[USRP-users] Why does the external 10MHz clock only works if I disconnect the 1-PPS signal?

Marcus D. Leech mleech at ripnet.com
Mon Oct 9 23:33:30 EDT 2017


On 10/09/2017 07:07 PM, Michel Chauvin via USRP-users wrote:
> Hi,
>
> I've been trying to use an external clock with the USRP B210 board. We have a time server which is locked to a GPS signal which provides both a 10MHz output and 1-PPS output.
>
> When I connect both 10MHz and 1-PPS output and use the external clock source the USRP B210 and OAI eNodeB have issues. (I get a large number of uplink re-transmissions between a Huawei B593 UE and a OAI eNodeB and a large number of these re-transmissions fail to decode completely which eventually leads to the UE being disconnected).
>
> Interestingly, if I disconnect the 1-PPS signal from the USRP B210 and only leave the 10MHz clock from the time server connected to the USRP B210, the USRP B210 works perfectly!
>
> I understand the 1-PPS signal is for time stamp alignment.
>
> Q1: Do I need to use the 1-PPS signal or can we leave it disconnected?
> Q2: If yes in Q1, what benefit do we lose, if any, if we disconnect the 1-PPS signal from the USRP B210? Is it needed/used in when the USRP B210 is used with an OAI eNodeB?
>
Since the vast majority of us on here are completely ignorant about what 
the needs of OAI eNodeB are (or, TBH, even *WHAT* that is), it's hard 
for anyone
   to say whether you need 1PPS in your particular application or not.

The 1PPS signal is used as a 1-shot "trigger" to allow time-of-day 
synchronization among multiple USRP devices (via the set_time_next_pps() 
API call
   in the UHD API).  Whether your application needs that or not is very 
much application-specific.









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