Soeren, what do you main in PLL controlling ?
Can you explain it ?
PLL = Phase Locked Loop.
DDS = Direct Digital Synthesis
The PLL looks at both the output from the DDS and your square wave signal and "locks" the two together.
If the signal drifts/deviates, the PLL will correct it,
The output from a DDS can be anything you can imagine, sine, triangle, saw tooth or something entirely different, like the quack of a duck
What's your purpose with the converted signal?