A few weeks ago, on Jul 24, Rana and I measured the phase noise of the FSS frequency box (aka the 'Kalmus Box'). See elog entry 3286.
That time, for some reason, we measured a phase noise higher than we expected; higher than that of the Marconi.
I repeated the measurement today using the SR785 spectrum analyzer. Here is the result:
(The measurement of July 24 on the plot was not corrected for the loop gain. The UGF was at about 30 Hz)
To make sure that my measurement procedure was correct, I also measured the combined phase noise of two Marconis. I then confirmed the consistency of that with what already measured by other people in the past (i.e. Rana elog entry 823 in the ATF elog).
This time the noise seemed reasonable; closer to the Marconi's phase noise, as we would expect. I don't know why it was so bad on July 24.
The shoulder in the Marconi-to-Marconi measurement between 80Hz and 800Hz is probably due to the phase noise of the other Marconi, the one used as LO.
I'm going to repeat the measurement connecting the setup to the DAQ, and locking the Marconi to the Rubidium standard.
Ultimately, the goal is to measure the phase noise of the new Sideband Frequency Generation Box of the 40m Upgrade.