Non-Reflective, High Power HF harmonic filter wanted - RF Cafe Forums
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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!
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.
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