This video demonstrate three methods to measure the voltage and harmonics of an AC power supply using a USB oscilloscope with passive probes. The three method are: (1) Earth-Grounded Single-Ended Measurement (2) Earth-Grounded Differential Measurement (3) Floating-Ground Differential Measurement.
[youtube]http://www.youtube.com/watch?v=oMqhrRUjxOg[/youtube]
Method 1
Method 2
Method 3