The first measurement here seems like a smoking gun for limiting noise. A mixer down converted noise of 1.3 µV/rtHz (directly out of the mixer) with a S*A (VCO + SR560) gain of 200 kHz/V would be 0.26 Hz/rtHz. That would be consistent with the noise floor of the more recent BN PSDs. The PLL open loop gain will change this estimate, but the 200 kHz/V gives a lower bound. This initial measurement of NF1811 noise was a factor of ten more than spec for this detector. It should be ~100 nV/rtHz (equivalent to input ref noise of 2.5 pW/rtHz at peak responsively). We should have traced back to the source of this earlier, but now that we know its a potential problem we should double check if we're having issues with BN noise floor not matching up with the noise budget.
Its weird that the second measurement of the same NF1811 seems to show it performing closer to its spec value. The setup for measurment was exactly the same. I should note that when we were making these measurements we switched the BN cable from the NF1811 to the resonant 27.34 MHz detector and back again. I also wiggled the power cable at the time. I've seen in the past that when there is a power connector issue, the NF1811 can produce something that looks like signal but with RF oscillations if there is a problem with one of the power rails. We probably should have checked at the time with an oscilloscope to see if there were any issues with the RF signal coming out of the NF1811. It could be that there was some kind of oscillation or other bad thing going on and the act of unloading and reloading the output with 50 Ω and wiggling the power connector some how got it back to proper operation.
For now we should should recheck the NF1811 dark noise (after mixing down) before making another BN measurement and not touch connectors on the detector side in case there is a flaky connection.
You definitely saw the higher noise state, so I wouldn't dismiss it yet. I guess we won't know if the reduced noise second measurement will bring down out noise floor until we see a BN measurement.
We suspected if the wideband RF detector New Focus 1811-FC could be the source of noise floor through its dark noise. So I took dark noise spectrum measurements today of this photodetector.
Edit Tue Jan 22 20:06:00 2019