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

[David Wilburn]

Nice explanation, thanks.  
-  

David Wilburn
k4dgw at k4dgw.com
K4DGW
K2 S/N 5982


On Mon, 2007-12-17 at 13:06 -0800, wayne burdick wrote:
> 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
> 
> 
> ---
> 
> http://www.elecraft.com
> 
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