On Wed, 18 Aug 2004 12:18:36 GMT, Liberals for Guns
wrote:

Can someone help me understand something about IF vs synch. I've read
that IF shifting can get rid of an interference that is occurring
slightly lower or higher than the desired frequency. So, if you've
got noise within the filter range and it's on the lower side of the
frequency than you simply shift everything up so the noise is out of
the "window". My question is that I'm not sure why you just couldn't
use sync in to get rid of the interference. By just listening to the
upper sideband the noise wouldn't be there. Right? Do I need to pay
for an advanced feature like IF shift when I could get away with
simply using a sync detector function?

One of the values of synchronous detection is that it can be effective
even on signals within the passband. Shifting the passband can result
in 'blocking'. In effect the intefereing signal may be outside the
passband, but is still seen, and drives the AGC, effectively reducing
the level of the desired signal.

It is also rare in most receivers for the skirts on the passband to be
sharp enough for this to work well. Usually a Q multiplier set on null
is a better approach, you just put a black hole in the passband at the
desired spot.

Synch other attraction, and perhaps the most attractive features is
something that passband tuning cannot help with. Selective fading of
the carrier. Sometimes it isn't really interference it is selective
fading causing the sidebands to have more energy than the carrier,
causing severe distortion. Synch detection insures adequate carrier
energy.

yes, interestingly, there was an article in 73 magazine IIRC some years
back which described how to retroactively add IF shift to some older radio
receiver(s) using only a handful of parts, basically just add ability to
tune or shift the IF oscillator up or down in frequency so as to move it
w.r.t. the IF chain etc. I had envisioned a far more complex circuit,
based on its relative rarity, but it turned out to be a lot easier circuit
than I expected...

fyi - bob monaghan
--
************************************************** *********************
* Robert Monaghan POB 752182 Southern Methodist Univ. Dallas Tx 75275 *
********************Standard Disclaimers Apply*************************

B i l l E v e r h a r t wrote:
On Wed, 18 Aug 2004 13:28:20 GMT, Doug Smith W9WI
wrote:


IF shift allows continuous adjustment of the passband, rather than just
selecting upper or lower. You could knock down interference 2KHz below
the desired station without deleting the entire lower sideband. Or,
while monitoring a SSB station.


Could you knock out the 2KHz interference with a notch filter as well?

Only if it's a single-frequency carrier. (like a Morse Code signal, or
a spurious carrier from a computer or something like that) A notch
filter removes a particular frequency very well but doesn't do much
against signals on other frequencies. Another SSB signal 2KHz below the
one you're wanting to monitor cannot be "notched".
--
Doug Smith W9WI
Pleasant View (Nashville), TN EM66
http://www.w9wi.com

Here in Sactown, Radio Australia's 9.590 mhz signal has an annoying hetrodyne
which can't be eliminated in either upper or lower sync sideband on a Sony
2010. On ICOM's R-75 hower, in regular am mode, that het whistle can be
eliminated with passband tuning.

yes, interestingly, there was an article in 73 magazine IIRC some years
back which described how to retroactively add IF shift to some older radio
receiver(s) using only a handful of parts,

Sounds like a worthy article, and a VERY worthy project. Is there somewhere I
could find this article online (or a similar project for retro-fitting IF
shifting to older receivers)?
Linus

GrtPmpkin32 ) writes:
yes, interestingly, there was an article in 73 magazine IIRC some years
back which described how to retroactively add IF shift to some older radio
receiver(s) using only a handful of parts,

Sounds like a worthy article, and a VERY worthy project. Is there somewhere I
could find this article online (or a similar project for retro-fitting IF
shifting to older receivers)?
Linus

Assuming the same issue had an article about adding an FM detector to
old shortwave receivers, I don't think the "IF shift" article is
worth tracking down.

Unless the description was horrible, the thing made no sense. My recollection
is that he put a varicap across an IF transformer, and called that IF shift.
Tha author showed no understanding of what IF shift was.

Michael

I'll try to find the cite to the 73 article, it is in my articles files
IIRC ;-) Can't be that complicated*, but whether it will work well with
all older radio receiver designs? Should be available via interlibrary
loan, or I can mail you a photocopy if you can't find or get it locally?

from google,
http://www.universal-radio.com/catal...hf/3450if.html
another reference to the 73 article,
http://lists.contesting.com/archives.../msg00290.html

grins bobm

*Since my Drake R4B has IF shift, in a tube/transistor hybrid, it can't be
that complicated to have IF shift in your radios ;-) ;-)
--
************************************************** *********************
* Robert Monaghan POB 752182 Southern Methodist Univ. Dallas Tx 75275 *
********************Standard Disclaimers Apply*************************

but whether it will work well with
all older radio receiver designs?

I guess I should have been a little less vague. I wouldn't want to add an IF
shift function to a tube model, I just thought it would be neat to try to
install such a function on a more modern radio which didn't have that feature.

Should be available via interlibrary
loan, or I can mail you a photocopy if you can't find or get it locally?


