[Elecraft] K3 and AM mode: using the DSP bandwidth controls

[wayne burdick]

A few of you have asked how to use and interpret the settings of the 
receiver's DSP controls when in AM mode (SHIFT/WIDTH and HICUT/LOCUT 
functions). I'll be improving the associated Owner's Manual section 
soon, but I thought I'd try to get the word out more quickly.

What the DSP controls are adjusting is the final audio passband: what 
you hear. This is true in all modes, and is immediately reflected in 
the DSP "graphic" on the LCD. When you rotate WIDTH and see "2.80", 
that means that the audio bandwidth is 2.80 kHz. When you rotate SHIFT 
and see "1.60", that means the center of the audio passband is 1.60 
kHz.

Of course there isn't just audio filtering in the K3. As you rotate 
these controls, we internally adjust the 1st and 2nd I.F. passbands 
optimally to match the audio passband. For example, in SSB mode, if you 
set WIDTH to "2.80", and have a 2.80 kHz crystal filter, then you have 
three filters all cascaded at this same bandwidth: a 2.8 kHz 1st-IF 
crystal filter (centered at 8.215 MHz); a 2.8 kHz 2nd-IF DSP filter 
(centered at 15 kHz); and finally, a 2.8 kHz DSP audio filter (centered 
at 1.6 kHz). We do all the math to make this as seamless as possible, 
selecting the ideal crystal filter as you cut the highs or lows or 
reduce the audio bandwidth.

With this in mind, I can now explain what happens in AM mode, which has 
to be handled differently.

An AM signal is comprised of a carrier and both sidebands. So it's much 
wider than an SSB signal, and this is why you need a 6-kHz crystal 
filter for best fidelity. But this filter bandwidth is only required 
ahead of the AM demodulator, that is, at the first and second I.F.s. 
After the demodulator, we're back to the audio passband -- what you 
hear.

So, when you rotate the DSP controls in AM mode, even though you'll 
typically see the 6-kHz filter selected ("FL1" on my rig), you won't 
see "6.00" shown on the LCD. Instead, you'll see the audio bandwidth 
("2.80", etc.), just like you do in other modes. And this is what makes 
sense, given that the controls are linked the AF passband.

This will be even more apparent when using FM (coming soon!). FM 
requires a filter around 13 kHz wide. As you can see, having the DSP 
controls show something like "13.00" when WIDTH is rotated wouldn't be 
very useful. You'll want to know how they affect what you hear, not the 
underlying IF filter selection.

Note: For now, I suggest that you enable only the 6-kHz crystal filter 
for AM. A future firmware revision will also properly handle the 2.8 or 
2.7 kHz crystal filters when DSP controls are rotated in AM mode. This 
will be most useful when doing hicut/locut.

73,
Wayne
N6KR


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