Hi Dan0,
The spectral window in zero-span mode is plotted using a width equal to the sample rate. The rolloff you see is the software filter we apply to the data. The filter cutoff (6dB point) is selectable in the zero-span controls. The filter is a standard windowed sinc filter using the blackman window. The size of the filter does vary based on decimation and there can be cascaded filters at higher decimations.
Let me know if you have follow up questions.
Regards
