[CQ-Contest] W3LPL RX filter - last update

Scott Ellington sdelling at facstaff.wisc.edu
Tue Oct 28 10:12:55 EST 1997

There's a simpler tuning method, well-founded in filter theory:

Connect filter between a signal generator and a receiver.

Shunt each LC but one with a 100 Ohm resistor.

Tune the un-shunted LC for maximum response at mid-band.

Repeat for remaining LC's

Remove resistors.


It sounds counter-intuitive, but it works.  The filter is designed
so that each inductor resonates at mid-band with the sum of all
the capacitors connected to it, including the coupling capacitors.
(The W3LPL filter is an experimental design, but I think the
procedure should still work.  The values aren't far from the
theoretical ones.)

For those old enough to remember, this was the Drake R4C preselector 
alignment procedure.  If you simply tuned each LC for maximum without
the resistors, the result was a very narrow filter.  But if you 
did it right, the result was a nice flat-topped bandpass response.


Scott  K9MA
sdelling at facstaff.wisc.edu

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