Solution
The PXIe-5842 combines a vector signal generator and vector signal analyzer into a single four-slot PXI Express instrument. This VST provides the fast measurement speed and small form factor of a production test box with the flexibility and high performance of R&D-grade box instruments.
To achieve such level of integration, the VST uses a Zero IF architecture. While this radio layout presents multiple benefits, it also forces that some of the hardware components found in traditional spectrum analyzers are implemented through digital signal processing blocks. This affects the way the front-end of the VST filters the power of the signals that reach the internal digitizer input.
More precisely, the VST doesn't have a way to filter out signals present at the operational bandwidth of the instrument. Therefore, even if the span of the instrument is configured to ignore the main signal, the power of such signal still reaches the digitizer. This means that the reference level of the instrument must be set to a value that takes into account the total power of the signal, and not just the power included within the configured span.
If a user needs to decrease the reference level to look for specific spurs far from the main signal, the best is to include external filters that eliminate the power of unwanted signals.