Configure LO Sharing

Since R2025a

NI™ USRP™ radios have multiple local oscillators (LOs) that support independent tuning of transmit and receive channels. By default, LOs are independent; they are not phase-coherent or phase-synchronized.

Note

To phase-synchronize multiple radios, the radios must be time synchronized. For more information, see Time-Synchronize Operations.

To achieve phase coherence or phase synchronization between channels that do not share an LO source by default, you can use some NI USRP radios that provide mechanisms for LO sharing. The table gives an overview.

Radio Device	LO Sharing Mechanism
USRP N310	Import LO signals for transmit (TX) and receive (RX) channel pairs from an external LO source
USRP N320	Import LO signals for TX and RX channels from an external LO source or USRP N321 radio configured for LO distribution
USRP N321	Onboard LO distribution that supports LO sharing scalable up to 128-by-128 channels alongside USRP N320 radios
USRP X300/X310 + TwinRX (since R2026a)	LO sharing between companion TwinRX daughterboards using MMCX connectors Multi-radio LO sharing by importing and exporting LO signals (requires chassis modification)

For details about the recommended connections and the phase stability that you can achieve with specific hardware configurations, consult the manufacturer's documentation.

To set up these radios for LO sharing, follow the instructions for your specific radio.

LO Sharing Using USRP N310 Radios

USRP N310 radios have four TX and four RX RF channels. By default, the radio uses four internally generated LOs that each serve two RF channels. To share an LO signal between all TX or RX channels on a single radio, or between channels across multiple radios, you can import an externally generated LO signal using the LO ports on the front panel of the radio.

These antenna ports correspond to each LO input port:

LO Input Port	Corresponding Antenna Ports
LO IN 0/1: TX	RF0 channel: TX/RX RF1 channel: TX/RX
LO IN 0/1: RX	RF0 channel: RX2 RF1 channel: RX2
LO IN 2/3: TX	RF2 channel: TX/RX RF3 channel: TX/RX
LO IN 2/3: RX	RF2 channel: RX2 RF3 channel: RX2

To configure LO sharing between multiple RF channels on a single radio or across multiple radios:

1. Connect an external LO signal to all LO input ports that correspond to the RF channels of interest. The frequency of the external LO signal must be twice the center frequency.

2. In the Configure Radio step in the Radio Setup wizard, set LO Source to external for all radios.

During initialization, the radio calibrates the LOs using a 5 GHz external signal. Perform operations in this order:

1. Before your radio is initialized, set the external LO frequency to 5 GHz.

   • If you are using a baseband or detector application object, the radio is initialized when you create the object specifying the Preload name-value argument. For example:

     bbtrx = basebandTransceiver(radio,Preload=true,PropertyName=Value)

     If you do not specify the Preload name-value argument or you set it to false, the radio is initialized the first time you call the transmit or capture function.

   • If you are using a usrp System object™ with a custom bitstream generated using the targeting workflow, the radio is initialized when you call the setup function.

2. Initialize your radio. The LOs are calibrated using the 5 GHz external LO signal during initialization.

3. Set the external LO to the desired value.

   • The supported external LO frequency range is 600 MHz to 8 GHz, which corresponds to a center frequency range of 300 MHz to 4 GHz.

Note

Phase coherence is not maintained after you retune or reinitialize the radio. Repeat the LO and phase calibration after you update an antenna or sample rate property on the application object or usrp System object.

LO Sharing Using USRP N320 Radios

USRP N320 radios have two TX and two RX RF channels. By default, the radio uses two internally generated LOs to serve the TX and RX channels respectively. To share an LO signal between channels across multiple radios, you can import an externally generated LO signal using the LO ports on the front panel of the radio.

The LO IN:TX port corresponds to the RF0 and RF1 channel TX/RX ports and the LO IN: RX port corresponds to the RF0 and RF1 channel RX2 ports.

To configure LO sharing between multiple RF channels across multiple radios:

1. Connect an externally generated LO signal to all LO input ports that correspond to the RF channels of interest. You can use an OctoClock clock distribution module or an exported LO signal from another USRP radio.

   Note

   USRP N321 radios include an LO signal splitting and distribution board that distributes an LO signal for large MIMO systems using USRP N320/N321 radios. For details, see LO Sharing and Distribution Using USRP N321 Radios.

