Why does rectification of a digital signal double the frequency content?
Just look at the waveform.

If a sine wave goes through a complete cycle every t, and you full-wave rectify it, there will now be two upward humps in every t.  Since each hump is a complete cycle, you've doubled the frequency:



More generally, a perfect full-wave rectification means the response function has even symmetry.  This means that, for a sine wave input, the output will consist of only even-ordered harmonics, starting at the second harmonic, which is 2&times; the fundamental.

http://en.wikipedia.org/wiki/Even_and_odd_functions#Harmonics