Abstract: A method of instantaneously determining or estimating the frequency of an input signal includes mixing a digitally sampled and quantized input signal (12) by a time shifted replica of the input signal (12), where the time shift equals an integer multiple of a sampling period of the input signal (12), thereby producing a mixed signal. The mixed signal is filtered with a low-pass filter producing a filtered signal, which is used to obtain an estimate of the frequency of the input signal. The invention extends to an apparatus (8) for implementing the method in accordance with the invention. The invention also extends to a method of determining or estimating the amplitude of an input signal (12).