USRP phase synchroniz​ation over large distance

[matthewsmb]

[Please excuse the redundant post: here seems to be the forum of choice.] Problem: Phase synchronize several USRP devices with large separation (>100m). Carrier frequency is about 1 GHz. Time-varying properties of the physical media preclude cabled Octoclock sync distribution even with frequent recalibration. Is there a preferred method of phase sync distribution over large distance (laser, fiber optic)? Seems that even optical fiber has stretching properties on the order of 100 ppm.

[kevins33]

Hi,

Which USRPs will you be using? The best way to synchronize USRPs separated by large distances is with the GPS-disciplined clock. The USRP 2930 and USRP 2932 have an integrated GPS-disciplined clock. Please see this document for more information: https://www.ni.com/en/shop/wireless-design-test/what-is-a-usrp-software-defined-radio/global-synchro...

Thanks!

[matthewsmb]

Thank you, Kevin. We require phase synchronization to 1mRad at 1 GHz, which I beleive is unachieveable from GPS with ~100m of separation. 

[muffin.vi]

Maatthewsmb,

Based on my calculations, 1mRad synchronization at 1GHz is equivalent to 15.91 pS of drift. Is that correct, or did I mess up the math? The GPSDO functionality of the USRPs is something like +/- 25 ppb without GPS lock, and <1 ppb with GPS lock. So, if everything went perfectly with the GPS lock, the clock accuracy would be close, but not quite to your specified synchronization levels. 

Why do you need such a high level of synchronization? You might be able to use another form of synchronization, but you would need some non-NI parts to do so. 

[ErikL]

You can't guarantee phase sync over large distances, but it can be 'good enough' over short time intervals. (applicaiton dependent of course.)  The best way to syncrhonize two devices is always by cable.  GPS is very good, but does not provide the same level of precision.

[ErikL]

Clarification: The first is related to GPS.

You can't guarantee phase sync over large distances with GPS, but it can be 'good enough' over short time intervals. (applicaiton dependent of course.) The best way to syncrhonize two devices is always by cable. GPS is very good, but does not provide the same level of precision.