http://elecraft.85.s1.nabble.com/K3-KX3-Operating-Tip-APF-audio-peaking-filter-for-weak-signal-CW-work-tp7558208p7558221.html
> Wayne,
> I was pleased to see the APF added to the K3, though I think it is
> not as useful on radios with such good basic filtering, as compared
> to older radios. OTOH: it's a favorite feature of my old FT1000D!
> 73, andy ae6y
>
> -----Original Message----- From: Wayne Burdick
> Sent: Tuesday, June 26, 2012 11:19 AM
> To:
[hidden email]
> Cc: Elecraft Reflector
> Subject: [Elecraft] K3/KX3 Operating Tip: APF (audio peaking filter)
> forweak-signal CW work
>
> 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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