[Elecraft] APF For K3

[Paul Christensen]

> "Dual PB" (press and hold XFIL) allows one, as the manual says, to "focus"
> on a given signal that is matched to the sidetone frequency while hearing
> other callers above and below that frequency at reduced level (the
> "context").  Doesn't that do what the waveforms described on AB4OJ's 
> website
> depict?

Icom's use of APF is different than that used in the FT-1000/D.   Icom more 
closely emulates the Dual PB feature in the K3 but without the "context" 
feature.  What's missing is high "Q" of the centered frequency.  The 
FT-1000/D's Q of the audio circuit varies with the tuning control, but it's 
generally greater than 10.  When the Dual PB tuning is enabled on the K3, 
and with aggressive NR engaged, I don't hear what sounds like a high Q 
equivalent.  The FT-1000/D's APF is more closely associated with a 
single-channel of a multi-band parametric equalizer where Q, amplitude, and 
frequency are all variable.  In the FT-1000/D, Q is mostly fixed, but 
amplitude and frequency are variable.

Although the FT-1000/D adds amplification of the centered frequency -- and 
even allows changing the peaking frequency, I think the most desirable 
attribute of the circuit is the high Q ability.  Fixed gain and variable 
peaking frequency may be a nice added feature to the K3 if APF is 
implemented, but these are secondary to achieving emulation of the high Q 
portion of the analog circuit in DSP.

Paul, W9AC