Introduced with firmware release 4.16, Elecraft offers an APF, audio peaking filter, to enhance the readability of weak cw
signals. The APF was based on the original high Q FT-1000 APF (not the later watered down APF). It has been an instant
success for the K3, especially among low band dx'ers.
It's important to tune in the weak signal carefully to match your sidetone frequency ("pitch") before engaging APF. When
you engage APF, engage "FINE" tuning at the same time. Tune very slowly through the signal with APF engaged and listen
for the signal to pop out of the noise.
Have a listen to the sound clip below for a good demonstration of its benefit.
K3 APF sound clips
RA0FF on 160M cw 6November2010 1100Z, NW Beverage 200hz selectivity with APF turned on and off to hear the difference
V73QQ 160M qso, thanks to K3 APF December 12 1145Z, NW Beverage 300hz selectivity
K3 APF passband spectrum - 200hz width with APF on (yellow) /off (red)
(ignore seeming spurs at 120/180/300hz - just 60hz noise harmonics)