R&D (FilterCavity)
MichaelPage - 11:46, Monday 20 February 2023 (3173)
New spectrum analyser in filter cavity cleanroom

We recently purchased a new Stanford Research SR785 Dual Channel Signal Analyser to be used in the FC cleanroom.

It is a bit complicated. One of the main features is that the measurement is independent of the display. This allows us, for example, to have a really precise measurement but with a really rough display, among other things. It can also save a lot more data into its internal buffer in one go, so we can then transfer to disk without having to redo a measurement.

By default it had a really irritating alert message that sounds like a phone ringing, so I turned it off in "Preferences -> Alarm noise -> Quiet". There is another menu option which says "Alarms -> off" but that controls the display of error messages. Very strange UI in my opinion.

It can write data to USB, however, due to weird stuff, it won't recognise storage devices over 8GB. This seems to be a common issue with making old tech forward compatible with FAT32 USBs. The grey USB drive in the FC cleanroom works fine and I already formatted it.

The spectrum analyser formats the USB to pretend that it is several hundred 1.44MB floppy disks. But unfortunately Windows only recognises the first "disk" in the sequence (labelled 000 in red digits on the front panel of the device next to the USB port), so it still runs out of space. Also at one point the spectrum analyser would refuse to re-format the USB, so I had to Full Format on Windows (slow...) and then re-format on the analyser. Shalika says that apparently you can program it to send data over wifi to the computer, which would be better. The manual talks about cable connections only (RS232 and GPIB) but maybe we can find a wifi attachment.

One thing that is a bit annoying, initially, is that there is no internal utility to save as an easily workable format (i.e. simple ASCII .txt). The device wants to save in some native format (.78C, .78D) with all of the settings in a header to save and recall at will. But this makes data analysis more annoying of course. Apparently the disk supplied with the device contains a command line file conversion utility, which in the manual is interchangeably called SRTRANS and SRT785. The instructions say to use SRT785 /Oasc [filename.78C/D] i.e. SRT785 /Option (ascii format) [native file save in device using whatever -> Disk]. I downloaded SRT785.EXE but it only works on 32 bit operating systems. The I went back and tried to find SRTRANS.EXE, which for some reason requires a bit of specific google searching. Anyway, that one worked and I could convert to .txt. format. It works in MATLAB and python, though for python np.fromfile has strange behaviour for data using scientific notation (i.e. in the format 1.00000e+00), so use np.loadtxt instead. The file conversion utility has some nice options so you can delete all headers, write headers to a separate file, manage data sets taken from whole measurement groups (i.e. more than just two columns), write with space/comma/tab delimiter, save to .MAT etc.

To do: Figure out a good way to remotely import data from the analyser using programmable commands, and then batch convert into some convenient data format. Personally I want to separate headers and data, and the delimiter doesn't matter to me. MATLAB also seems to not mind this operation. But maybe others also have preferences for dealing with data.