Re: [Discuss-gnuradio] multi usrp, 1 odd channel, HW problem?

[Steven Clark]

On Thu, Oct 14, 2010 at 12:47 PM, Matt Ettus <address@hidden> wrote:

Probe R201 on all 4 WBX boards with an oscilloscope while running.  One side of the resistor is ground and the other side is the reference clock.  They should all be around the same amplitude, and have the exact same frequency and phase.

Matt

On 10/14/2010 09:13 AM, Steven Clark wrote:

Hi all-

I have a multi-usrp setup with 2 USRP 1s and 4 WBX daughtercards. I have
performed the clock synching described here:
http://gnuradio.org/redmine/wiki/1/MultiUsrp

I'm doing 4-channel receive, using a power splitter to send the same CW
signal into all 4 d'cards, and tuning them all identically.

3 of the 4 channels are nicely phase-locked. The 4th seems a little...off.

ClockMasterUSRP (Serial #5821): Side A: "red" channel
ClockMasterUSRP (Serial #5821): Side B: "green" channel
ClockSlaveUSRP (Serial #2087): Side A: "blue" channel
ClockSlaveUSRP (Serial #2087): Side B: "black" channel <--- problem with
this guy

Please see these 3 images to see the problem:
http://picasaweb.google.com/steven.p.clark/MultiUsrpGlitches?feat=directlink

You can see the problem in both the frequency domain, and in the time
domain.

I tried swapping daughtercards around, and the problem is not tied to
any one daughtercard, but rather to slave USRP side B.

Any idea what could be causing this? Shouldn't the same clock be going
to both sides of the USRP? (why does blue look fine, but black does not?)

-Steven

Matt-

Sorry for the delay in getting back to you. I took a look at R201 on all dboards as you suggested. All 4 looked fairly similar -- a clean-looking 64MHz sine wave. Amplitudes were close, in the 700-800mV p-p range. Comparing between 2 dboards on the same USRP, the phases matched pretty well (within a few degrees I would say). Comparing from one USRP to the other, there was a definite phase offset (~90degrees). Since the SMA cable bringing the clock from master to slave is a few feet long, this seems reasonable.

What's the next step?

-Steven