Solution
NI-RFSA and RFmx drivers only allow us to set the downconverter frequency offset up to half of the instantaneous bandwidth. You can obtain the instantaneous bandwidth value based on the hardware specifications.
For example, NI PXIe-5840 (
table below) with I/Q carrier frequency set at a value range of 1.3 GHz to 2.2 GHz will have instantaneous bandwidth of 500MHz. Therefore, the downconverter frequency offset will be up to a maximum of 250MHz.