I have been following this post for quite some time now and, imho, noticed that ground loops are mainly the root cause for all evil and tribulations.
I have had a very twisted experience with noise in the recent past; which to my conclusion, was dependent on the "time window" available in a day.
The number of sensors (Pressure, Strain, Temperature, Position, etc) used was in 4 digits and a huge amount of time was spent to make sure "check, recheck & confirm" everything was right electrically.
We observed that noise occurred in between a specific one and half hour time window. ( Of course, after much debate the temperature factor was ruled out with fathomable Trial and Error results).
The test span was 3 months and the way it turned out was during the first month it was unacceptable to run a test in that time window and much to our chagrin, the pattern flipped on us, that is, by the time we were in the 3rd month of testing, there was no noise in that one and half hour time window and considerable/unacceptable noise every other time.
The test was conducted in an open space, and yes, continuity, ground loops, signal conditioning, floating power supplies, interferences from neighboring power supplies, floor plans and for that matter, even Dish setups were diligently looked at and ruled out for fathomable reasons.
So, I was wondering if you or any one else here had any similar experiences and what do you think might be a possible cause for the noise?
If left unexplained, sounds like a great bar story.
Remember, The devil lies in the details...
Sounds like something is occurring in that time window. I have a story for you about something like that. Got a call from a customer saying that measurements taken during the day were different than at night. The stand was in an unconditioned environment. Turns out we were seeing the thermal expansion of the stand. The position measurements were smaller during the cooler night and larger during the hotter day.
I would try to think of what is changing during that time window. Hopefully that line of thinking will lead to the solution.
We recently had a problem with noise. Very sparadic, no pattern. Drove me nuts. Turned out we had a test running that was being controled by engineers in Australia and South Africa. Test included a VFD that had its input filter disabled. I also saw a problem years ago that involved a cleaning lady plugging her vac into the wrong outlet every evening while she cleaned the office. It would reset all the computers and kill the overnight simulations the digital designers were running.
I would recomend that you get hold of the electrical schematics for your facility. Figure out how your equipment is being powered and then look at where any noise sources are being powered within the building. You can also use a good power analyzer to monitor your incoming power to look for issues. Most modern buildings are filled with potential noise sources. SCR controls on electic heating elements, VFD's used all over the place including the HVAC system for the building. Make sure that any large VFD's have a Wye-Delta transformer installed on the input to help control conducted emissions.
Can you share the schematic of the complete circuit after the problem was solved?