Fwd: [Elecraft] Sherwood on Roofing Filters, Transmitted IMD and Receiver Performance

[Ed K1EP]

I think that that comment was taken out of context.  In his talk, I believe
that he remarked that some xtals lacked quality control.  I think it was
said or implied that both the manufacturing process and testing process
wasn't always perfect and that some xtals in the filters had some
contamination or process defect that would cause them to have higher IMD
products than they should.  If that particular xtal was at the high signal
end (input) of a ladder filter, it would produce IMD products.  However, if
it was at the other end of the filter, most of the strong signals would have
been filtered out by that point and you would not see the problem.  Since
the filters were supposed to be symmetrical, theoretically it shouldn't
matter which direction they were used.  But given the lower quality of some
of the individual xtals in the filter, he noticed the difference.  That is
what I got from his talk.  I may not have interpreted it correctly.  The K3
filters only plug in in one way, so if they were tested properly and the
xtals manufactured properly, there shouldn't be any problems.  Yes, the
problem can still exist if the xtals are not manufactured properly.


On Wed, Mar 12, 2008 at 8:33 AM, K3KO <alsopb at nc.rr.com> wrote:

>
> One thing stuck out.  "Inserting filters in one direction in his testing
> for
> some rigs resulted in an 8db difference relative to the opposite direction
> in a performance parameter."
>
> Are K3 filters properly polarized so this won't happen?  Has the filter
> technology improved so this isn't an issue?  Or is it a crap shoot with a
> 50/50 chance of getting it right even with polarized filters?
>