Share LO Signals in USRP-29x5 Devices

One can share the local oscillator (LO) signals either within one USRP-29x5 device, or between multiple USRP-29x5 devices. This article will go through how to set-up the hardware as well as software in order to share the LO signals, and some considerations that need to be made.

1. Set-up hardware like the following images below depending on the number of USRP devices used.
Note: Requires external amplification and splitting for higher quality LO signal.

For a single USRP-29x5 device: 


For multiple USRP-29x5 devices: 


2. Set-up software by first creating a LabVIEW project from the "NI-USRP Simple Streaming with USRP-2945/2955" sample project

3. Once project is created, open up the "Rx Streaming (Four Channels) (Host).vi" and set the LO Configuration for RF0 as Import and for RF1 as Reimport

The LO signals outputted from LO OUT1 (IF1, IF2) are amplified by an external amplifier (such as PXI-5691), and then go through an external splitter to input into the two sets of LO IN (IF1, IF2). 
Note: When the external amplification is removed, the LO level drops accordingly, which in turn makes it seem like the signal, inputted through the Rx terminal, has dropped. 

Next Steps

There are other possible LO configurations for sharing LO such as the following: 

• Independent—Each channel will use its own LOs. The LO input ports will be disabled.
• Import—Both channels will use the LOs provided at the corresponding import ports. If you are using an external LO, use the Coerced Configurations tab on any host VI front panel to see what frequency LOs are required for your configuration.
• Shared and Export—Both channels will use the same LOs from first channel. The first channel's LOs will also be exported to the LO output ports on the device back panel. NOTE: Only RF 1 can export to the LO output ports.
• Reimport—Both channels use the same LOs from the first channel. This mode offers maximum coherence between channels since it shares the same locally generated and reimported LO signal. The LO ouptut connectors on the device back panel receive the exported LOs from the first channel. Both channels may need external amplifiers to reimport this signal through the LO input connectors. You can share this LO signal across all the channels on the device through the LO input connectors using RF splitters and amplifiers. Note: Only the RF 1 daughterboard can export to the LO output connectors.
• Shared—Both channels will use the same LOs from first channel.

Refer to the following diagram for the possible LO configurations for the different settings: