Abstract: Apparatus and methods for measuring physical quantities making use of resonant structures, the resonant frequency of which changes with changes of the physical quantity. Techniques for detecting resonance are described. In one embodiment the apparatus comprises detecting circuit which has an arming circuit including a receiver which tracks the frequency of excitation signals and on the basis of the detected amplitude of the signals in that frequency range arms a comparator to compare signals indicative of the relative phase of the excitation signal and a signal reflected by the resonant structures which in turn allows resonance to be detected.