Doug,
I have never seen a crystal filter that is truly symmetrical. That is
just the "facts of life". The real question is, just how much asymmetry
is the user willing to tolerate. Since this is a roofing filter, and
not the ultimate filter for the K3, I tend to believe that the 5 pole
filter is adequate for my purposes. YMMV.
73,
Don W3FPR
Douglas G. Bonett wrote:
Table 2 of the ARRL review finally answered my questions about the relative
performance of the 2700 and 2800 Hz filters at 1 Hz and 2 Hz spacing. The
results are very impressive. Can someone explain to me why the above/below
results for these two filters are so disparate at these narrow spacings? The
review suggests that the observed asymmetry is quite unusual. Is the performance
of a radio most accurately described by the larger or the smaller of the
above/below values? Is the asymmetry due to a design problem in the roofing
filters or something in the K3 circuitry? If the asymmetry could be corrected by
a change in roofing filter design, would the above/below values both become
similar to the average of the two values? For instance, would the 94/81 IMD
values at 1 kH spacing for a redesigned 2800 Hz filter approach 87.5/87.5, which
would be a truly spectacular result for a radio with a single 2800 Hz roofing
filter.
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