The PXIe-5840 supports acquisition and generation of RF signals from to . For both the RF Input and RF Output channels, the transition from low frequency to high frequency is implemented with a passive diplexer scheme. This passes RF energy below 120 MHz to the low frequency path and RF energy above 120 MHz to the high frequency path. Because the diplexer has a frequency response of its own, it is possible to extend RF frequency coverage above or below 120 MHz in both paths.



While the automatic selection of the signal path using the NI-RFSA or NI-RFSG driver is optimal for most use cases, some applications may require you to select a particular mode of operation. Example situations where this may be useful include measurements or applications sensitive to speed, phase noise, relative gain, or dynamic range.

For example, the high frequency path is implemented with a wide range of amplification and attenuation as a result of more stages within the signal chain. This is useful for applications that may require a significant amount of analog attenuation or amplification. The high frequency path also mixes the RF signal to and from baseband, so phase noise may be degraded compared to a directly sampled signal. Some performance parameters may have more or less impact depending on the nature of the application. Refer to RF Input Subsystem and RF Output Subsystem for more information about the PXIe-5840 components and frequency paths.

RF Input Signal Path Selection

The primary method of controlling which path is used on the PXIe-5840 is based on center frequency. For RF Input, this frequency is tied to the NI-RFSA IQ Carrier Frequency property in I/Q mode. Refer to the NI RF Vector Signal Analyzers Help for more information about using the PXIe-5840 in I/Q mode.

The NI-RFSA Downconverter Center Frequency property, NI-RFSA LO Frequency property, or NI-RFSA Downconverter Frequency Offset property can be used to further specify which path the device uses in addition to the NI-RFSA IQ Carrier Frequency property. You can use the NI-RFSA Downconverter Center Frequency property, or, for an external LO, the NI-RFSA LO Frequency property to lock the device into the low-frequency or high-frequency path despite the value specified for I/Q carrier frequency. When you set the NI-RFSA Signal Bandwidth property, NI-RFSA may choose a downconverter center frequency for you. Refer to the Automatic Frequency Offset section for information on how to automatically offset the downconverter frequency.

Note The downconverter center frequency is coerced to 0 Hz when it is set to any value less than 120 MHz. Refer to the Programming Reference section in the NI RF Vector Signal Analyzers Help for more information about the NI‑RFSA Downconverter Center Frequency property.
Figure 1. I/Q Carrier Frequency or Center Frequency Set to 100 MHz


Figure 1. I/Q Carrier Frequency or Center Frequency Set to 125 MHz


Figure 1. I/Q Carrier Frequency or Center Frequency Set to 100 MHz and Downconverter Center Frequency Set to 125 MHz


Figure 1. I/Q Carrier Frequency or Center Frequency Set to 150 MHz and Downconverter Center Frequency Set to Less Than 120 MHz


Note Calibrated performance is bound by the equalization bandwidth at a given frequency. As such, the ability to digitally tune to a frequency with the NI‑RFSA IQ Carrier Frequency property or NI‑RFSA Center Frequency property is restricted to within ± 0.5 x Equalized Bandwidth of the downconverter center frequency. You can exceed this limitation by using the NI‑RFSA Allow Out of Specification User Settings property.

RF Output Signal Path Selection

Similar to RF Input, the RF Output channel is divided into a low frequency signal path and high frequency signal path. RF energy is diverted to either path through a diplexer with a crossover frequency of 120 MHz. The signal path is automatically configured when the NI-RFSG Frequency property is modified. If you set the NI-RFSG Frequency property value below 120 MHz, the low frequency signal path is used, while frequency values greater than or equal to 120 MHz are generated with the high frequency signal path.

Similar to RF input signal path selection, in I/Q mode you can override the signal path used for RF output with the NI-RFSG Upconverter Center Frequency property. Using the NI-RFSG Upconverter Center Frequency property locks the device into the low-frequency or high-frequency signal path despite the specified frequency value. If you set the NI-RFSG Signal Bandwidth property, NI-RFSG may select an upconverter center frequency for you. Refer to the Automatic Frequency Offset section for information on how to automatically offset the upconverter frequency.

Note The NI-RFSG Upconverter Center Frequency property is coerced to 0 Hz when set to any value less than 120 MHz. Refer to the Programming Reference section in theNI RF Signal Generators Help for more information about the NI‑RFSG Upconverter Center Frequency property.

The following figures illustrate examples of configuring the signal path selection based on the NI-RFSG Frequency and NI-RFSG Upconverter Center Frequency properties.

Figure 1. Frequency (Hz) Set to 100 MHz


Figure 1. Frequency (Hz) Set to 125 MHz


Figure 1. Frequency (Hz) Set to 100 MHz and Upconverter Center Frequency Set to 125 MHz


Figure 1. Frequency (Hz) Set to 150 MHz and Upconverter Center Frequency Set to Less Than 120 MHz


Note Calibrated performance is bound by the equalization bandwidth at a given frequency. As such, the ability to digitally tune to a frequency with the Frequency property is restricted to within ± 0.5 × Equalized Bandwidth of the upconverter center frequency. It is possible to exceed this limitation by using the NI‑RFSG Allow Out of Specification User Settings property.