This gave direct transfer function from FP1Test to Mixer Out as in second plot.
• Using the two measurements, I calculated the open-loop transfer function for the NPMC Servo Loop.

Inference:

• My hope was to see some activity or unity gain around 433 kHz where we see the spike of noise.
• But the unity gain is much before around 6 kHz with phase margin of about 90 degrees.
• There is a small dip around 433 kHz but again it seems like a reaction to the problem rather than the source of the problem.
• The measurement, however, is pretty trashy. I could have used SR785 for the lower frequency region to make it better.
• I think we are back to square one. Any comments to infer more or suggest further tests are welcome.

Data and Analysis