Zero Span mode can be done through our S240 software, there is a zero span option as shown below.
By default, the signal display area plots the frequency (in MHz) on the x-axis and the power level (in dBm) on the y-axis. The Zero Span plot changes the x-axis to the time domain (in seconds), while the power level remains on the y-axis. The spectrum plot updates continuously along the x-axis as each capture is received.
Enabling zero span forces the the S240 GUI to use the 10 MHz Span mode, which provides the most accurate amplitude level.