NI-USRP: Using GPSDO OCXO as 10 MHz Reference

Updated Mar 10, 2025

Reported In

Hardware

  • USRP-2930
  • USRP-2932
  • USRP-2955
  • USRP-2953
  • USRP-2952
  • USRP-2954
  • USRP-2950

Driver

  • NI-USRP

Issue Details

I have an USRP with a GPSDO (293X or 295X), the datasheet gives a very good accuracy for it (ppb), but when I measure the accuracy with an external instrument, the value is far worse than expected (ppm)
I think that the USRP is not using the OCXO as a reference but the VCXO, how can I change it ?

Solution

By default, the USRP will use the built-in VCXO as a 10 MHz reference.
To configure the USRP to use the OCXO reference (which is part of the GPSDO component, mounted on 293X and 295X) you need to configure it using the driver.
 
LabVIEW 
Reference Frequency and Timebase Clock Selections

C API

niUSRP_SetAttributeString(TxSession, channelList, niUSRP_Attr_ReferenceFrequencySource, niUSRP_Val_Terminal_GPS);

Additional Information

You can use the OCXO reference even if no GPS signal is connected to your USRP
The GPS signal improve the OCXO long-term stability, but even without this improvement the OCXO will be better than the default VCXO

Related Links

Global Synchronization and Clock Disciplining with NI USRP Software Defined Radios - NI

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