You can set the PXIe-5831 to automatically offset the downconverter center frequency from the carrier frequency when sufficient instrument bandwidth is available. For example, the signal must fit within half the device instantaneous bandwidth minus an LO guard band. The actual frequency offset applied can be determined by reading the coerced downconverter center frequency. For this mode to function, you must specify the signal bandwidth. Additionally, when the downconverter frequency offset mode is enabled and the signal bandwidth is known, NI RFSA can further optimize the dynamic range of the instrument. Using the instrument when downconverter frequency offset mode is enabled also ensures the gain accuracy is maintained, as noted in the specifications document for your instrument. Otherwise, using the user-defined offset mode, the gain accuracy with an offset is limited to the accuracy specification +/- the flatness specification.

Note When using a shared LO source across NI-RFSA and NI-RFSG, you must ensure the frequency configuration, including any frequency offset, matches.

The following example shows how to programmatically set the PXIe-5831 to automatically offset the downconverter center frequency from the carrier frequency, if possible. Refer to the Downconverter Frequency Offset Mode and Upconverter Frequency Offset Mode properties for more information.





In-Band Retuning

In-band retuning allows the instrument to quickly tune between signals of interest or generation within a given device instantaneous bandwidth. The advantages of in-band retuning include reduced settling time for both frequency and amplitude. However, in-band retuning can only occur within the supported instantaneous bandwidth of the instrument. You can use digital tuning in combination with LO tuning to more efficiently sweep over a spectrum for supported applications.

By setting device instantaneous bandwidth and leaving the downconverter center frequency fixed, you can significantly reduce reconfiguration and settling times. When you set the I/Q carrier frequency away from the downconverter center frequency but still within the device instantaneous bandwidth, the instrument digitally shifts the waveform of interest. This is referred to as in-band retuning.

The following example shows how to programmatically set the PXIe-5831 for in-band retuning.



The following figure shows three acquisitions with only digital frequency shift reconfigurations.