I will look around, starting from the leads you provided. If you do find a
copy, let me know. I'll pay for the trouble! :-)
Linus

(Bob Monaghan) wrote in message ...
I'll try to find the cite to the 73 article, it is in my articles files
IIRC ;-) Can't be that complicated*, but whether it will work well with
all older radio receiver designs? Should be available via interlibrary
loan, or I can mail you a photocopy if you can't find or get it locally?

[stuff deleted]
grins bobm

*Since my Drake R4B has IF shift, in a tube/transistor hybrid, it can't be
that complicated to have IF shift in your radios ;-) ;-)

But there's a world of difference between something built from the ground
up, and trying to modify an existing radio.

The basic concept is tuning the receiver so the unwanted signal is out
of the IF passband. Technically you could do it manually, and indeed the
concept existed before receivers had it as a feature. For an AM
signal, it is simply a matter of mistuning the receiver; people doing
this without thinking of the process. But once you "mistune" the
receiver so the unwanted signal is knocked out, an SSB or CW signal will
not be tuned properly. You have to adjust the BFO so it beats the wanted
signal down to it's proper frequency. Imagine tuning the receiver up
1KHz, what was a 1KHz beatnote before now becomes a 2KHz beatnote, so
you've got to set the BFO so it's again a 1KHz beatnote.

Go back enough decades, and the BFO was tuneable, so one could play around
with this. But once BFOs became crystal controlled, it wasn't possible and
even with a tuneable BFO the technique can be cumbersome.

I seem to recall that one of the early Collins SSB receivers had it,
and did it simply by having a control that tuned both the main tuning a
bit and the BFO frequency. But that requires a tuneable BFO and an analog
main tuning.

One, or some, of the Drake receivers actually tuned the filter. This
was possible because those receivers used an LC-tuned filter at 50KHz,
and so unlike a crystal filter, one could add a front-panel control
to tune that filter.

But most applications of IF shift require some kind of conversion stage,
either in the signal chain or outside it, in order to use the BFO as
both the BFO and a control to shift the IF passband.

The Drake R4-C (I couldn't easily dig up a block diagram for the "B")
has three IF frequencies, one at 5.645MHz (a somewhat wide crystal
fitler), one at 6.695MHz (a mode-specific crystal filter there)
and the third at 50KHz (an LC filter).


A B
-----5.645MHz---Mixer---5.695MHz---Mixer---50KHz---Product
filter | filter | detector
| | |
| fixed |
| osc |
| ---------------------------------|
variable
50KHz
oscillator

So as the oscillator signal into Mixer A is varied, it tunes the
rest of the receiver through the passband of that 5.645MHz filter.
But, since that oscillator also feeds the product detector, the
beat note remains constant.

One can get away without so many conversions in the signal chain,
but it still requires mixers. The Kenwood TS-820, for instance,
is single conversion. But the BFO signal is mixed with the local
oscillator signal (and filtered carefully) before going to the
mixer between the antenna and the IF stage. So again, as
the BFO is varied, the filter is moved but not the beat note.

There is no simple way to add IF shift to a receiver. At the very least
it requires bringing the IF out, and passing it through some stages
of mixers and a filter, and then feeding it back, into the receiver.
I've seen such articles, and it risks degrading the receiver, if you
don't use the right conversion frequencies or good mixers.

Or you modify the whole local oscillator chain, so it works like
the Kenwood above.

Neither is an easy step.

Like I said, my recollection of that 73 article was that it was
nonsense. I'm sure it simply added a varicap to an IF transformer
in the existing receiver, as if tuning one LC circuit would allow
for tuning the whole IF strip. Even if I'm misremembering, or
missed the explanation (as I said, the article did not give a good
indication that the author understood IF shift), merely making an
oscillator variable does not make for IF shift.

Michael

yes, I found the brief article and the cite is 73 Amateur Radio Magazine,
August 1989, p.53, "IF Shift, Cheap - Easy IF Shift Add-on to your Older
Rig" by Terry F. Staudt, LPE, W0WUZ (LPE is licensed professional
engineer, IIRC?). He reviewed a number of 1970s upgraded receivers (A, S,
or Mk II versions ;-), and discovered how they did IF shift or passband
tuning in those updated models. The modification uses a varicap Motorola
MV 1872 and a pot and trimmer capacitor to vary the first IF transformer
frequency. He added it to a Galaxy V mk II transceiver, noting "the
circuit works wonders". You do have to make two adjustments to the
modified IF transformer to preserve selectivity etc., which he describes
briefly ;-)

If you can't find the article locally, let me know, email me directly with
your address to and I'll mail you a photocopy.

Drake IF tuning, IIRC, the R4C had an odd "feature" (aka bug ;-) in
that it didn't provide IF or passband tuning in all modes, as the earlier
Drake R4B did, although you could modify the R4C to do so (and other
mods too).

I probably shouldn't note that I found another 73 article in my search
that discusses how crystal filters could be built to provide any desired
CW or SSB bandwidth (though you might have to grind a crystal or two to
get precisely the bandwidth you want ;-) ;-)

grins ;-) bobm

--
************************************************** *********************
* Robert Monaghan POB 752182 Southern Methodist Univ. Dallas Tx 75275 *
********************Standard Disclaimers Apply*************************