Measurement of motor rotor position or speed

A measurement circuit includes first and second inputs for receiving first and second input signals which vary cyclically with the rotational position of a rotor of an electric motor; a phase detector circuit; and an observer circuit that estimates the frequency and phase of the first and second input signals. The phase detector circuit determines a phase difference between first and second estimated signals with a phase difference dependent on the phase difference indicated by the estimates, the first and second estimated signals varying in quadrature with each other and the first and second input signals by adding a multiplicative combination of the first input signal with the first estimated signal to a multiplicative combination of the second input signal with the second estimated signal to result in a sum signal dependent upon the phase difference, The observer circuit varies the estimates dependent upon the sum signal.