For the Reference Input, you need a waveform with a fast rising/falling edge, and only one threshold crossing upwards, and one threshold crossing downwards per period. The threshold is where the input signal is recognized as either a "1" or a "0"
Dividers have rectangular outputs with fast rise/fall times, and satisfy that criteria.
For signals with harmonics, you have to look at them on an oscilloscope and see what those harmonics are doing. If they line up at the right phase to make a rectangular pulse, you are all set. If they line up to make a random set of superimposed sine waves, all bets are off.