--- On Tue, 12/29/09, Al Lorona <alor...@sbcglobal.net>
wrote:
Originally, IF shift was defined as moving one IF passband within another IF
passband, making the resulting passband the intersection (not the union) of the
two passbands.
When you do that, you effectively reduce the width *and* the center
frequency of the IF passband... it has nothing to do with manufacturers failing
to make it work correctly.
That's not the way I understand it. What you describe
is a way to narrow the composite filter bandwidth. If done correctly, the
center frequency doesn't change.
Take two pieces of paper and cut a square in each. Hold them up to a window,
and slide one square horizontally across the other one, and note how the width
*and* center of the opening shifts left or right. This is what I mean when I
say, "IF shift". We might be talking about two different things.
If you are trying to adjust bandwidth, then *both* filters
are effectively moved in opposite directions, leaving the center frequency
fixed. But this isn't "IF Shift".
IF Shift is when the pass band of a filter (or composite
filter) is effectively moved with respect to the carrier (or suppressed
carrier) frequency, i.e. BFO.
A better term for this is Passband Tuning, first used AFAIK, in the Collins
75A-4. Since the VFO was very linear in degrees of rotation of the tuning
knob with respect to frequency, turning the VFO housing was the same as turning
the knob in the opposite direction.
The BFO frequency was also linear with respect to knob rotation, so Collins
mechanically slaved the rotation of the VFO housing to the rotation of the BFO
knob. So the BFO could be moved from one side of the passband of the
mechanical filter to the other but since the VFO tracked the frequency change,
the beat note of a signal wouild remain constant. This gave the illusion
that the mechanical filter passband was being moved with respect to the BFO,
which in turn appeared to be fixed in frequency since a signal's beat note did
not change*.
* Actually, since the two oscillators didn't move exactly the same amount due
to minor non-linearities, there was at some frequencies a perceptible beat note
change. For those who were not faint of heart (me), the linearizing widgets
(have to be seen to understand) in the Collins PTO could be tweaked to improve
this. The K3 uses almost simultaneous shifts in the 'BFO" and VFO to
do the same thing. I say "almost" because the DDS step size
changes with frequency so the shifts aren't exact and a beat note change can be
seen when changing filter shift.
Wes N7WS
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