OK this my best understanding of ddr-

MHz is a frequency, so it is based on a sine curve, so it has a creast/peak and a trench/base or whatever you want to call it.

Since it is only positive half the time, you can double it by sending a second signal offseted by one half of a wavelength. So that when one wave is positive the other is negative, and vise versa.

If you don't know what a sine curve is just think of waves in water.

If anyone knows anything otherwise, please let me know.