Keysight N9912A User Manual page 59

Minimum gate span is twice the cutoff time. Each gate shape has a minimum
recommended gate span for proper operation. This is a consequence of the finite
cutoff rate of the gate. If you specify a gate span that is smaller than the minimum
span, the response will show the following effects:
 distorted gate shape that has no passband
 distorted shape
 incorrect indications of start and stop times
 may have increased sidelobe levels.
Time Domain - Option 010 59