Today, I finally used the trigger generator to trigger near the start of the maximum of PDH error signal to get an FFT when the PDH error signal is near maximum.

Setup:

• All connections are as in PSL:2326 except for the following changes.
• The Mixer Channel output is fed into the TTL trigger generator box and the trigger is read in CH4 while Mixer Output is read in CH2 of the oscilloscope.
• OUT1 of FSS box is read in CH3. Please refer PSL:2326  for detailed definitions of these connection points.
• I took spectrum from HP4395A at 30 kHz Res BW and 197 points spanning 1.2 MHz around 37 MHz. This was done to keep FFT time low to 17.37 ms.
• I set the trigger level such that FFT is done near PDH error peaks as shown in the first two time series plots taken from the oscilloscope.

Spectrum result:

• The 37 MHz frequency was found to be at -15.61 dBm level. This corresponds to 104.87 mVpp at 50 ohm termination. This number matches well with the time series measured value from OUT1 which is supposed to be 0.36 times RFout (see PSL:2326).
• I see two sidebands sort of at 36.72 MHz and 37.28 MHz which are created with 280 kHz mixing into 37 MHz.
• My guess right now is that this 280 kHz is some mechanical oscillation in PMC which is getting mixed with 37 MHz at the EOM.
• This 280 kHz modulation will remain in the signal even after mixing down and low passing of our FSS and was seen in time series data in PSL:2326 .
• There is a tunable notch present in FSS box at D040105 through capacitor C46. I think it is here to be tuned to suppress such peaks but I'm not sure.
• However, we still haven't found any damaging cause to RFout. There is no significant distortion and its level being spit out of the RFPD is actually low.
• I measured input light on the RFPD at 2.08 mW. I measured modulation index using optical powers of the carrier (1.4V at transmission PD) and sidebands (58.3 mV at transmission PD). The transmission PD has a dark output of 27.2 mV. Using these numbers and taking ratio of Bessel's function, I found beta to be 0.298, close to expected 0.3.
• These numbers say that RFout after divided down due to 50Ohm voltage divider should be 1.05 Vpp when we are measuring just 125 mVpp (OUT1/0.36) :(.
• All things point to RFPD but nothing seems wrong with it. Should I go to 40m again and free space TF again?

DATA_DIRECTORY        ANALYSIS_CODE

Reply to Rana:

It is not a beat frequency, it is rather some other oscillation mode (probably mechanical oscillation through PMC) at 280 kHz mixing at the EOM as I mentioned above. Modulation frequecy is 37 MHz for this path. 14.75 MHz is the modulation frequency for PDH of locking South PMC.