Re: [Discuss-gnuradio] digitizing a digital data stream with USRP2

[Jeff Brower]

John-

> I have a USRP2 and I'm interested in doing an experiment where I'd like to
> simply use the USRP2 as an A/D and D/A device, essentially disabling the RF
> tuning portions.  The signal I'm trying to digitize is a stream of digital
> TTL pulses.  I assume that the best way to interface to this is through the
> DC-30MHz LFTX and LFRX daughtercards that would pass DC.  Could anyone
> provide me with additional guidance on how to run this test?  Thanks in
> advance.

I'm not intimately familiar with GNU radio hardware and the experts here will 
hopefully correct me if needed, but I
think you would face these issues:

1) The default FPGA logic doesn't offer a demodulation enable/disable, so you'd 
have to make some logic changes and
rebuild.

2) Assuming the A/D converters on the LFTX and LFRX are > 8 bits and <= 16 
bits, then the maximum sampling rate you
could move over GbE would be around 60 MHz.  Is that enough for your 
application?  If you need faster rates but only
in bursts (like a radar situation) then possibly some memory (block RAM) in the 
USRP2 FPGA could be used for buffers. 
I'm not sure how much is available... a build without demodulation would 
probably free up some mem.

3) Continuous streaming to HDD file could be difficult, or at least would 
require careful configuration of the HDD
file system, real-time scheduling and other system considerations, and of 
course careful programming.

-Jeff