Hardware options for synchronization

Question

Was wondering if someone could help "rank" these approaches for synchronization.
- TCXO with shared references/PPS (i.e. - MIMO cable or OctoClock)
- OCXO
- OCXO with GPS
The "standard" TCXO models require extra stuff (MIMO or OctoClock or external sources) to phase lock and time synchronize. 
The OCXO gives you better accuracy...but it is unclear the degree to which this has an effect on phase coherency (and over how long a time).  Presumably, you'd still need something like a PPS to time synchronize?
With the GPS disciplined devices, are you basically getting the phase coherency and time synchronization from the GPS signal rather than external 'stuff'?  Or are there instances where you'd still need something like the MIMO cable or OctoClock to further nail down timing and synchronization?
Brandon
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Answer 1

Hi Brandon,
Thank you for the extra information! I have been looking further in to this issue, and a lot of your question depends on your hardware and system setup. JamesB, awesome post! This is great information.  To add to this, for our GPS USRPS (NI USRP 293x) unfortunately you cannot use the Ref In inputs, so splitting out the external clock is not going to be an option on this hardware. With this in mind, option (c) will likely be the easiest since that is exactly how our USRP's work. Are you using an NI USRP? If so and you're trying to sync up 8 of these USRP's together, GPS might be the only real option. 
As far as direct cabling vs. GPS for accuracy, direct cabling is more accurate if you are using an external clock that is is more accurate than the onboard clock and if you're using a USRP that can use an external reference clock 
The NI USRP 2930 and 2932 both have OCXO's which alone are more accurate but there is no way to extract the 10MHz source to another device, except for a MIMO cable which is only good to use for 2 devices. Therefore the direct cabling method only exists for specific scenarios:
1. Using MIMO cable for up to 2 devices
2. Using an external clock, and a splitter.
However, the internal OXCO does not do you any good for more than two device synchronization. Using a GPS antenna, clock disciplining and a clear view of the sky will let each USRP utilize it's own OXCO and still be synchronized. So it is not just GPS, it is the act of using GPS information to adjust the already great OCXO time reference which required each device to adjust it's own clock. The following document shows the specifications for a USRP without a GPS antenna (just the OXCO accuracy) vs. the OCXO accuracy in conjunction with the GPS disciplining in section 4 (Synchronization with GPS Disciplined Oscillators on NI USRP 293x):
http://www.ni.com/tutorial/14705/en/
While I was looking in to this, I also found this forum post that had a similiar question:
http://forums.ni.com/t5/USRP-Software-Radio/Synchronization-of-three-USRP-2932/m-p/2835772
I hope this was helpful!
Stephanie S.
Application Engineer
National Instruments