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RE: SDR Prototype transponders



Answering a couple of questions about yesterday's SDX (Software Defined
Xponder) tests:

> Mode A would certainly be widely welcomed.

Our initial testing was Mode-A because we could make it happen with the
available SDR-1000 radios. One had a 2M transverter built in, while the
other only went thru 6 Meters with the stuff in the box. Our next tests
will be Mode-B with a 70cm converter in the "HF" radio input.



> I noticed that the transponder bandwidth was given as just 45 kHz,
> what is the reason for restricting it to this bandwidth rather  
> than the 100 or 150 kHz that has been used on previous satellites ?

The tests used "stock" SDR-1000s (see http://www.flex-radio.com/)
with the A/D and D/A conversion done using a high quality audio card
(M-Audio Delta-44) in the LINUX PC; the sampling rate was chosen
was the rather standard sound card 22 kHz (in both the I & Q channels, 
resulting in the 44 kHz bandwidth.  Needless to say, this is NOT the 
hardware we would fly, it was just a convenient place to start.



In case you are interested, the audio from the first SDX QSO can be
fetched from ftp://ftp.cnssys.com/pub/amsat/Eagle_SDT_1st_Contact.mp3.
In this audio clip you will hear Rick (W2GPS), Bob (N4HY) and me.
We were in Rick's shack on the 2nd floor, with the transponder in the
basement. The only "cheat" was that we were all sharing the same
microphone in Rick's shack ;<}

As Bob mentioned and as I stated a few weeks back, our plan is to have a
Mode-B version running at the AMSAT meeting in Louisiana in October. We
plan to be able to select a linear transponder vs. 2 or 3 FM/FM
"easysat" channels on either half of the passband, with an "AMSAT
Standard" 400BPS PSK beacon in the middle. With luck, the 2M TX in the
transponder will have enough power to be heard for a few miles around
the convention hotel. And, depending on the Internet connectivity at the
site, it is likely that we will have Echolink access from remote
locations and an 802.11g wireless LAN. By showing all these capabilities
running on one platform, we want to demonstrate the inherent flexibility
of the SDX approach.

Please don't think that this is a finished project! We wanted you on
AMSAT-BB to be able to participate in the first "birthing" of what we
think is a very exciting new project.

73 de Tom, W3IWI
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