If your'e using an oscilloscope to check square wave distortion (i don't know of any other way) make sure that your probe is correctly compensated. There should be a little jack that outputs a square wave from your oscilloscope, connect your probe to that. Then there is a little adjustable capacitor in the probe that you use to compensate it. If your probe is overcompensated, you will get sharp peaks, and if it is undercompensated, you will get rounded off edges. It should be in between . . . a SQUARE wave :lol:
If this procedure is not done, you will get incorrect measurements. This isn't only for squre waves, your scope should always be compensated correctly.