The Full Span function will set the span to the 
full frequency range.  
 
This function will set the start and stop 
frequencies to 0Hz and 3GHz respectively. 
The Zero Span function will set the frequency 
range of the sweep to 0Hz and fixes the start 
and stop frequencies to the center frequency. 
The Zero Span function measures the time 
domain characteristics of the input signal at the 
center frequency. The horizontal axis is 
displayed in the time domain.