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RE: In band transponders



> 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.

It's not that bad.  The filter in the receiver side of the duplexer just needs to attenuate the transmit signal to a level that it doesn't overload the receiver.  And the filter in the transmit side of the duplexer needs to have enough attenuation at the receive frequency to reduce transmitter broadband noise below the receiver noise level.  With good transmitter and receiver design, something on the order of 50 dB or so should be sufficient for both filters, assuming a 1W transmitter.

> 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.

I don't believe you can buy a 24-bit A/D converter that runs at multi-MHz sample rates.  Above 1 msps, around 16 bits seems to be the state of the art.
http://www.analog.com/en/subCat/0,2879,760%255F788%255F0%255F%255F0%255F,00.html

You can do better by oversampling.  Each factor of 2x oversampling buys you half a bit (3 dB) effective resolution.  For example, if your RF bandwidth is 100 kHz and you are using a pair of 100 MHz A/Ds with 14-bit performance in a quadrature demodulator, the effective resolution theoretically would be about 19 bits, but you likely would't do quite that well in practice.

Al N1AL

> -----Original Message-----
> From: owner-AMSAT-BB@AMSAT.Org [mailto:owner-AMSAT-BB@AMSAT.Org]On
> Behalf Of David Carr
> Sent: Thursday, October 21, 2004 11:14 PM
> To: amsat-bb@AMSAT.Org
> 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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