What is causing this distortion on a 20MHz control signal?
I would not say that 20MHz is high frequency, that's why I didn't care about impedance and termination. Could you please confirm this in addition to your opinion on the signal?


but your square wave contains components to 100's of MHz, and you do care about those if you want a square wave response. 

It looks like you a) don't have a properly compensated scope probe; b) have reflections from your load (or transmitter); c) have too long a scope GND lead, or don't have it sufficiently close to the signals, or d) have bad grounding on your board - that's equally as important as the direct signal path.