Abstract: A node (20) of a communications network extracts a reference signal from a transport network for use as a reference signal for the node. The node comprises a frequency locked loop (22) which filters the reference signal to the node, as well as a calibration system (24) which determines a tuning sensitivity factor (a) for the frequency locked loop. The calibration system performs a calibration procedure which includes the calibration steps of (1) obtaining a first error measurement (f1) when a first tuning data value is applied to the frequency locked loop; (2) obtaining a second error measurement (f2) when a second tuning data value is applied to the frequency locked loop; and, (3) using the first tuning data value, the second tuning data value, the first error measurement, and the second error measurement to determine the tuning sensitivity factor for the frequency locked loop.