(Bob Monaghan) wrote in message ...
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

Thanks for digging that up, it saves me looking for the issue that's
somewhere in a box.

Then, as I remembered and had intended to write the magazine at the time,
it's not IF shift. At the very most it's a tuneable peak within the
passband of the receiver.

He's tuning a single tuned circuit. That IF transformer will not provide
much selectivity, more important won't provide good skirt selectivity.
One tuned circuit won't do that, even if the IF frequency was low enough,
he'd need to be tuning all the tuned circuits in the IF strip. And given
that there is a crystal filter, and in the example in the HF range, the
IF transformers cannot supply anything narrower than the crystal filter.

He's not adjusting the crystal filter so the unwanted signal falls out
of the passband. The crystal filter supplies the ultimate selectivity.
So the good shape factor of the filter is not part of the "IF shift";
he is only shifting a far inferior selective element through the
passband of that crystal filter.


The article can provide no solution to "easy" IF shift.

Michael

Hi Michael,

you may well be right, though there weren't any followup complaints or
correction IIRC on the article. With 73 magazine in bankruptcy court now,
it is too late for you to publish a correction ;-)

I don't think the idea was to replace or supplant the crystal filter skirt
selectivity with IF shift, or to change or shift the crystal passband.

Since some radio receivers only have one IF stage and one IF transformer,
as an example, shifting its frequency would be shifting the entire IF ;-)

Then again, some of the 455 Khz IFs use murata style resonators which are
at least as good and selective as some quartz crystal filters (so I'm
told, anyway ;-) In such a case, it would seem possible that detuning the
first IF transformer so an interfering signal is above or below the main
IF passband and out on those many dB down skirts, while leaving the
desired signal within the IF passband, with resonator skirts or subsequent
crystal filtering etc. providing the selectivity, as usual, could work?

Again, I can't reject the idea out of hand, for all radios or setups. When
an LPE (licensed professional engineer) tells me he has reviewed many
example cases and studied how IF shifting works, it carries more weight
than some of the stuff I see on and off-line ;-) ;-) At least the cost of
the experiment is low (~$5?), so it may be worth testing for some
experimenters?

regards bobm


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