[Elecraft] K2 CW Filter Characteristics

[Don Wilhelm]

Bill,
The K2 CW filter is variable via the menu - it is varaible for not only the
bandwidth but the relationship of the BFOs to the filter passband.

There will be some variability from one K2 to another in the filter
charateristics - largely due to the crystal motional inductance and the
tolerance of the capacitors - and perhaps a small bit due to builder
technique.

The crystals currently supplied by Elecraft are more tightly speced on Lm
than at the time the ARRL tests were run, and the crystals are sorted for
frequency by Elecraft, so a filter on a current build K2 will be better than
that used in the ARRL test.

The 'nominal' bandwidth of any particular filter bandwidth setting may not
be exactly the numbers shown on the K2 menu, but for your particular K2, you
can get a sense of the relationship when you use Spectrogram for setting the
filters - if one wants a 200 Hz wide filter at the 6dB points, just use the
Spectrogram display to read the width and dial in whatever width matches
those points.  Wayne Burdick is likely the only one who can say why the
bandwidth numbers shown by the firmware are not closer to the actual
bandwidth - but then there have been several firmware upgrades since the
ARRL test was conducted, so perhaps some correction factor has been applied
already.

I find my K2 filter response excellent from a width of 100 Hz to 1000 Hz -
and quite usable up to about 1800 Hz (or down to 40 Hz).  The shape factor
is characteristic of a 5 pole filter (approx 2:1 for 6/60dB) and makes nice
copy for CW with no ringing, and yes the passband ripple becomes greater as
the filter is widened.  Anyone wanting extremely steep skirts would be wise
to install the DSP option, but I personally like the wider skirts because it
makes listening less tiring than an 'ideal' filter.

I realize I have not given you the precise numbers you requested, but since
each filter may be slightly different and the shape factor will vary
somewhat with the filter width, such numbers may not be the best way to
characterize the K2 filter - it is good and very usable for crowded band
conditions - see some of the posts dealing wiht Field Day operation as
testimony to that fact as well as Earl's recent reports on A/B comparisons
for digging out weak signals on 160 nd 10 meters.

73,
Don W3FPR

----- Original Message -----

>
> Questions:
>
> 1.  What is the actual 6 dB BW, combined 6/60 dB shape factor (X7-11
> plus X5-6) and passband ripple for various "selected" bandwidths from
> minimum (200 Hz?) to about 1 kHz?  What I'm looking for is actual data
> from measurements (Spectragarm if it has 60 dB DR or spectrum analyzer)
> similar to the following:
>
> Selected BW   Actual BW  Shape Factor  Passband Ripple
>       700            350         2.1:1           3 dB
>
> 2.  What is the minimum "selectable" BW  (Spec sheet says only "~200 Hz")
> and what is its actual BW?
>
> 3.  Why aren't "selectable" and "actual" bandwidths closer?
>
>