System for analysing the frequency of a signal, a method thereof and a system for measuring the relative phase between two input signals

A method for analysing the frequency fi of a signal, comprising:

• inputting a signal with frequency fi into a system, wherein the system comprises an analogue to digital converter having a sampling frequency fs, a first band pass filter and a second band pass filter, the first band pass filter and the second band pass filter being arranged in parallel with one another, and a feedback loop from an output to an input of the system;
• setting an initial centre frequency of the system equal to a nominal frequency f0;
• sampling the signal at sampling frequency fs using the analogue to digital converter;
• feeding back the output of the system into the input;
• and
• revising the centre frequency fc of the system based on the output of the system,
• wherein transfer functions of the first and second band pass filters have the same poles,
• and wherein the system is configured such that, when the centre frequency fc equals the frequency of the signal fi, the transfer function of the first band pass filter is in phase with the input, the transfer function of the second band pass filter lags the transfer function of the first band pass filter at a constant phase of π/2, and the two band pass filters have equal gain amplitude.