TF Measurement Method

• I switched on Test1 switch on RF Board to cut off RFPD input to FSS.
• I switched on Test2 switch to connect the excitation port at the summing point.
• I switched on Ramp Engage for FSS to ensure paths to PZT and EOM are connected.
• Then, I disconnected and shorted EOM input when taking the measurement for PZT and vice versa.
• COM Gain was 6dB and FAST gain was 10 dB.
• TF was measured from Test2 EXC Port to the output of PDs. This was divided by 10s averaged DC value of PD output to get TF to RIN.
• Measurements were taken in turns for low and high frequencies with IFBW 10Hz and 1 kHz.
• Again, PDs are  Thorlabs PDA10CS placed at (110,44) at dumped end of a PBS before PMC and at (64,28) at dumped end of Input PBS of Faraday Isolator after PMC.

Calculations:

• The noise spectrum in FSS loop was measured at OUT2 port on the common board.
• This was divided by 50/(453+50) voltage division caused by 50 Ohm input impedance of AG4395A. This gives spectrum at the output of the U3 adder stage in the common board.
• The TF measured above are divided by 392/1200 which is the gain from Test2 EXC to output of U3. So the TFs are no referenced from the output of the U3.
• The measured noise spectrum is multiplied by the TF from that point to RIN at After and Before PMC due to PZT or EOM.
• These are summed quadratically to get total estimated RIN ASD (before and after PMC) due to the noise from FSS.
• Finally, I also plotted the RIN ASD measured in CTN:2502 before and after PMC when FSS was ON.

Inferences:

• The measured RIN, both after and before PMC are much higher than the estimated noise due to FSS.
• In particular, the bump around 10kHz in measured RIN before and after PMC is not present in transfer functions from FSS to RIN or the noise spectrum itself in FSS.
• The notorious 433 kHz peak is present everywhere, but there's order's of the magnitude of difference between FSS contribution and seen RIN. So it is mostly the other way round, the RIN causing these peaks in FSS.
• Either that, or there is something wrong with the way I took measurements or did the above calculations.
• Also, I'm aware that the RIN is actually fed back into the FSS through PD and it is not an open-loop. But I did this simpler analysis first. Maybe that's what is wrong.

Data

Edited on Mon Dec 23 15:21:50 2019 .