[Elecraft] APF For K3

[Richard Ferch]

Here is an operational scenario:

You are CQing in the CQ WW CW contest. There are other loud stations 
CQing on either side of you. You have adjusted your SHIFT and WIDTH 
controls to eliminate the adjacent-channel interference while still, you 
hope, being wide enough to catch anyone calling you.

A weak station replies to your CQ, but he's some distance off your 
frequency, although still within your bandpass. You'd like to be able to 
put an audio peaking filter on his signal without affecting the rest of 
your setup.

You don't want to change your sidetone pitch; that's a separate question 
for a separate discussion. You don't want to use RIT or CWT, because 
that will bring one of the adjacent loud signals into your bandpass. 
Anyway, the desired signal is too weak to trigger CWT. Likewise, you 
don't want to shift your main DSP filter bandpass (the "context"); 
you've got this set up for where you are and you don't want to change it 
just for this one 30-second QSO. If you didn't have an APF, you might 
try narrowing the DSP filter bandwidth, but he's far enough 
off-frequency that doing so would run the risk of losing his signal, 
meaning you would have to fiddle with both SHIFT and WIDTH to home in on 
him, and that takes time to do, plus more time to restore them after the 
QSO.

What you would really like to be able to do is to adjust an audio peak 
filter's frequency so that it is on top of the received weak signal, but 
without affecting the context (your DSP shift and width, your sidetone 
pitch, or your transmit frequency - these should all stay where they were).

My suggestion would be to use one of the buttons (DUAL PB seems to be a 
possible candidate here) to turn the APF feature on, and while the 
feature is on, the SHIFT and WIDTH controls would adjust the APF 
frequency and width (or Q) and would have no effect on the context 
filtering. RIT and SPLIT would still work normally, it's just SHIFT and 
WIDTH that would have been subverted to do the APF adjustments while the 
APF is on. If you really did want to adjust the context filter width or 
shift, you would have to turn APF back off to do so. Once the QSO with 
the weak station had been successfully concluded, you could either turn 
APF off, or else leave it on with its width adjusted to include the 
entire context bandpass, and carry on CQing.

My 2c worth.

73,
Rich VE3KI