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, -js _______________________________________________ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
