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Re: Mode S - Mixing to baseband



David Green <david.green@BAEsystems.com> wrote:

>Just wondering if anybody has tried converting the
>mode-S beacon direct to baseband (audio)? Perhaps as
>a version of the R2 image-rejecting direct conversion
>receiver.
>
>Any obvious problems with this approach?

KK7B did a paper at Microwave Update 1999 on the subject.
Accurate phase quadrature takes some care, and direct-
conversion radios work lots better if there is no
oscillator running at the operating frequency. A
passive doubler feeding a modern (0 dBm or lower)
mixer is one approach.

This also permits mast-mounting of the RF stuff, with
the cables to the shack carrying baseband signals only,
permitting all kinds of mix-and-match mischief.
You can process the baseband signals as you wish.

S53MV has done a series of radios along similar lines,
but using the 3rd method (he calls it zero IF) instead
of phasing for image rejection. Have a look at the
1999 AMSAT proceedings (San Diego), or visit
<http://www.hamradio.si/notune.html>. The same issues
apply: Matjaz's radios use harmonic mixers for the
final frequency multiplication.

Laura Halliday VE7LDH     "Que les nuages soient notre
Grid: CN89mg                     pied a terre..."
ICBM: 49 15.042 N 122 59.053 W       - Hospital/Shafte

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