The PXIe-5842 can automatically offset the downconverter center frequency from the carrier frequency when sufficient instrument bandwidth is available.

For the PXIe-5842 to apply automatic frequency offset, the signal must fit within the following limits, depending on the bandwidth hardware option:

  • 2 GHz and 4 GHz bandwidth options: (Instantaneous Bandwidth - 200 MHz LO Guard Band) / 2
  • Lower bandwidth options: Bandwidth Option / 2

A table of the maximum available bandwidth for a given I/Q frequency when using automatic frequency offset is available in the instrument specifications in Equalized Bandwidth.

When Offset Mode is Automatic (the default behavior) and Signal Bandwidth ≤ Maximum Offset Bandwidth, as defined in the Maximum Offset Bandwidth table in the specifications, the PXIe-5842 automatically applies the offset. When Offset Mode is Automatic and Signal Bandwidth > Maximum Offset Bandwidth, the PXIe-5842 does not apply an automatic offset.

You can determine the actual frequency offset that was applied 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 that 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 Amplitude Accuracy Specification ± Frequency Response Specification.

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

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

Note The following figure illustrates frequency and bandwidth concepts by portraying a generic example instrument of 1 GHz bandwidth; values and figures might not necessarily reflect the frequencies and bandwidth of your instrument.