🚀 go-pugleaf

In article <1992Dec14.064445.9591@ips.oz.au>, bruce@corona.syd.ips.oz.au
           (Bruce Paterson) wrote:

> I need a VERY sharp notch filter (or actually 3, if sharp enough) to reject
> FM broadcast stations from an antenna signal. The requirements are tough:
> a. VERY sharp - I need to measure spectra of broadband noiselike signals and
> the present LC filters make a far too visible hole.
> b. No other holes (or even ripples) in the pass band - 57 to 180MHz.
> c. Low loss.

I live in a rural area where my distant, favorite FM station is wiped out
by a nearby station. The solution I found most promising was written-up
in the August 1991 issue of RF Design, "VHF Active Bandpass Filters", by
Eugene Mayle, pp. 58-65. I haven't had time to try it out yet.

Mayle discusses the use of a grounded-gate JFET as a "Q multiplier". (This
is almost anachronistic since most of us hams thought Q multipliers died
a peaceful death 30 years ago). At any rate, the performance of circuits he
presents in his article are quite remarkable, with one stage typically
capable of a Q of 1,000; this corresponds to a bandpass filter bandwidth
of 0.1%.
Your requirement should be easily met, since this corresponds to a 100 kHz
notch at 100 MHz.

The notch is obtained by paralleling one of his bandpass filters with a
broadband phase-shifting network. At the center frequency, the two
networks' phases cancel. He claims 50 dB notch depth is routinely
achieved. The 200 MHz notch filter he demonstrates shows a <2 dB insertion
loss in the passband.

My concern is that his filters are operating on the fine edge of
oscillation, and might need to be compensated for changes in temperature.
I haven't tried them, so I don't know.  If they do work, though, they would
be a substantial improvement over the two or three large canisters which
would be necessary for a resonant cavity solution.

In regard to the "open-stub" notch that someone else suggested, I suppose
it is something inexpensively verified, although my calculations told me
that the notch width will be too broad for the application. It will remove
a TV channel but will obliterate a large portion of the FM broadcast band

Marshall Jose
josemj1@aplmail.jhuapl.edu