"SB"-Series AM Filter...

[Steve Harrison]

At 07:14 PM 1999-12-30 -0500, Gerald C. Lemay wrote:
>I've been unsuccessfull as well. The one thing remaining for me to try is a
>filter converter such as described in an old HR radio article for the R-4C.
>You get an AM filter for some other IF frequency and convert the exixting IF
>to it such that the filter does its thing. Then you up convert again back to
>the original IF. It uses one xtal oscillator. Biggest problem would be
>leakage. Fortunately, I found the SB-310 AM filter last summer and it's
>awsome.

If you do the above, you will have to make certain that your isolation
between input and output of the outboard module and its installation
exceeds the desired filter specs. Otherwise, you will get leakage around
the filter, negating the high isolation of many filters.

W7ZOI wrote several articles some years back on the design, testing and
construction of homebrew crystal filters. The article which I remember in
particular was published in the ARRL's Solid State Design Handbook, long
out of print (probably to avoid discouraging any future would-be hams from
thinking that there is actually any "design" involved in homebrewing
Amateur Radios).

To obtain any reasonable amount of skirt selectivity, numerous poles are
required in a filter; the simple two-crystal filter that the original
correspondent mentioned will perform little better than a high-Q LC tank.
W7ZOI mentioned buying dozens of crystals then testing and categorizing
each according to several parameters. Chosen crystals were then assembled
into a prototype filter which was "tweaked" for optimum performance.

The ARRL handbooks of yore have also had good information on the design of
crystal filters.

I believe Ham Radio also published several articles on their design and
construction.

As for the old Fox-Tango company: it now exists as IRCI and their web site,
for Heath filters, is at http://www.qth.com/inrad/heath.htm

Unfortunately, the catalog doesn't show any wideband AM-type filters; and
the stock models are already $100, more than the value of almost any SBxxx
radio. IRCI has, in the past, custom-built filters, so perhaps that might
be an option.

73, Steve K0XP

Sponsored by the City of Tempe 

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