Just to check, you used the Aglient to do this so to the 50 Ω loading will affect the mesurment. From the picture it looks like the mixer output is loaded and the excitation (50 Ω assumed) is plugged into a high impedance input of the SR560. The post injection point readout is the 50 Ω port, but that will be reporting half the value that the marconi high impedance input sees. There are a couple of different factors of two dividing and mulitplying but I think maybe its a factor of four off between the excitation and actuation out signals. Loading the mixer with an extra 50 Ω halves the value of the loop gain compaired to when it is not connected there.
With the help of the PLL autolocker, I retook the OLG of the PLL with a frequency modulation level of 10 kHz and 3 kHz.
I did this because we have new beatnote spectra in the can at both levels of frequency modulation and we would like to calibrate and add to the noise budget. The new spectra are in the ctn_labdata/ git repo under ./cit_ctnlab/ctn_labdata/data/20171108_TransBeatnoteSpectrum/
I calculated this PLL OLG the same way I did so previously through the SR560 preamp. This result is plot 2.
I wasn't convinced this was right, so I did it again my calculating the PLL OLG from the mixer error signal through some OUT1/EXC = G/(1 + G) math. This result is plot 1.
The results matched up pretty well:
The 3 kHz FM Deviation PLL OLG had a UGF of ~2.3 kHz in both plots 1 and 2.
The 10 kHz FM Deviation PLL OLG had a UGF of ~5 kHz in plot 1, and ~4.3 kHz in plot 2.
I was doubtful because this is so much less than our previous PLL OLG UGF, which was ~30 kHz at 10 kHz FM Devn. The preamp gain is down to 100 from 200, but not clear otherwise why the PLL UGF is down 6 times as opposed to just 2 times...
With this in hand we should be able to easily calibrate the low-actuator noise spectra we took earlier today.