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In band transponders

I was reading Bob's notes on a 10meter in-band transponder and got to 

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 

David Carr
Oscar Satellite Status Page
Sent via amsat-bb@amsat.org. Opinions expressed are those of the author.
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