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Re: [Discuss-gnuradio] FPGA time

From: mleech
Subject: Re: [Discuss-gnuradio] FPGA time
Date: Wed, 29 Aug 2012 15:18:14 -0400
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On 29 Aug 2012 15:13, Josh Blum wrote:

On 08/28/2012 03:24 PM, Anisha Gorur wrote:
Sorry for the confusion. We are trying to synchronize 3 usrps for collect. The devices seem to be time aligned in that the samples are timestamped with the same metadata, so we believed that synchronization had been achieved. However, when the data collected from the usrps was correlated, samples that should have had 0 time difference of arrival were off by as many as 5 samples, at a sample rate of 6.25MS/S. So even though the timestamps are time aligned, the data does not seem to be. The devices have been synchronized PPS times, not uspr.get_time_now(). Thank you, Anisha
Can you tell me more about the correlation? Are you sending a impulse
split to all 3 devices and determining the pulse arrival. Is the error
in time of arrival consistent between runs or does it seem to be random?

If you ask all N USRPs to stream at time X, the time reported in the
metadata will still be X, even if the internal tick count in each device
is not marching in lock step.

I have a few suggestions:

1) I think you have 1 GPSDO per USRP providing each a different
reference. I would first try the experiment with a shared 10 MHz
reference and PPS to all devices to confirm the algorithm. You will need
to move the 10 MHz reference jumper back so you can provide an external
ref via SMA.

I've observed, in a previous life, phase-hits between two GPSDOs connected to the same antenna, watching the same cluster of satellites.  Never figured out why, which is why for phase-sensitive work, it makes sense to use a common reference, (like an external GPSDO), rather than a GPSDO-per-unit.



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