Can I Share LO Between Multiple USRP-29X5?

Updated Jan 20, 2021

Reported In

Hardware

  • USRP Software Defined Radio Device
  • USRP Software Defined Radio Reconfigurable Device
  • USRP-2945
  • USRP-2955

Software

  • LabVIEW Communications System Design Suite
  • LabVIEW Communications MIMO Application Framework
  • LabVIEW

Driver

  • NI-USRP

Issue Details

I'm currently using an USRP-2945/2955 and I know that it can share local oscillator(LO) between two daughter-boards in itself. 
Is it possible to share LO between multiple USRPs?

Solution

The USRP-2945 and USRP-2955 both have independent receiver channel that support LO sharing for phase coherency. It's possible sharing LO between different devices if your test environment satisfies several conditions. Full signal based synchronization using the USRP-29X5 needs 3 signals:
  1. PPS Trigger
  2. 10MHz Reference Clock
  3. Local Oscillator (LO) (power amplified)
     
To specifically answer why you need the PPS Trigger, it is used to ensure that data is actually generated starting at the same time. Like in TCLk, the USRP sends out a sync pulse, when a USRP receives the PPS Trigger it knows that it needs to start generating or acquiring on the next 10MHz clock edge.

The LO output port only provides enough power to drive a single TwinRX daughterboard.
So, external splitting and amplification of the LO output signal is required to drive any other TwinRX device.

Additional Information

If you use CDA-2990(Octoclock), the 10MHz clock and the PPS can be supplied by it, which makes the wiring and configuration a lot easier. However, they still need to use a PA and splitter to correctly route the LO.

If you are using a USRP-2945/2955 to supply LOs to other USRP-2945/2955 devices, you may require power amplification. 
Using LO Reimport, the connections for Multi-Device Reimport LO Configuration are:

  1. Connect the USRP-2945/2955 (master) LO OUT 1 IF2 back panel connector to an RF power amplifier IN connector using an SMA(m)-to-SMA(m) cable.
  2. Connect the RF power amplifier OUT connector to a four-way RF power splitter.
  3. Connect the four RF power splitter RF output connectors to the LO IN 0 IF2 and LO IN 1 IF2 back panel connectors of all USRP-2945/2955 devices.
  4. Repeat steps 1 through 3 using a separate RF amplifier and RF power splitter to reimport and share the IF1 signal.
  5. Share a common reference and PPS signal between the USRP devices using a clock distribution device CDA-2990.

Example configuration may use Mini Circuits ZFRSC-4-842+ RF Power Splitter and PXI-5691 RF Amplifier.