[rana,gautam]

Summary:

Last night, Rana fact-checked my story about the coil driver noise measurement. Conclusions:

1. There is definitely pickup of strong lines (see Attachment #1. These are hypothesized to come from switching power supplies). Moreover, they breathe. Checkout Rana's twitter page for the video.
2. The lines are almost (but not quite) at integer multiples of 19.5 kHz. The cause of this anharmonicity is to be puzzled out.
3. When the coil driver board is located ~1m away from the SR785 and the bench supply powering it, even though the lines are visible in the spectrum, the low frequency shape does not show the weird broad features I reported here. The measured noise floor level is ~5nV/rtHz, which is consistent with LISO noise + SR560 input noise (see Attachment #2). However, there is still some excess noise at 100 Hz above what the LISO model leads us to expect. 
4. The location of the coil driver board and SR560 relative to the SR785 and the bench power supply I used to power the coil driver board can increase the line heights by ~x50. 
5. The above changes the shape of the low frequency part of the spectrum as well, and it looks more like what is reported in elog13870. The hypothesis is that the high frequency lines are downconverted in the SR560.

Note: All measurements were made with the fast input of the coil driver board terminated with 50ohms and bias input shorted to ground with a crocodile clip cable.

Next steps:

The first goal is to figure out where this pickup is happening, and if it is actually going to the optic. To this end, I will put a passive 100 kHz filter between the coil driver output and the preamp (Busby Box instead of SR560). By getting a clean measurement of the noise floor with the coil driver board in the Eurocrate (with the bias input driven), we can confirm that the optic isn't being buffeted by the excess coil driver noise. If we confirm that the excess noise is not a measurement artefact, we need to think about were the pickup is actually happening and come up with mitigation strategies.

RXA: good section EMI/RFI in Op Amp Applications handbook (2006) by Walt Jung. Also this page: http://www.electronicdesign.com/analog/what-was-noise