When you measure a modulated waveform there are several ways to picture the changes. If you only look at the time vs level domain it's hard to see the changes. What you need is a way to see the other domains of the signal.

http://www.naic.edu/~phil/hardware/M...r_basis_of.pdf

https://www.youtube.com/watch?v=Lp6sT-n55N4