Digitizing a digital data stream with USRP2

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.

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

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.

no changes are necessary. He can just set the tuning frequency to 0.

  1. 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.

There are no AD converters on the daughterboards, they are on the
motherboard. And they are 14 bits.

  1. 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.

We suggest turning off journaling in the filesystem. If that isn’t
enough, an SSD or RAID should be able to keep up.

Matt

On 02/17/2010 09:38 AM, John Carlson wrote:

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.

Yes, the LFTX and LFRX are the best boards for what you are looking to
do. You’d want to use usrp2_rx_cfile.py to record samples to disk.
Depending on your data rate and how long you intend to record for you
may need very fast disks. SSDs work well.

Matt

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