Configuration:Advanced:LO Frequency Property

NI-USRP Help

Edition Date: March 2018

Part Number: 373380J-01

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Short Name: LO Frequency

Property of niUSRP

Specifies the local oscillator (LO) frequency, in Hz.

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.

Note  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.

Default Value: -1

Remarks

The following table lists the characteristics of this property.

Datatype fD
PermissionsRead/Write
High-level VIs N/A
Channel-based Yes
ResettableYes

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