[Elecraft] [K3] Questions about ARRL review

[Lyle Johnson]

> 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?

There are really two questions here.

The first question has to do with the skirt selectivity of the roofing 
filter.

If you look at the larger values of attenuation (134 vs 115, or 113 vs 
93) the result is mostly due to the difference in shape factor of the 
roofing filter.  Expressed differently, an 8-pole filter is likely to 
have a steeper slope in its selectivity curve than a 5-pole filter.

The second question has to do with the difference between the above and 
below values for a given filter when the signal spacing is less than the 
nominal width of the filter.

This asymmetry is due to the way in which the K3 aligns the roofing and 
DSP filter passbands.  The signal is not necessarily centered in the 
roofing filter passband; rather, the signal is shifted towards an edge 
to maximize the use of the roofing filter's skirt selectivity.

Why is this important?

Consider the case in which you are using a wider filter in a crowded 
band while operating CW.  A huge signal appears very close by, perhaps 1 
or 2 kHz away.  The use of CW REV or CW may make a considerable 
difference on the impact of that signal on the receiver.  If the 
passbands were centered, this tool would be less effective.

And if huge signal are on both sides?  Time to get a narrower roofing 
filter!

73 and Happy Thanksgiving,

Lyle KK7P