IF Shift vs. Passband Tuning?

[Jeff-229]

Hi gang,

Are IF shift and passband tuning (PBT) the same thing?  I assume that they produce the same effect.  I have a Ten-Tec Argonaut V that has PBT, but I have never used that feature, thus I don't miss it on my K2.  I guess I just don't grasp what PBT would do for me that I can't do with RIT and the variable width DSP filter of the Argonaut V.  However, I'm a CW op; someone commented that IF shift is more useful on SSB than on CW.  Why is that?

Thanks, 73 & 72,
Jeff
WB5GWB
Long Island, NY
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[Don Wilhelm-3]

Jeff,

As far as I know, IF shift and Passbasnd Tuning are the same thing - just
different names for the advertizing hype folks to enjoy.

The major benefit of Passband tuning is that an interfering signal can be
moved off the edge of that reciever passband without changing the pitch of
the received signal.  IMHO, the same thing can be accomplished by reducing
the receiver bandwidth - it matters not whether it is CW or SSB.  If the
offending signal is on the low side of the wanted signal, it will be
necessary to reduce the low frequency content on the wanted signal - OTOH
with a voice signal, there will be a lot of information (intelligibility)
lost if the passband does not contain energy in the 300 to 500 Hz range, so
cutting the low frequency offending signal will result in a loss of
intelligibility for the wanted signal.  Cutting the high frequency end is
quite feasible, and can be accomplished easily by using the variable
bandwidth filter provided in the base K2.  I can set the normal IF filters
to a bandwidth of 1600 Hz and still maintain intelligibility for the male
voice (the female voice begins to loose intelligibility at a bandwidth of
1800 Hz or less).  Set the low frequency corner of the passband at 300 Hz
and accept whatever the high froequency end may be - this is not passband
tuning, but is reduction of the high frequency end of the received signal,
and I find it as useful (if not more useful) than true passband tuning.

I normall set the SSB IF filters to OP1 for the FL1 position, then 300 Hz
less for the FL2 position - note that the numbers displayed on the K2 may
not be a good indication of the actual bandwidth for wide filter settings,
use Spectrogram to determine the actual filter width.  With a 2300 (or 2400
Hz) OP1 bandwidth, I set Spectrogram markers at 300 and 2600 Hz then center
the passband between those markers.  Then FL2 thru FL4 are set to
progressively more narrow bandwidths - FL2 = 2200 Hz, FL3 = 1900 Hz, FL4 =
1600 Hz.  The important thing is to keep he low frequency corner of the
bandpass  at 300 Hz to maintain good intelligiblilty.  Be aware that the
filter bandwidths indicated by the K2 may be substantially different than
what is indicated by the K2 display - use Spectrogram to determine the
actual filter bandwidth - If the K2 indicates 2200 kHz for the filter
bandwidth, it may acually be 2600 Hz wide and quite ragged in the passband -
sett it for the actual width as observed o the Spectrogram display.

OK, all the above is valid for SSB - for CW, I find it sufficient to simply
switch to a more narrow IF filter - if you center the passband at your
chosen sidetine pitch, bothe the low and high requencies will be reduced by
switching to a more narrow filter.


73,
Don W3FPR

> -----Original Message-----
> Are IF shift and passband tuning (PBT) the same thing?  I assume
> that they produce the same effect.  I have a Ten-Tec Argonaut V
> that has PBT, but I have never used that feature, thus I don't
> miss it on my K2.  I guess I just don't grasp what PBT would do
> for me that I can't do with RIT and the variable width DSP filter
> of the Argonaut V.  However, I'm a CW op; someone commented that
> IF shift is more useful on SSB than on CW.  Why is that?
>
> Thanks, 73 & 72,
> Jeff
> WB5GWB
> Long Island, NY
>

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[wayne burdick]

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


---

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[Ken Alexander-2]

--- wayne burdick <[hidden email]> wrote:

> 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.

I'd love to go for a ride in a biplane, but you'll
never catch me going near a semi-plane!

"Is that the same as the difference between flammable
and inflammable?  Boy, I found out about THAT one the
hard way!" -- Woody from Cheers

73,

Ken Alexander
VE3HLS
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[jmeade]

The first time I saw the term "passband tuning" was on the Collins 75A4
receiver.  It shifted the BFO in one direction and the main PTO in the other
direction. The two were linked together with a mechanical strap. The idea
was that the pitch of the received signal would not change nor would the
actual selectivity bandwidth.  It was useful in both CW and SSB.  The
background pitch changed, but not the tone of the signal being received.

Then the Drake receivers came along, and did the same thing with the
variable L/C filters in the 50kHz IF.  A different approach, but the effect
was the same. The passband could be adjusted from one side of zero beat to
the other.

I thought that PBT a great feature.  Then I got a Kenwood TS-930S which had
"VBT" - "Variable Bandwidth Tuning".  This feature allowed for the
adjustment of the difference between two edges of two filters - like opening
and closing a sliding door - the "gap" could be adjusted wider or narrower,
thus changing the selectivity.  The center frequency does not change. You
get continuously-variable selectivity this way and can respond to any band
condition easily. Now I am used to that feature and miss it on other rigs

73,  John W2XS.
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[Darrell Bellerive]

Could the SSB filter on the SSB Option board be made into a variable-passband
filter rather than modifying the CW filter?


On August 8, 2006 11:18 pm, wayne burdick wrote:
> 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.

--
Darrell Bellerive
Amateur Radio Stations VA7TO and VE7CLA
Grand Forks, British Columbia, Canada
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[Geoffrey Mackenzie-Kennedy-2]

 On Wednesday, August 09, 2006 at 7:18 AM Wayne Burdick wrote:

 A much better approach to the "Filter Shifing" problem would be to
 actually *narrow* the first crystal filter, protecting all subsequent
 stages.

 -----------------------------------------------------------------------

May I second Wayne's suggestion. In practice I have found that it is very
 useful to be able to select roofing filters having different bandwidths, at
 the moment down to 1.5 kHz, with good stopband attenuation. In spite of the
 protection offered by *narrow* roofing filters, I believe that it is wise
to
 have all the stages before the second IF filter(s) capable of handling
 strong signals, i.e. IIP3s of  +40dbm or better, to reduce the crud
 generated by those unwanted signals that are very close in and get through
a
 narrow bandwidth roofing filter. One price is high current draw which is
 probably not acceptable if the rig is to be used portable. The phase noise
 of the LOs must also be suitably low to avoid compromising dynamic range
 etc. Lastly the filters themselves must also be capable of handling strong
 signals.

 73,
Geoff
GM4ESD




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