Re: [Discuss-gnuradio] USRP and ADS-B

[ldoolitt]

Andrew -

On Wed, Nov 08, 2006 at 01:15:50AM +1000, Andrew Rich wrote:
> I bought a bitscope device which did 40MS/s and had a 32 kb buffer.
> Was disappointed and sold it.
> What I really need is a FPGA and fast A/D solution.
> What I am trying to do is 
> 1) Capture up to 200us of window data
> 2) Capture at a rate of 40MS/s or better
> 3) Being able to interface this to LINUX and use PERL to process data.
> 4) My voltage levels are up to 1 volt.
> 5) I am sampling the video signals from a RADAR receiver.
> 6) Trigger on a voltage change ?

You need 200*40=8000 words of capture, suppose 12 bits,
single channel?  That's 96 kBits.  Even a lowly XC3S400
(288 kBits) can manage that.  The EP1C12 on the USRP holds
234 kBits, so you should be able to manage.  The stock
USB interface logic absorbs 64 kBits (one 2k x 16 each for
transmit and receive), leaving 170 kBits for your use.

There are FPGA evaluation/prototyping boards with much larger
external memory chips, but they are more expensive and harder
to deal with.  Better to leave that for people who really need it.

Do you need DC response?  You need to know before selecting
a daughter card.  I, personally, detest working at DC.  With
only a 200 us pulse, you can probably AC couple, and do baseline
reconstruction in software.

In principle, you can do everything you want with a USRP
(probably plus a LFRX daughtercard), but you will have to
adapt the Verilog programming to your needs.  This group
can help you, but won't do it for you unless you pay
exorbitant consulting fees.

There is no existing GnuRadio/USRP firmware to do (6) Trigger
on a voltage change.  That will require thought, design,
simulation, and debugging.  If you understand what your
application needs, and what an FPGA can do, it is both
eminently do-able, and could be a lot of fun!

    - Larry