[Elecraft] K2 Filter Alignment: USB vs. LSB, CW vs. CW-Rev

[Ron D'Eau Claire]

Hi, Michael!

Criteria 1) is not possible at the wider filter bandwidths without losing
single-signal reception (hearing the signal on only one side of zero beat).
Consider this: you choose a sidetone of 600 Hz. If the filter bandwidth is
2000 Hz and you center the signal in the bandwidth, then the filter will
pass frequencies from 1600 Hz down to zero beat and up to 400 Hz on the
wrong sideband! The better approach is to place the bandpass so that it
never gets closer than about 200 or 300 Hz from zero beat. Filters widths up
to about 1000 Hz can then be centered on the beat frequency. Wider filter
bandwidths will be set off-center as needed. 

2) and 3) are limited by the resolution of he digital to analog converters
(DACs)in the K2. When you do the filter alignment, the K2 stores a tuning
voltage value that will put the BFO back on the right frequency each time
you choose a filter setting or switch from normal to CW REV. The DAC's have
a resolution or "resettablity accuracy" of about 30 Hz. That means that you
may hear a 30 Hz difference in beat as you move from one filter setting to
another. You can often reduce the error by having the K2 re-measure and
store the tuning voltage value for a filter setting again until it happens
to hit a value that produces close enough to the right frequency to please
you. It is possible, by pushing a few buttons repeatedly while doing the
alignment to get the filters within 10 or 20 Hz. That includes switching
from one sideband to the other (normal to cw rev) as well as switching from
one filter to another.

Ron AC7AC

-----Original Message-----
  Once I get to the Stage III alignment on my K2, I plan on using
Spectrogram to align the CW and SSB filters/BFOs. It seems that proper
filter alignment is a juggling act between the following:

1) Proper centering of the signal within the filter's passband: For the most
part, CW should be aligned in the center or the largest peak, SSB should be
aligned for best audio quality (usually with the bottom at about 300Hz).

2) Constant pitch across the different bandwidth filters for a given mode.
In other words, you should hear very little pitch change (<= 20Hz or so) as
you move from the widest filter for a given mode to the narrowest filter for
that same mode.

3) Constant pitch as you switch between USB/LSB on an accurately tuned AM
signal like WWV and constant pitch as you switch between CW and CW-Reverse
on an accurately tuned CW signal.

  Is it difficult to satisfy number 3 if you tune the filters with
Spectrogram based on number 1? Has anyone come up with a reliable
methodology for doing this, or is it just something that you pretty much
play with until you are satisfied?

  Thanks, Michael N9BDF
          K2 #4137