I read in !emc-pstc that Robert Wilson <[email protected]> wrote
(in <3FF57405336C9B4C976A1819F860A2560F6929@xng_tirsys.TIRSYS.COM>)
about 'Safety Margins based on NSA premises', on Thu, 28 Feb 2002:
>A 10 dB margin? That's easy for the guy doing the test to say! Heck; why
>not 20dB?!
I think there's another way to look at this. 10 dB below the limit has
been a *target* in the context of radiated emissions for a very long
time; from before widespread control of emissions. 20 dB is too far off
to be even a target.
>For the designer who is sweating bullets trying to keep
>within cost and size requirements, however, this may be quite another
>matter.
Much of *radiated emission* control is about technique rather than cost-
and-size. Unfortunately, there are few circumstances where a synthetic
approach to emission control is available, so a solution has to be
sought analytically ('cut and try', if you like), and this takes time.
These days, time is costly.
>
>Considering the original specifications are somewhat arbitrarily derived
>in the first place, it seems to me a more pragmatic approach is
>warranted that meets the spirit of the regulations, rather than just
>overdesigning to meet ill-defined "worse case" scenarios that may or may
>not actually exist.
There is a need for balance. A product that appears to be 3 dB under the
limit *might* unfortunately be placed next to something sensitive that
gives rise to a complaint of interference. If subsequent tests show that
the emission is in fact 5 dB above the limit, this may result in
anything from temporary embarrassment of the manufacturer to a financial
disaster. It dends on which country is involved and the interpersonal
attitudes of the manufacturer's staff and that of the regulatory
authority: the latter is a particularly fraught and unpredictable
factor.
--
Regards, John Woodgate, OOO - Own Opinions Only. http://www.jmwa.demon.co.uk
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