On Wed, Oct 20, 2010 at 7:41 PM, Steven Clark <[email protected]>wrote:
> On Thu, Oct 14, 2010 at 12:47 PM, Matt Ettus <[email protected]> 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 > New info. All 4 channels look fine when capturing individually on them. Furthermore, frequency-wise, all the individual captures look different from their corresponding simultaneous captures. For example, with individual captures my CW input might show up as energy in FFT bin #514 of 1024 for all 4 chans, whereas with a combined capture, the energy shows up in bin #535 for Chans 1-3, and in bin #536 for Chan 4. This is consistent with all 4 channels dropping occasional samples when used jointly, but with chan 4 dropping them slightly more frequently than the others. So I would say the problem is not as simple as "one channel has HW problems", but rather there is something system-level going on. Sample rate was 250kHz, which should be plenty doable... Does anybody have any thoughts as to what might be going on? -Steven
_______________________________________________ Discuss-gnuradio mailing list [email protected] http://lists.gnu.org/mailman/listinfo/discuss-gnuradio
