[Elecraft] APF vs 10 Hz DSP, why they don't sound the same

[William Carver]

The "DUAL PASSBAND" makes a pass at providing an APF type of function
where you make the DSP passband as narrow as you choose, but there's a
wider passband that's 20 dB down. Undoubtedly it isn't the same shape as
APF amplitude v.s. frequency, but conceptually "a signal peak floating
on a sea of weaker undesired signals" is similar.

G3XJP implementated a DUAL PASSBAND function (that terminology hadn't
been invented then) in his STAR transceiver, but there the amplitude of
the wider passband can be dialed to be -20, -30, -40, .... -60 dB. It
probably isn't too big a deal to make the "skirt attenuation" number a
user-adjustable variable but I enjoyed being able to dial it up and down
as QRM changed....which would ite up another programmable functio
button.

For those who suggested kicking the gain compensation up when narrower
roofing filters are switched in, that's exactly what I did. I'm well
aware that   IF   the roofing filter is setting the IMD capability of
the receiver adding pre-filter gain reduces the intercept/increases IMD
WHEN  that IMD happens to occur, and   IF  it happens to fall into the
passband with the desired signal. I intentionally emphasized the string
of IF..WHEN..IF.. because depending upon you penchant for living in
pileups, or turning the rig off on big contest weeks, a 4 dB IMD
increase  IF all of those things happened to occur may, or may not, be
that big a deal. 

A post-roofing filter gain increment would not affect intercept or IMD
and could be a smoothly varying increase that could conceivably be
matched to a users memory of APF or Heath QF-1 function.

Bill