Wes wrote:

Specing and measuring feedthru filters was one of the most pointless
things we ever did. The specs all measure attenuation in a 50 ohm
system and the filters are never used in a 50 ohm system. In fact,
nobody ever knows what the source and load Z are at the test
frequencies so they don't know what they need or what they get.

Wes makes a valid point about the source and load impedances being
unknown in practical applications. The filter performance is going to be
different from the performance measured in a 50-ohm system.

But that doesn't make a 50-ohm measurement completely "pointless". In
order to select a filter, you have to make *some* attempt to
characterize the performance of the available options, under some kind
of standard conditions.

If the source and load impedances are totally unknown, a 50-ohm
environment is actually not a bad choice for a standard test, since it's
neither extremely high nor extremely low. If you have a better handle on
the practical source and load Z, then it makes sense to define a
different standard test environment - for example, the standard Line
Impedance Simulating Network (LISN) used for AC line/mains filters has a
much lower source Z.

The common-sense solution is to have standard tests, but understand
their limitations. Unfortunately - as Wes knows, all too well - military
spec writers aren't noted for either common sense or understanding.


--
73 from Ian G3SEK 'In Practice' columnist for RadCom (RSGB)
http://www.ifwtech.co.uk/g3sek