Oscilloscope AC coupling and PWM signal
set the AC coupling in oscilloscope 


This is what setting the oscilloscope to AC coupling does: It puts a capacitor in series with the input, so that the average voltage of the signal is removed and you see only the AC components.

If you had a 0 to 9 V square wave with 55.6% duty cycle, the average voltage would be 5 V, and the AC coupling would subtract 5 V from the signal before it reached the scope's sampling circuit. So the signal you'd see on the scope would range from -5 to +4 V.