Yes, that is exactly the point I was trying to make! You can have it as
you like it (I am speculating a bit about the control options in the
firmware, of course). Some people like a wider width at the first IF to
hear signals approaching, even if it opens the window for AGC blocking
or desense. Others would want the two passbands to be exactly aligned
and as narrow as possible, which would provide the best IMD and AGC
performance, but then the signals will drop off the cliff as you tune.
What you appear to have done is allow nuance in these adjustments, which
I think is quite clever and well thought out. I can't wait to play with it.
I didn't mention the audio passband shaping, but that makes it just that
much better. Well done. Look forward to seeing you at FDIM.
73,
Larry N8LP
Lyle Johnson wrote:
Hello Larry!
...Therefore, the "roofing filter" should be termed the "passband
tuning enabling filter", or "PBTE" filter ;-)
Thankfully, I think Elecraft has done a brilliant job of giving us
the options we want without compromises. By tying the DSP bandwidths
and PBT functions to the "roofing filters", we have the ability to
have the combination of 1st and 2nd IF BW we want,,, and with the
variable "roofing filters", I think we will be able to almost set the
relative BWs between the two... allowing a window for PBT or not as
we choose.
In the olden days, analog radios would use two IF filters and slide
them back and forth against each other to form variable bandwidth
filters. This may be some of the PBT you were referring to.
In the K3, you can set the IF selectivity almost anywhere you like.
The conversion oscillator and the DSP's internal software BFO will be
moved around to ensure that the narrowest available 8.215 MHz IF
filter is selected. The edges of the DSP IF filters and the crystal
filter aren't worked against each other. In general, we try to keep
the passband of interest centered in the IF filters. The
variable-width crystal filters will make this matchup fairly precise :-)
Thus we can offer passband tuning without opening up the crystal
filter bandwidth wider than required for the ultimate passband chosen
by the operator of the radio.
Oh, and did I mention we also shape the audio passband according to
the filter selection? And that is overlaid with the 8-band Rx EQ?
73,
Lyle KK7P
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