[Elecraft] K2 CW Filter Characteristics

[Bob Tellefsen]

Hello Bill

This is anecdotal info, rather than measured, but it might be of interest.
I was a Field Tester for the early K2 (mine is s/n 12), and did some
tinkering
with the filter after a suggestion by Wayne Burdick.  I made small changes
to
the matching transformers at the filter input and output for a better match
at the
narrowest bandwidths, and greatly reduced the insertion loss in the process.
Now
signals are heard at about the same volume at any bandwidth.

Later on, wo0b (I think) posted a mod to reduce the filter blowby by
reducing
the inductance of the crystal can grounding wires.  This was later verified
by
others as making a noticeable but not large improvement.  So I added it to
my
K2 also.  At that point, I think my K2 was as sharp as it was possible to
make
it at the time.  I've since received the new matched crystal filter set, but
have
yet to install them.

The filter works well, but I've never bothered to try matching the indicated
BW with measured BW.

I expect the discrepancy between the desired/indicated BW and the measured
BW can
be attributed to crystal variances and to variances in the varicaps that
change
the BW.  Over time (in his spare time :-), Wayne might be able to create a
better
match between the dial and the performance.  Since it is not a performance
problem,
I expect it will take a low priority though.

The most pragmatic advice seems to be to treat the indicated BW as a logging
scale,
and use Spectrogram to find those settings that give the BW you want.  Then
record
or remember them.

73, Bob N6WG