The PXIe-5841 can 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 the LO guard band). A table of the maximum available bandwidth for a given I/Q frequency when using automatic frequency offset is available in the device specifications. In auto-offset mode, the NI-RFSG filters are scaled so that a CW tone will not overflow if the frequency is within the maximum available bandwidth or ± 50 MHz, whichever is less. To prevent an overflow at any offset, you can set Prefilter Gain to less than or equal to -2.0. Outside of auto-offset mode, the filters are scaled so that no CW tone will overflow, even with Prefilter Gain equal to 0.0.

The actual frequency offset applied can be determined by reading the coerced downconverter center frequency. For Automatic Offset 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 device specifications. Otherwise, using the user-defined offset mode, the gain accuracy at an offset is limited to the amplitude accuracy specification +/- the frequency response specification.

Note To use automatic frequency offset with an external LO, including sharing an LO between acquisition and generation, refer to the Using an External LO section.

The following example shows how to programmatically set the PXIe-5841 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 using digital upconversion or downconversion to digitally tune the I/Q carrier frequency. 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.

You can use in-band retuning by setting the device instantaneous bandwidth and leaving the downconverter center frequency fixed, which significantly reduces reconfiguration and settling times.

The following example shows how to programmatically set the PXIe-5841 for in-band retuning.



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