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
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