These terms are often used interchangeably. It's like the terms
"biannual" and "semi-annual." One is supposed to mean "every six
months" and the other "every other year," but some dictionaries list
them as synonyms, because you can make a case for either word having
either definition.
Similarly, there's nothing about either the term "I.F. Shift" or
"Passband Tuning" that helps the average operator tell which one you're
talking about. They're too vague. But there really are two different
concepts here. I'll explain what I think they are, and then explain why
most implementations don't really protect you against out-of-band
signals.
There are at least two kinds of "shifting" covered by these terms:
- In one case you're just shifting the pitch of the I.F. without
changing the bandwidth. A more accurate term would be "Pitch Shifting."
- In the other case you're shifting *two* I.F.s in opposite directions,
reducing the degree of I.F. filter overlap from 100% to something
lower, so that the ultimate bandwidth (at audio) is reduced. The center
pitch of the passband may or may not change at the same time. Let's
call this "Filter Shifting."
Either approach may appear to reduce QRM. But the QRM may in fact still
be there. How can this be? It's because the bandwidth first filter has
not actually changed; you're just listening to the portion of its
passband that's making it through to the second I.F. The rest of the
first I.F. filter's passband is still there, exposing subsequent stages
to overload, AGC pumping, etc. That's why some seemingly modern radios
break down in contest situations: they use a fairly wide first filter
(often without telling you), then tighten or shift the *second* I.F.
filter, which you perceive as a reduced bandwidth. But if a signal
within the first passband exceeds the signal-handling capability of a
subsequent stage, it sounds like, well--crap. (That's another name for
serious in-band IMD.)
A much better approach to the "Filter Shifing" problem would be to
actually *narrow* the first crystal filter, protecting all subsequent
stages. This is why the K2's variable-passband filter is such a great
tool. You can narrow it down at the same time you narrow the DSP or
audio filter. Of course the variable-passband filter is most useful in
CW mode because it's optimized for narrow bandwidths, but it can
certainly be used in SSB modes as well.
In the case of the K2, ignoring audio filtering, the "quantized"
version of I.F. shift that I mentioned earlier (using different BFO
settings with the same crystal filter) is like the "Pitch Shifting"
case. However, as soon as the pitch-shifted passband starts to hit a
wall (namely the basic audio response of the radio and/or the DSP
filter passband), the bandwidth starts getting narrower, too. This has
some of the effects of "Filter Shifting," whether intended or not.
Ideally you'd have variable-passband filters usable in all modes, along
with several controls, e.g. SHIFT, WIDTH, HI CUT, and LO CUT. It would
then be possible to optimize the passband intuitively, as required to
deal with QRM (that's the point). And like I mentioned, you'd like to
have the benefit of not just shifted filters, but filters that get
narrower as required.
How does all this apply to the K2?
Obviously there isn't room for a suite of passband controls. But you
might be able to emulate this functionality using a computer, given the
proper remote control commands. (Sounds like something else for the
wish-list.)
You could also modify the K2's variable-passband CW filter to optimize
for wide bandwidths (SSB/DATA) rather than narrow. The same crystals
are used both in the main (stock) filter and the KSB2. All you'd need
to do is use smaller varactor diodes, preferably a matched set. You
might be able to get a 1200-2400 Hz passband range with acceptably low
ripple.
Food for thought.
73,
Wayne
N6KR
---
http://www.elecraft.com
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