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Re: [Discuss-gnuradio] 4 RX with 2 WBXs?

From: Scott Storck
Subject: Re: [Discuss-gnuradio] 4 RX with 2 WBXs?
Date: Fri, 10 Sep 2010 21:38:06 -0400

On Fri, Sep 10, 2010 at 5:11 PM, Jason Abele <address@hidden> 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:


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?


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