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

[Bill W4ZV]

N1EU wrote
> I suspect that when the K3 APF was first engineered, the Yaesu FT-1000D
> APF
> (original hi-Q version) was used as the model because many prominent
> dx'ers/topbanders always raved about it.

I lifted the "original" FT-1000D APF from the schematic in 2009 and Al W6LX
did a circuit simulation on it.  Al's description provided the roadmap for
what Elecraft eventually implemented in the K3's DSP in 2010.  

http://www.mail-archive.com/elecraft%40mailman.qth.net/msg105227.html

As Wayne has suggested previously, APF works best if you use a relatively
wide DSP setting (I use 400 Hz).  I trick my K3 into using my 200 Hz XFIL at
DSP=400 by setting the XFIL BW to 400 (similar idea to what Wayne recently
suggested with the 2.7/2.8 XFIL for better NB).  The wider DSP BW keeps the
ringing down and gives your ear/brain a better background to discriminate
signals through the 30 Hz APF without ringing while the wide (5 pole) 200 Hz
XFIL still gives good protection from nearby strong signal overload. 
Setting the VFO is not as critical compared to narrow DSP settings, and I
use APF virtually all the time...even in contests.  This signal path may
help visualize what I'm doing:

signal >> 200 Hz XFIL >> 400 Hz DSP >> 30 Hz APF >>  ear/brain

73,  Bill  W4ZV



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