Bandwidth Effect

The Bandwidth function enables EMI Analyst to simulate the effect of a spectrum analyzer or EMI receiver, so that calculated values will be presented in a format that is similar to what is reported by the measurement instrument.  The Bandwidth function takes into account the theoretical thermal noise floor of the measurement instrument and the effect of the instrument bandwidth.

The following two figures show conducted emissions, first accounting for measurement bandwidth (upper figure), and second ignoring (bypassing) measurement bandwidth (lower figure).  The peak amplitudes in the lower figure are the same as those in the upper figure, but the upper figure accounts for the measurement instrument noise floor and the "skirting" effect of the measurement bandwidth.

More:

Specifying Bandwidth