[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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