Yes, your PicoScope 212/50 has a 50MS/s sampling rate in real time and the wave your sampling is 20Mhz, so you're close to the minimum Nyquist sampling rate here (http://www.cs.cf.ac.uk/Dave/Multimedia/node149.html
is a good site to explain this).
However, with this continuous signal, you can use ETS (Equivalent Time Sampling) on your scope to obtain a smoother image - see attached screen shot. This was performed with a timebase of 20ns/div. ETS is turned on with the Trigger option at the bottom of the screen.