When referring to the switching behavior of a signal, the term “phase coherence” defines the state of the signal’s phase once the switching process is complete. In order to illustrate this, lets consider

Two phase coherent signals (Signal 1 and Signal 2), at Frequency F1 with a Relative Phase of ΔФ or P1.

If Signal 2 is switched to Frequency 2 (F2) and then back to Frequency 1 (F1), the relative phase between the two signals will be ΔФ or P1 again. 

This represents a coherently switched signal. Let’s review another video.