Method

I've wrote this script, nonlinTF.py which controls a Marconi 2023A and SR785 together. Marconi is used to providing a carrier frequency which is mixed with the Source Out signal from SR785 before feeding into the TEST2 input port on D040105 of TTFSS boxes. Then OUT1 port on D040105 of TTFSS box is used to read back at channel two of SR785 (channel one being fed with a copy of the Source Out signal). So SR785 is effectively measuring any downconversion in the loop (due to some nonlinearity) from micing of CF-IF, CF and CF+IF probe signals injected into the loop. The effectively closed-loop transfer function between TEST2 and OUT1 should be G/(1+G), so this injected signal should not suffer any suppression, nor should it affect the locks. The locks were maintained without any problems during the whole measurement. The CF frequency was stepped by 100 kHz from 100kHz to 10 MHz and then by 1 Mhz upto 100 MHz.

Mixer ZX05-1LHW (level +13 dBm) was used for the mixing and IF peak voltage was set to 30 mV. The configuration of the measurement for the transfer function is present in the configuration file in the folder.

Results?

• I had to upload plots in png as they were aggregated result of a lot of data, pdf would have been very heavy.
• North Side looks good with only minor bumps in 60 Hz harmonics. I'm not even sure if this just came because of SR785.
• The South side however looks very dramatic. A lot of stuff happening when carrier frequency was near 50 MHz.
• But I'm not sure if this measurement even makes any sense. South side is better in performance then North side, this wasn't expected.
• Also, the measurements are long-time measurements, so a lot of things could have changed between North and South (they were taken on different days too).
• I'll take more verifying measurements near the suspected frequencies tomorrow.
• I'm also thinking of taking spectrums instead of swept sine transfer functions next time.

Data