(BTW I wrote the following ~3 years ago before APF was changed from HOLD to
TAP on DUAL PB):

##############################################################
I use APF virtually all the time and never use RIT, SHIFT, etc.  Just HOLD
DUAL PB to actuate APF and use the VFO to zero beat.  COARSE (10 Hz) VFO
steps is usually close enough but you can switch to FINE (1 Hz) if necessary
for ultra weak signals.  I also strongly endorse N6KR's suggestion below: 

"I suggest setting the passband width to about 250-300 Hz when using APF." 

This is extremely important since bandwidths too narrow will introduce
additional ringing to the APF which already borders on ringing.  I actually
use a 200 Hz XFIL but set to actuate at DSP 400 Hz.  With WIDTH set to 400
Hz, the signal first goes through the relatively broad (4.0 shape factor)
200 Hz XFIL, then a 400 Hz DSP and finally the narrow APF.  This makes the
cascaded XFIL/DSP seem more like 250 Hz, reduces ringing, still allows you
to hear off-frequency callers (although attenuated) and helps your ears
discriminate weak signals from noise by providing a wider background sample
of noise.
#######################################################

The last phrase "helps your ears discriminate weak signals from noise by
providing a wider background sample of noise" is key.  Narrower bandwidths
are not best for the human ear/brain...no matter what theory says about
narrower bandwidths having better S/N.

73,  Bill  W4ZV



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