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Non-Reflective, High Power HF harmonic filter wanted - RF Cafe Forums
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MOSFET Blues
Post subject: Non-Reflective, High Power HF harmonic
filter wanted
Unread postPosted: Sun Feb 13, 2005 10:34 pm
Using MRF154 x2 in a 3-50Mhz, 1KW HF amp. Currently using 7 pole
Butterworth LC filters. The reflections, harmonics, from this type of
filter slowly smokes the highpower FETs (MRF154). Any ideas on a filter
that would block these reflected harmonics from the FETs?
Thanks
for any ideas!
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Post subject: Filter
Unread postPosted: Wed Feb 16, 2005 8:46 pm
If I understand
you correctly, what you need is what's variously called a "branching"
filter, a "complementary" filter, or a "dissipative filter". The power
that doesn't go out the output port has to go somewhere. Right now,
it's reflected back and absorbed by your output stage transistors. The
only other place it could go is into a resistive load.
There's
a discussion of this kind of filter in the book by Anatol Zverev, "Handbook
of Filter Synthesis". It's kind of pricy, but a nearby university library
may have it. What you need is only a few pages.
Good Luck!
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Post subject: mrf154 filters
Unread
postPosted: Wed Mar 16, 2005 9:01 pm
Hi--I am using 7 section,
eliptical filters with my mrf154's with no problems. The steeper roll
off of the eliptical allows you to move the corner frequency further
from the operating frequency and thereby allows you to look into a better
impedance at the operating frequency. My 2nd harmonic was down about
55 db and the 3rd was down 15 db with a matched pair of mrf154's without
the filters. This allows you to move the corner out even further.
I hope this helps
Posted 11/12/2012
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