[Discuss-gnuradio] sample time alignment in GRC

[Lapointe, Benjamin - 1008 - MITLL]

Hi,

 

I have two USRP X310 devices that I am trying to time align in GNU Radio Companion.  One X310 has a GPSDO that is sending 10 MHz reference and 1 PPS signals to the other one. The GPS is locked.  Ideally I would have matched length cables for 10 MHz reference and 1 PPS, but I think my setup is close enough. (Input signal from sig gen = pulsed 10.005 MHz, input is split with matched length cables, USRP output sampling rate = 5M, USRP center frequency = 10M.) 

 

I am using WX GUI Scope Sink to look at the magnitudes of each stream from the USRP devices.  I expect to see no/minimal delay between the two signal streams, but I am seeing delays of 24, 13, 9, 0, 3, 6, 25, 24 samples from run to run between the two signal streams.  The period of the signal is 50 samples, so the maximum delay difference is 25 samples.  Am I missing something in my configuration?  Since I am using a 10 MHz reference and 1 PPS signals, I expect time alignment between the two sample streams.  Is there a GRC block for forcing time alignment? 

 

Thanks!

-Ben

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