2. In the Configure Radio step in the Radio Setup wizard, set LO Source to external for all radios.

LO Sharing and Distribution Using USRP N321 Radios

USRP N321 radios include an LO signal splitting and distribution board that distributes an LO signal for large MIMO systems using USRP N320/N321 radios.

To configure your radio to use the internal RF source for channel 0 as a master LO source, set LO Exported to true in the Configure Radio step in the Radio Setup wizard. The radio routes the internal LO from RF channel 0 to a 1:4 splitter on the LO distribution board, which allows other modules to receive this LO.

To enable the output ports on the LO distribution board, set LO Distribution to true in the Configure Radio step in the Radio Setup wizard.

For information about how to connect your radio for LO distribution, see USRP N320/N321 LO Distribution.

LO Sharing Using USRP X300/X310 with TwinRX Daughterboards

Individual TwinRX daughterboards include two RX channels that, by default, are served by their own internally generated LO. They provide the capability to import and export each LO, which enables LO sharing between two TwinRX daughterboards on a single USRP X300 or X310 radio, or between multiple USRP X300 or X310 radios with TwinRX daughterboards.

Note

The TX/RX port corresponds to RX1 in the manufacturer's documentation. Wireless Testbench™ uses the front-panel label to refer to the antenna name.

Companion LO Sharing

To configure your radio to share an LO signal between both RX channels on an individual TwinRX daughterboard, set these LO synchronization options in the Configure Radio step in the Radio Setup wizard.

• Set one LO source to internal.

• Set the LO source for the companion channel on the same daughterboard to companion.

LO Sharing Between TwinRX Daughterboards on Single Radio

USRP X300 and X310 radios with two TwinRX daughterboards support LO sharing between the boards by using MMCX connectors. This configuration enables phase coherence across all four receive channels on a single radio.

To make the physical connections, open the chassis and attach four MMCX RA male cables between the LO ports on each daughterboard.

Make these port connections:

TwinRX in Slot A	TwinRX in Slot B
J1 (LO2 output)	J2 (LO2 input)
J2 (LO2 input)	J1 (LO2 output)
J3 (LO1 output)	J4 (LO1 input)
J4 (LO1 input)	J3 (LO1 output)

To configure your radio to share an LO signal between both TwinRX daughterboards on a single radio, set these LO synchronization options in the Configure Radio step in the Radio Setup wizard.

• Choose one of the internally generated LOs to act as the LO source for all channels. Set LO Source to internal and check the box to export this LO signal.

• For the companion channel on the same daughterboard, set LO Source to companion.

• For the channels on the other daughterboard, set LO Source to external.

This example uses the internally generated LO that corresponds to the RFA: TX/RX antenna as the LO source for all channels:

Corresponding Antenna Port	LO Source	Exported?
RFA:TX/RX (RX1*)	internal	true
RFA:RX2	companion	false
RFB:TX/RX (RX1*)	external	false
RFB:RX2	external	false
* Wireless Testbench uses the front-panel label for the antenna name, TX/RX, in this property. The antenna name in the manufacturer's documentation is RX1.

External LO Sharing

USRP X300 and X310 radios with TwinRX daughterboards can be modified enable external LO sharing. With a radio modified for external LO sharing, you can use the LO OUT and LO IN ports to share an LO signal between multiple USRP X310 radios with TwinRX daughterboards. For details, see Modifying an X310 Chassis for External LO Sharing. The application note describes the configuration options.

To configure your radio to import an LO signal, set LO Source to external in the Configure Radio step in the Radio Setup wizard. To configure your radio to export an LO signal, set LO Exported to true.

In the Configure Radio step in the Radio Setup wizard, use the LO synchronization settings to configure your radios for multi-radio LO sharing. For each TwinRX daughterboard on the radio, two RX channels correspond to the TX/RX and RX2 antenna ports. For each channel, you can select the LO source from the following options:

• "internal" — Use the internal LO signal of the radio.

• "external" — Use an LO signal from an external signal generator.

• "companion" — Use an LO signal from the companion channel on the same TwinRX daughterboard.

Additionally, you can choose to export an internally generated LO on the LO OUT port by selecting the Exported option.