Is there a reason they require device specific tests?
It's the "unknowns" not accounted for in the models that are the (potential) issue. If you have time read through this Ars Technica thread where a bunch of these questions are hashed out: http://arstechnica.com/civis/viewtopic.php?f=23&t=116333...
For those without the time, I'll highlight one post from page 5:
"Unintentional radiators.
The reason they ask you to power down ALL electronics, and not just turn off radios, is that the oscillators (the clock that runs all digital devices) on the circuit board can act as miniature radios, in and of themselves, via clock signals on circuit traces. They emit at the clock frequency, and in some cases many higher level harmonics of those frequencies.
As I've stated several times, it's actually pretty easy to mitigate against known frequencies and signalling techniques...it's the ones you don't know about that are the problem. EMI can be downright spooky.
As an example: I was once testing a medical device (for the Medical Device Directive) that was required to failsafe, since it would literally be touching a patient (it was a combination pulseoximeter and a few other things all rolled into one.
There were numerous tests for both emissions and immunity, and things were going along ducky until we noticed two separate failure modes that weren't considered failsafe. At the time, we were testing ESD immunity up to 20kV. It would pass one time, and not another. We thought maybe we had a bad unit, so we got a few more from the manufacturer. That entire week we kept trying to figure out how to make the failure repeatable, without luck. My coworker and I went in over the weekend, and could simply not make any of the units fail at all, with the exact same test.
That's what triggered our thought process...what else could be causing the issue during the work week but not on a weekend?
Other immunity tests! Turns out, a dozen or so meters away, a different device was undergoing a different test...that wasn't required for the MDD. It was a conducted immunity test (it may have been Electric Fast Transients, can't recall) but the actual test signal was leaking out of that lab, and into our lab, via AC lines in the building. Our chief engineer submitted a proposal to add whatever test it was to the MDD, but I don't know whatever became of that.
I've seen simple clocks inside electronic gear cause CPUs to go haywire...in effect a single system interfering with itself. I've also seen extremely low power, yet very high frequency harmonics invade and corrupt function of another device, several meters away."
Why then do they not have people remove the batteries from their watches? Can't they also be unintentional radiators?
what spa942 said.
From the linked thread:
"The low power in a digital watch is certainly a factor but the main thing that keeps them from interfering is very low operating frequency. In older, simpler watches this was 32 kHz! Only one octave above the audible range. In more modern multifunction watches with stopwatches that go to 0.01 seconds (as if anyone can push a button that precisely, but never mind that) they do have to be clocked faster, but they are still clocked as slowly as possible, to save power.... typically 1 MHz or less.
The RF from these things is all but undetectable unless you put a pickup directly against the watch. "
It's the "unknowns" not accounted for in the models that are the (potential) issue. If you have time read through this Ars Technica thread where a bunch of these questions are hashed out: http://arstechnica.com/civis/viewtopic.php?f=23&t=116333...
For those without the time, I'll highlight one post from page 5:
"Unintentional radiators.
The reason they ask you to power down ALL electronics, and not just turn off radios, is that the oscillators (the clock that runs all digital devices) on the circuit board can act as miniature radios, in and of themselves, via clock signals on circuit traces. They emit at the clock frequency, and in some cases many higher level harmonics of those frequencies.
As I've stated several times, it's actually pretty easy to mitigate against known frequencies and signalling techniques...it's the ones you don't know about that are the problem. EMI can be downright spooky.
As an example: I was once testing a medical device (for the Medical Device Directive) that was required to failsafe, since it would literally be touching a patient (it was a combination pulseoximeter and a few other things all rolled into one.
There were numerous tests for both emissions and immunity, and things were going along ducky until we noticed two separate failure modes that weren't considered failsafe. At the time, we were testing ESD immunity up to 20kV. It would pass one time, and not another. We thought maybe we had a bad unit, so we got a few more from the manufacturer. That entire week we kept trying to figure out how to make the failure repeatable, without luck. My coworker and I went in over the weekend, and could simply not make any of the units fail at all, with the exact same test.
That's what triggered our thought process...what else could be causing the issue during the work week but not on a weekend?
Other immunity tests! Turns out, a dozen or so meters away, a different device was undergoing a different test...that wasn't required for the MDD. It was a conducted immunity test (it may have been Electric Fast Transients, can't recall) but the actual test signal was leaking out of that lab, and into our lab, via AC lines in the building. Our chief engineer submitted a proposal to add whatever test it was to the MDD, but I don't know whatever became of that.
I've seen simple clocks inside electronic gear cause CPUs to go haywire...in effect a single system interfering with itself. I've also seen extremely low power, yet very high frequency harmonics invade and corrupt function of another device, several meters away."