What is the fastest pulse risetime that I can measure with my spectrum analyzer in zero span?

The measurement risetime may not be faster than the best risetime of the spectrum analyzer. The analyzer’s risetime can be determined from this equation:

Tr = 0.66/max RBW,

where RBW is resolution bandwidth.

For example, in the PSA (E4440A, E4443A, E4445A, E4446A or E4448A), the maximum RBW is 8 MHz. Therefore, the fastest risetime is:

0.66/8 E6 = 82.5 nS.

However, RBW filter width uncertainty is ± 15% nominal (at center frequencies ≤3 GHz), so the risetime range is 71.7 nS to 97 nS.

Refer to the Data Sheet or Specifications Guide for your particular spectrum analyzer.