EMRFD Message Archive 185
Message Date From Subject 185 2006-11-07 10:44:15 Nick Kennedy Up-conversion If one were designing a (conventional; analog) receiver or
transceiver for HF, what are the advantages of up-converting to an
IF around 70MHz? I've been re-reading Ulrich Rohde's QST articles
and see that scheme in his analysis of a TS-50 and in his own
proposed design. I guess it's common these days.
I can see that having the LO tune one continuous range, always in
the same direction, might make you want the IF to be out of range of
the desired band.
I ask because of the necessity of having a narrow (as possible),
steep sided filter following the IF. For a ham of modest means and
test equipment, a 9 MHz filter would be much easier to design than a
70 MHz one.
So are there reasons other than what I stated for preferring the IF
so high? Before I even push the 'send' button, I suppose that image
rejection is going to be a big one. What else?
186 2006-11-07 12:14:42 larry allen Re: Up-conversion Actually, having an upconverter as you first IF is a good idea...This way
you can also have your receiver receive frequencies like 2 meter and 450..
The first IF is high enough to descourage images....
Second, I would not worry about needing a narrow filter at 70 mhz... as
most receiver designs put their filter money into the lowest IF... eg
digital IF's tend to be as low a frequency as practically possible.. eg
IF's of 70 mhz... mixed down to 9 mhz .. mixed down to 2 mhz.. mixed down
to 455.. mixed down to 100 khz... With technology as refined as it is
mixer noise is very low....
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188 2006-11-07 16:30:35 jr_dakota Re: Up-conversion Image rejection and eased front end filtering requirements ... also it
makes continuous LF-HF recepti
199 2006-11-12 13:54:57 Luiz Amaral Re: Up-conversion It is a great advantage to use an IF above the maximum receiving frequency
because it is possible to get a ?all HF? radio without the problems:
1 - reception of any frequency (if you have a 9 MHz IF, for example, signals
of 9 MHz cannot be received without zero leakage to the IF circuitry).
2 - The image rejection becomes much better because they are separated from
the desired signals 2 x IF, or 140 MHz.
Luiz - PY1LL
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