Richard, I have read and heard this line of reasoning for the limit lines and obviously the limits were somehow set to protect the broadcast and emergency broadcast services among other things. But I have yet to find any derived data or studies on the subject. I do not believe that a group of people sat down and decided that 30 was a nice number but I also believe that a group of reasonable people would have built a margin of protection into the limit line thus perhaps negating the necessity/importance of measurement uncertainty. Alas, it seems that we will never know. And if "subjectivity" enters into this thing, then I am very very sorry that I asked the question.
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(Embedded
image moved "WOODS, RICHARD" <[email protected]>
to file: 10/27/98 02:10 PM
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Please respond to "WOODS, RICHARD" <[email protected]>
To: [email protected]
cc: (bcc: Robert E. Heller/US-Corporate/3M/US)
Subject: RE: EMC Limits
TVs and radios have a specified signal rejection. That is, they must
continue to operate with a specified minimum signal to noise ratio and a
specified minimum signal. That noise signal may be a spurious emission from
another product. That derived noise signal is projected to a certain
protection distance such as 10 m. Now you have an emissions limit which
will
vary with frequency. Measurement uncertainty does not enter into the
equation. But, of course, the initial basis for deriving the maximum noise
signal is subjective.
----------
From: [email protected] [SMTP:[email protected]]
Sent: Tuesday, October 27, 1998 12:32 PM
To: [email protected]
Subject: RE: EMC Limits
Which brings to mind one other question....................
I have yet to find someone who can tell me exactly how the limit
lines were
determined. In other words why does the 10 meter limit line start at
30
dBuV/m and not 29 or 31. but regardless of how they were determined,
there
has to be uncertainty of measurement in that
determination.....................what is the measurement
uncertainty of
the limit line itself? Are we trying to hit a moving target?
I do agree with Chris D. on the fact that we should sit back every
once in
awhile and look at the spirit of what is trying to be accomplished.
We are
too often blinded by minutia (you know that old "trees and forest"
thing.
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