On 09/04/2017 04:54 PM, Michael M via USRP-users wrote:
I'm having an issue with my Ettus E310 where I'm seeing inconsistent
timestamping from one run to another.
First, I set up a reference transmitter that is designed to output a
waveform at the top of each second. The transmitter has been tested
and shown to be within a few nanoseconds of GPS time with other test
equipment. This transmitter is connected to the RX2 input of the USRP
E310.
I am then setting the time source on the E310 to GPSDO and the clock
source to INTERNAL. I am waiting until the gps is locked and the
reference is locked. Once this is done, we are getting the gps time
from the E310 and setting the clock to what the GPS time should be at
the next 1PPS. This is all working fine and the time looks correct.
Then, I receive the signal, write it out to files with the time stamps
and correlate with the signal that is being transmitted by the
reference transmitter. Within one run, I'm getting a standard
deviation of under 50ns for arrival time relative to the top of each
second. If I stop receiving and start again, I am again getting a
standard deviation of under 50ns for arrival time. The problem is
that that arrival time varies as much as a couple of microseconds
between separate recordings.
My question is, can anyone think of what might be the cause of this
variance from one run to another? Could it be something I'm doing
wrong? Could the process of locking the digital PLL to the 1pps
reference have enough jitter that once locked it is tracking a value
that can be off by a couple microseconds? I'd like to be able to get
repeatable results from one run to another (within 100ns).
Thanks,
Michael
How are you starting your receive process? How is that time-coordinated?
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