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Re: In band transponders



The problem is that ADC dynamic ranges are the ratio between the largest and
smallest signals that they will detect. Its like blocking dynamic range for an
analog circuit. The ratio between a large signal and the distortion products
that the ADC generates is much smaller. 110 dB of harmonic distortion for a
single tone is the best that I've seen for audio ADCs and IMD isn't even
specified. ADCs that are capable of digitizing a 2 or 4 MHz band have IMD and
harmonic distortion that is 90-100 dB down. Of course these numbers are typical
at 25 C and the guaranteed specifications over 0-70 C are usually 10 dB worse.

73,

John
KD6ZOH

----- Original Message -----
From: "David Carr" <dc@dcarr.org>
To: <amsat-bb@AMSAT.Org>
Sent: Friday, 22 October, 2004 06:13 UTC
Subject: [amsat-bb] In band transponders


> I was reading Bob's notes on a 10meter in-band transponder and got to
> thinking....
>
> As I understand it, the reason that AMSAT doesn't fly coventional
> in-band transponders/repeaters is that amount of filtering required to
> isolate the uplink reciever from the downlink transmitter is too heavy
> or too dificult to achieve.  Think of the size of the required filter
> cavities used on a 2M repeater.
>
> Some numbers from the AO-51 link budget:
>
> Max range recieved signal level (2m): -107dbm
> TX output level at 1W: 30 dbm
>
> This means we need at least 137db isolation to get equal amounts of TX
> "noise" and recieved signal.  Thats a pretty tall order...
> I'd say this is a very tough spec to meet using conventional
> analog/cavity filters and we'd need a pretty big satellite to cram it in.
>
> Consider this... the theoretical maximum dynamic range of an
> analog-digital converter of B bits is (2B + 6)db.
> This means for a 24bit converter in theory we have 146db of dynamic
> range.  Using numbers from the above example, with digital filters this
> converter could yield 7db SNR in a situation where there was no analog
> isolation between the TX and RX.  Thats fairly impressive.  Moreover,
> all of this filtering can be done on a 2cm x 2cm DSP that consumes very
> little power.
>
> In actuality we have a few problems though.  Real converters don't
> achieve their full theoretical dynamic range (120-130db for a good
> converter).  Also 7db SNR isn't very good.  However it might be possible
> to build a reasonably-sized analog filter that can achieve a more modest
> amount on rejection (a few 10s of dB).  Also, using orthogonal RX/TX
> antenna polarizations might give us a little more.  Perhaps a bit of
> analog isolation in concert with a good analog-digital converter might
> make it possible to build a single-satellite, low power in-band transponder.
>
> I enjoyed playing this idea out in my head and thought that perhaps some
> others might be able to chip in on whether or not any of this is
> reasonable...
>
> 73s,
> David Carr
> KD5QGR
> ----------------
> Oscar Satellite Status Page
> oscar.dcarr.org
> ----
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