I did some theoretical calculations using the datasheet value of second harmonic SFDR from MAX4107 and the transfer function I measured from Test IN ports of RFPDs (using 100 kOhm series resistance).

Calculation:

Attached notebook has all the calculations. In all places, "Omega" refers to the modulation frequency.

I first calculated power at 0, 1-Omega and 2-Omega from the resonance frequency in the light incident on the cavity using Bessel functions.

I used reflection function from the cavity (using transmittance of 5 ppm, cavity length of 1.45" and assuming lossless cavity):

Using this, I calculated reflected power at 0, 1-Omega and 2-Omega from the resonance frequency.

Then I used -40 dBc SFDR for second harmonic generation mentioned in MAX4107 datasheet when used with a gain of 10. I used following formula to calculate this:

Finally, I also calculated ratio of 1-Omega signal (actual beat between Sideband and Carrier) and the 1-Omega Distortion signal (generated) due to the non-linearity of MAX4107.

Inferences:

These are purely theoretical calculations but I can put in the second harmonic SFDR by measurement tomorrow.

The suppression of 2nd harmonics by the notch in the RFPD circuits is very bad. It only suppresses the second harmonic by 25-30 dB.

This is not enough as this 2-Omega signal would mix a lot with 1-Omega and would also go forward into the TTFSS box after downconversion.

The ratio of Signal at 1-Omega to the distortion at 1-Omega is about just 8 at 100 Hz. This doesn't reach 100 even till 1 kHz.

And below 10 Hz, the distortion dominates the 1-Omega signal from RF out.

This looks like a big limitation to our FSS loop. The notch filters in the RFPDs need to be doing much better job then they are doing.

Edited Wed Nov 20 14:43:07 2019: Corrected an error in code.