The PLL inside the DW1000, used to reduce the frequency difference between the internal frequency and the received frequency.
Does the phase detector trying to reduce the phase shift as well. It is important, due to that if the PLL receive two signals and both signal have the same frequency but different phase the PLL integrator require different amount of time for the correction. In this way the PLL integrator would be not independent from the distance.
(Maybe you can provide the PLL design)