Re: [amsat-bb] LONG and long overdue report

[Tom Clark <w3iwi@xxxxxxxx>]

As an expansion of Bob McGwier and Rick Hambly "work in progress" notes 
telling about our (N4HY/AB2KT/W2GPS/W3IWI) transponder effort, let me 
spill some more of the beans about the initial Mode-A/B demo we plan. I 
have dubbed the activity the SDX (Software Defined Xponder).

The SDX Demo testbed consists of a pair of FlexRadio SDR-1000s connected 
to a LINUX PC equipped via a high performance (M-audio Delta-44)24-bit 
4-channel "sound card". SDR-1000 #1 is equipped with a built-in 2M 
transverter which feeds a few watt PA. SDR #2 listens on 70 cm thru a 70 
cm to 10M converter (or is connected to a 10M antenna for Mode-A).

When #2 is the RX and #1 is the TX, we will emulate AO-7's Mode-B 
transponder that the old timers remember so well. We plan on beacons in 
the middle of the ~50 kHz wide passband including a 400bps PSK signal 
with M-blocks. At the flick of a (software) switch, the transponder can 
be either all linear, or it can be half linear + half with 2-3 "easysat" 
FM (with the halves split around the beacon -- i.e. ~25 kHz wide each), 
or it can be all FM .

With the same switch, the linear transponder can be made inverting or 
non-inverting. Probably the only difference you will notice is that 
there will be a fixed fraction of a second RX-to-TX delay due to 
computer latency; this shouldn't be much different than the delays you 
experienced when HEO satellites were near apogee. We expect that there 
will be Leila-like AGC on the linear transponders so alligators won't 
kill it for everyone.

With a minor antenna cable swap, #1 can be used for RX and #2 for TX, 
making a Mode-A SDX transponder with all the same capabilities. Of 
course, since the entire transponder is defined in software, the entire 
personality can be reconfigured at will (remembering the old adage, IT'S 
ONLY $OFTWARE!!) ;<}

So when will this be on the air? This coming week is disrupted by the 
Surrey & Colorado Springs meetings, but I'm hoping it will come together 
in early August.

Our current "big" goal is to have the system on-the-air for all to 
see/hear/use during the October AMSAT meeting in Louisiana. It is quite 
likely that there will be local on-the-air tests in the MD/DC and NNJ 
areas also. We hope to have the LINUX box connected to the Internet and 
equipped with an 802.11x wireless LAN so that people will also be able 
to operate it as a virtual SDX. Plan to bring your radios.

We'd be happy for others to test clone the demo SDX in your area -- you 
will need to supply the two SDR-1000s plus a LINUX PC with Delta-44 
board. Let one of us know if you are interested.

73 de Tom, W3IWI
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