[time-nuts] Commercial software defined radio for clock metrology

Sherman, Jeffrey A. (Fed) jeff.sherman at nist.gov
Wed Jun 1 11:45:56 EDT 2016

Jim Lux:
If you pick the right USRP models, you can lock the sampling clocks
together or distribute the clock.  I don't know if that distribution is
sufficiently high quality for time-nuts kinds of applications.

A bit of extra detail related to this but not reported in print... The N210 has two means of locking the sampling clocks of two SDR units together. First, a 10 MHz reference signal can be split and input into the two units' reference ports. Second, a "MIMO link" can be made between the two units with a SFP-style "direct-attach" cable. I don't know the details of this digital link, but it supports data transfer (so only one of the two SDR units requires an Ethernet connection) and reference frequency/time transfer.

Is the digital MIMO-link method any worse than the analog splitter method? Over short averaging intervals (1 us through ~30 us), we resolved no degradation in a 1-channel measurement. This is consistent with the advertised bandwidth of the PLL ~3 kHz. Over intervals 1 ms through 10 ms, the MIMO-link reference method resulted in about a factor-of-2 worse time deviation (in this test, the NCO was turned for a heterodyne frequency of approximately 8 Hz, which also leads to an oscillation peak in this range). Beyond 10 ms, both methods showed the same ~150 fs flicker floor that we've attributed to the ADC aperture jitter.

Finally, the SDR also has a PPS input, which can be used to "name" a 100 MHz master clock edge as an epoch (with 10 ns resolution). Although I didn't test this, I think this epoch can be synchronized over the MIMO link.

Best wishes,

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