1. A calibration device for a phase locked loop arranged to generate an output frequency based upon a first frequency range of an input signal applied to a first input and a second frequency range of the input signal applied to a second input, the calibration device comprising an estimator arranged to use a two dimensional estimation algorithm with a signal value indicative of a mismatch between the first input path and the second input path to determine an estimate of the mismatch to allow matching of the first input path and the second input path.