[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.
>
>


Interesting..

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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