[Elecraft] Spectrogram

[Ron D'Eau Claire]

Initially I used background noise when I adjusted my K2 filters.  Since
then I have used a noise generator cobbled up on a bit of perf board
which resulted in a much better display.  A nice big S9+ noise lets you
see further down the slope of the filter, especially when checking out
the crystal filter grounding mods.

Turn OFF the AGC on the K2.

Mike VP8NO
#1400
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Good point! Also, as John KI6WX pointed out to me a while back, if you
want to take a critical look at the bandpass, be aware that the
capacitors in the headphone output will cause the response to roll off
at the higher frequencies if you plug your sound card directly into the
phones jack. This produces a very definite downward slope when looking
at the 2 kHz bandpass of the SSB filter.

I plug my computer into the auxiliary speaker jack on the back of the K2
and use the "line input" on my sound card to avoid this effect. If  one
wants to use the phones jack, John recommends putting a 10-ohm resistor
to ground across the headphone output

After aligning the filters on a good wide band noise so you can see
where your "sidetone" frequency appears in the bandpass, tune in a
carrier (the birdie at 7 MHz is my choice) and see if there is any shift
in frequency as you step through the filters or change from Normal to
Reverse. If so, simply step through CAL FIL once again, moving the
frequency of the BFO at each filter position away from the desired value
and then putting it back to make the uP recalculate the BFO frequency. 

That takes care of any excessive shifting. Keep in mind that the BFO
frequencies are not controlled accurately enough to guarantee that the
beat note is be EXACTLY the same in each filter position. The DAC's used
in the K2 limit the accuracy to 30 Hz, I believe it is. You might get
closer, but it's pure luck of you do. I don't have any problem getting
mine to within 20 Hz after a couple of tries. That's about all my "tone
challenged" ears can detect anyway.

Ron AC7AC
K2 # 1289