[time-nuts] HP Cesium Standards in the International Atomic Time Scale, the legend of Felix Lazarus, and the "top cover effect".

[ed breya]

Yup, I've seen plenty of "top cover effects," and other strange things. 
That sort of thing is evident even when you don't have to reach E-12 
resolution levels - like a million times less. The instrument cabinet 
and hardware are indeed a part of the electronic system and circuits. So 
are what it's hooked up to, and its various characteristics, and so on.

Coincidentally, just the other day I was working on reducing residual 
line noise in my HP8568 SA and its homemade tracking generator project. 
This involved line frequency and its harmonic spurs just above the noise 
floor, around -95 dBm on screen. Minor changes to grounding, hardware 
mounting, position, signal paths, etc, can make large differences in 
result, due to effects on ground current distribution. Sometimes bad 
things even cancel out, but only when just right, under certain conditions.

And that's just an electrical aspect - there are thermal and mechanical 
stress and other effects in play too, as you go ever higher in resolution.

Ed