[Elecraft] Filter setup

[Tom Hammond NØSS]

Hi Earl:

>When the K2 is first tuned on the bfo settings are roughly 110 lower and
>200 upper (reverse)  I set up the filters initially by ear. Wasn't happy
>with the settings and run spectrogram.  The settings are no where near
>the default settings.  I now get 91, 10, 4,3 on CW and 150, 134, 128 and
>126 on CW rev.   The cw settings in fact are a little high at 4 and 3
>but I can go any lower without putting the bfo on the wrong side.
>
>Hope someone has an idea what I am doing wrong here.  I have tried this
>several times and still run out of numbers on the bottom end.

To what frequency do you have your sidetone set?

None of your CW Normal settings look correct, though your CW REV figures 
don't look all that bad.

When you're in CAL FIL, try pressing the DISPLAY button, to display 
frequencies, rather than the DAC values. The DAC values can vary more 
radically than the actual frequency, so we'll be more able to compare your 
frequencies against our frequencies when any of us wish to respond.

Here are what I hope will be some representative BFO frequencies, taken 
from my K2 with new xtals (marked 3.7 on the envelope):

         Bandwidth    Filter
           (Hz)       Center       CW Nor      CW Rev

   FL1     1800      4914.41      4913.50     4915.12

   FL2     1000      4913.88      4913.28     4914.44

   FL1      400      4913.82      4913.15     4914.33

   FL4      200      4913.75      4913.11     4914.29

Note that, as you narrow the filter bandwidth, the UPPER end of the filter 
passband shifts downward much more quickly than the LOWER end of the 
passband shift upward. As a result, the center frequency of the filter also 
shifts downward as the passband is narrowed. Sooo, you cannot merely use 
the center frequency at the widest chosen bandwidth as the center frequency 
for all of the other passband settings. If you do, you will wind up with 
some of your CW Rev frequencies (appearing to be) lower than the center 
frequency. This drove me nuts early on (actually until about a month ago), 
until I realized that the filter frequency moves too.

If your measured frequencies are anywhere close to those I've shown above 
and you still have very low DAC values, it would appear that your VCO may 
be centered at too high a frequency.

73,

Tom Hammond   N0SS