Table Of Contents

Coerced LO Frequency

Last Modified: May 22, 2018

Reads back the coerced local oscillator (LO) frequency, in Hz.

Data type: datatype_icon

Default: -1

Long Name: Configuration:Advanced:Coerced LO Frequency

Class: NI-USRP

Permissions: Read

Channel-based No
Resettable Yes

Considerations for Specifying Carrier Frequency and LO Frequency

The overall carrier frequency is realized in hardware as a combination of frequency translation in the local oscillator (LO) and an additional frequency translation implemented in the digital signal processing on the FPGA. You can allow NI-USRP to automatically calculate the value for the LO frequency, or you can specify it with the LO Frequency property.

If you only specify a value for the Carrier Frequency property, NI-USRP sets the LO frequency as close to the carrier frequency as possible and implements the remainder of the frequency translation using digital signal processing. Any time the carrier frequency changes, NI-USRP adjusts the LO frequency. However, each time the LO frequency changes, NI-USRP must wait for the LO frequency to settle before it adjusts, which causes a delay.

When changing carrier frequencies within a narrow range, you can minimize retuning time by locking the LO frequency to one value using the LO Frequency property. In this case, changes to the carrier frequency do not affect the specified LO frequency, and NI-USRP only changes the digital signal processing frequency to account for the difference between the carrier frequency and LO frequency. To unlock the LO frequency and allow NI-USRP to automatically calculate it, set the LO Frequency property to -1. Specifying the LO frequency allows you to offset the LO from the carrier frequency, which avoids LO feedthrough that can distort your signal.

The range of available values is hardware dependent. If you specify a value outside the range supported by your device, NI-USRP coerces the value to the nearest supported value. Writing to this property specifies the value you want to use. Reading from this property returns the actual value, which may have been coerced to the capabilities of the device. NI recommends that you read the value after you write to this property to determine the actual setting for the device.


The difference between the LO frequency and the desired carrier frequency must be less than half the device bandwidth.

Carrier Frequency = LO Frequency - DSP Frequency

For example, if you want to sweep through a 25 MHz band starting at 2.4375 GHz, set the LO Frequency property to a value halfway through the band using the following equation:

2.4375 GHz + (25 MHz / 2) = 2.45 GHz

When you set the LO Frequency property to 2.45 GHz, as the carrier frequency changes, NI-USRP only changes the DSP frequency as it sweeps through the bandwidth. The following figure illustrates how NI-USRP sets the frequency values on the first step of the sweep.

Where This Property Is Available:

Desktop OS: Windows

FPGA: Not supported

Web Server: Not supported in VIs that run in a web application

Hardware Support:

This property supports the following hardware:

  • USRP-2900/2901
  • USRP-2920/2921/2922
  • USRP-2930/2932
  • USRP-2940/2942/2943/2944/2945
  • USRP-2950/2952/2953/2954/2955

Recently Viewed Topics