Posted by
wayne burdick on
Jun 26, 2012; 6:19pm
URL: http://elecraft.85.s1.nabble.com/K3-KX3-Operating-Tip-APF-audio-peaking-filter-for-weak-signal-CW-work-tp7558208.html
Both the K3 and KX3 have an APF control (audio peaking filter). APF
can be extremely effective when you're trying to copy weak CW signals
buried in noise.
Most DSP filters have very steep skirts; they're often referred to as
"brick-wall" filters for this reason. This is the type of filtering
associated with the WIDTH control (K3) or PBT control (KX3).
In contrast, the APF is a filter with a very narrow peak (about 30 Hz
at the -3 dB points) and very broad skirts. The narrow peak brings up
the signal amplitude slightly, while the broad skirts prevent noise
from being amplified and delayed in such a way as to dominate the
signal. The APF function used in the K3 and KX3 is a modern DSP
reinterpretation of the hardware APF circuit found on some legacy
transceivers, which was very effective.
On the KX3, the APF switch is labeled "APF" (tap), while on the K3,
it's labeled "DUAL PB" (hold). The K3 actually provides two different
special filter functions; use the DUAL PB menu entry to set up the
switch for APF.
When you turn on APF, the DSP graphic changes to remind you of the
nature of APF, with its narrow peak and broad skirts.
I suggest setting the passband width to about 250-300 Hz when using
APF. The DSP graphic still shows an approximate representation of the
width even with APF turned on.
73,
Wayne
N6KR
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