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

jimlux jimlux at earthlink.net
Wed Jun 1 14:07:03 EDT 2016

On 6/1/16 8:45 AM, Sherman, Jeffrey A. (Fed) wrote:
> 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.


We were doing some work with the earlier USRPs and wanted to actually 
run the ADC tied to an external clock at a peculiar rate - which it 
turns out the USRP doesn't support: you can lock the internal clock to 
an external reference (or to a signal from another USRP), but there's 
that PLL in the mix, so you have the usual issues.  I think the MIMO 
link actually transfers the same clock around (so you can guarantee 
synchronous sampling).

But ultimately, we wound up going another direction - we needed the 
"external clock" input.

I think the MIMO transfer doesn't go through the PLL

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