After the Notch improvement, I took 20 dB coupled outputs of the RF out ports of the FSS RFPDs, SN009 (North) and SN010 (South), when cavities were locked or unlocked.

Measurement:

• Configuration files for all measurements are present in the Data directory.
•  ZFDC-20-5-S+ 19.5 dB directional coupler was used.
• 19.5 dB was added to all measurements to correct back from coupled value.
• Note that in case of measurement with unlocked cavities, attenuation of 10 dB was used for North side as it was overloading Ag4395A. hence different noise floors.

Inference:

• No significant oscillations above 100 MHz, in particular around 300 MHz or 350 MHz.
• When unlocked, the second harmonic is 50.7 dB lower than the first harmonic for North side and 53.7 dB lower for South side.
• However, when locked, the second harmonic is only 18.1 dB lower than the first harmonic for North side and 6.9 dB lower for South side.
• Last measurement was taken in CTN:2470 where the difference between first and second harmonics during lock was ~12 dB for North and ~17 dB for South.
• But I don't have very good confidence with the last measurement as it was foolishly taken in Noise mode and had different IF Bandwidth than the present measurement.
• With -50 dBc SFDR (Spurious Free Dynamic Range) of MAX4107, I expect that the distortion signal generated at the 1-Omega frequency would be -90.95 dBm for North and -96.5 dBm for South.
• This suggests that the non-linear distortion component in the error signal would be about 59 dB lower than the actual signal for North side and 53 dB lower than the actual signal for South side.

Data

Relevant elog post:

CTN:2470 FSS Diagnostics - RFPD RF Ouput under inspection