[Boatanchors] NEED HELP ON IF FILTERS

Horace W. Hall [email protected]
Sat, 22 Jun 2002 01:56:22 -0400


>
Since it is single conversion, adjacent channel rejection leaves
something to be desired. Seems the easest way is a 455kc 8 pole filter @
6kc (and maybe another @ 4kc) for AM mode
>

Its not the IF itself that is the problem. The problem is a common one fo=
r
that type design (single conversion, low IF freq). What it lacks is
rejection of the unwanted image frequency presented to the converter stag=
e.
This is because the image freq is only separated from the desired frequen=
cy
by twice the amount of the IF freq. =


The only way to fix an image problem (without rebuilding it as a double
conversion set!) is to add additional selectivity between the antenna and=

the converter stage. By the time the signals get to the IF, the image is
part of the IF signal and no amount of added IF filtering can get rid of
it. A really good preselector might help some, or a fully tuned =

preamplifier -- make sure its a TUNED one, not a broadband piece of junk.=


Wayne Hall
WB4OGM =