[Elecraft] APF (audio peaking filter) on the K3/K3S/KX3

[Wayne Burdick]

Hi all,

This morning I was hearing lots of very weak signals in the 40-meter CW band, right at today's noise floor of about S-5. (Among others, I copied BG4GOV in Shanghai.)

Many of the signals were not copyable until I turned on APF (audio peaking filter). This filter's center peak is very narrow (around 30 Hz), but the response broadens out quickly below the 3-dB points in the curve. This brings up the desired signal without allowing noise to cause ringing--something often heard with narrow filters of the "brick-wall" variety. On a noisy band, the effect is just short of magic.

APF is very narrow, so you'll need to tune in 1-Hz increments when it is turned on. Also, APF works best when used with a DSP/crystal filter WIDTH setting of 300-500 Hz.

Use of APF differs for the three transceivers.

KX3: 1-Hz tuning is selected automatically on the when the APF switch is pressed. 

K3/K3S: APF is accessed via a hold of the XFIL switch. This function is labeled "APF" on a K3S, or "DUAL PB" on the K3). Use CONFIG:DUAL PB to make sure the selected filter function is APF. See the K3/K3S owner's manual for a description of the alternate setting. 1-Hz tuning can be selected by tapping FINE.

You might want to give APF a try if you haven't already.

73,
Wayne
N6KR