On Fri, Sep 10, 2010 at 5:11 PM, Jason Abele <ja...@ettus.com> wrote:

> On Fri, Sep 10, 2010 at 03:59:34PM -0400, Scott Storck wrote:
>
> > The other option that would work for me is connecting antenna 0 to WBX 0
> and
> > connecting antennas 1-3 to WBX 1 through a commutator to switch them at a
> > sub-1 kHz frequency.  Can you tell me if any of the unpopulated headers
> on
> > the WBX board are digital outputs that could potentially be used to
> generate
> > a switching signal and/or low rate ADC inputs that could be used to
> sample a
> > switching signal generated elsewhere?
>
> You could already switch between the 4 antenna ports (2 on each WBX)
> using the antenna switches built into the WBX at sub-kHz speeds.  If you
> really needed to solve the antenna switching problem externally, there
> are GPIOs available on the some of the 0.1" pin headers, check the
> schematics here:
>
> http://code.ettus.com/redmine/ettus/projects/public/documents
>
> Thanks, I'll definitely try to use the internal switching, but I'm not sure
if it'll work for my application--my original goal was to measure relative
phase and amplitude of antenna 1 vs 2, 3, 4.
Looking at the schematics, I see that the several of the WBX's unused
io_rx/tx signals (io_rx[15:14] and io_tx[15:8]) are available on the
simple_gdb's J15.  Am I on thre right track?  If so, that will be perfect.
Marcus mentioned that GPIO support was "spotty" though.  From the USRP FAQ
page on FPGA registers (
http://gnuradio.org/redmine/wiki/gnuradio/UsrpFAQFpgaRegs) it seems like it
would be straighforward to toggle two of the pins to control the
commutator.  Is there something I'm missing?


> > Finally, what it would take for 2 USRPs to operate a total of 4 WBXs
> > coherently?  I assume this would at least require connecting Clock Out on
> > USRP0 to Clock In on USRP1, but is that enough to get all 4 WBX PLLs
> locked
> > together?  Also, does using two USRPs simply mean that the max USB data
> rate
> > for each USRP is halved, or is the effect more severe?
>
> If you want 4 coherent RX using WBX, what you want is 4 USRP2's, 4 WBX's
> and a GPSDO to supply 10MHz Refclock and PPS signals to all the USRP2's.
> You would also need 4 separate Gigabit ethernet ports on your host PC.
> Then, you will have 25Msps from each receiver.
>
> That sounds good, but I'm afraid I don't have the budget for that much
hardware on this project.  I can afford another USRP and 2 more WBXs
though.  Can you tell me how close would that get me to my goal of 4
coherenet receivers if I use the first USRP as the reference for the second?
>From what I've read, it sounds like the 4 PLLs will not drift relative to
each other, but that there will be random phase offsets that must be
calibrated out after each tune?  If so, that would be fine as I already need
to do a calibration before each data collect to account for other sources of
phase offset.
The signal I'm trying to measure is less than 200 kHz bandwidth, so I can
afford to decimate by 128 in the USRP.  Hopefully this means that USB
bandwidth won't be a problem even with two USRPs on the same PC?

Thanks,
Scott
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