[Elecraft] [Elecraft_K3] curious behavior of the K3/P3, etc.

[Richard Ferch]

OK, if you're talking about CW, the K3's normal CW is on lower sideband.

In CW the filter behaviour gets tangled up with the sidetone pitch 
setting, which is also the default FC (centre frequency) for the filters 
in CW mode. Suppose the sidetone pitch/FC is 500 Hz and the BW 
(bandwidth) is 2000 Hz. Given those numbers, the calculated hi-cut and 
lo-cut frequencies would be 1500 Hz (FC + BW/2) and -500 Hz (FC - BW/2). 
That's a minus sign on the lo-cut frequency, meaning it's on the other 
sideband. The firmware should not allow that; it needs to place the 
lo-cut frequency on the proper sideband far enough above zero to give 
adequate carrier and opposite sideband suppression. IIRC, the low-pass 
cutoff is adjustable in the latest firmware - I believe it can be set to 
100 Hz, 200 Hz or 300 Hz.

So, with the low-pass cutoff set at 200 Hz, FC at 500 Hz and BW at 2000 
Hz, the actual filter bandpass will be from 200 Hz to 1500 Hz. Adjusting 
the Width control would only affect the upper limit, as the lower limit 
is fixed by the low-pass cutoff. If the Width setting is sufficiently 
narrow for the calculated lo-cut frequency to be at or above the 
low-pass cutoff (100, 200 or 300 Hz), the effect of the Width control 
becomes symmetric about the centre frequency, as you might have 
originally expected.

Because CW is on the lower sideband, adjusting the Width control only 
affects the lower RF (=higher audio) edge of the bandpass - the upper 
edge is fixed by the low-pass cutoff.

Does this help make sense of your observations?

73,
Rich VE3KI


AA0AA wrote:

> Sorry Bill and all--I was describing the behavior when on CW.=A0 I more oft=
> en use these adjustments on CW, but if it is important for understanding th=
> e question (and for me to understand what's going on!) I will check with SS=
> B when I get home this evening.