>>BUT -- the same cautions about impedance matching being necessary
>>still apply, and that's the fly in the ointment. Not only must you
>>be matched at the operating frequency for the filter specs to apply,
>>you must also be matched at the harmonics! The chances of that
>>occuring are slim to none unless you happen to be working into an
>>antenna that is resonant at the harmonics. We DO use some of those,
>>but it certainly isn't all that we use, and it would only apply to
>>single band filters.
>>
>>Jim Brown K9YC
I missed the start of this - not sure why the messages did not appear,
unless someone has changed the title.
It is possible to design filter that present a constant impedance over a
wide frequency range. See for example
http://www.minicircuits.com/appnote/filters8-2.pdf
They are often used on the IF port of mixers, where it necessary to have
constant impedance. The downsides are that they are pretty poor
frequency response compared to filters designed without the requirement
on constant impedance.
I've never tried designing one, and know little about them, but I assume
that since they don't reflect out of band signals and don't pass them
either, then there must be resistive elements which absorb the unwanted
power. There can not just be L's and C's, but it will need R's too.
Without knowing the context in which the thread started, I can't make
any further comment, but just wanted to point out you can make filters
that present 50 Ohms over a wide range of frequencies.
--
David Kirkby,
G8WRB
Please check out http://www.g8wrb.org/
of if you live in Essex http://www.southminster-branch-line.org.uk/
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