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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