Abstract: A digital filter for obtaining quadrature components in a periodic signal, such as the recovery of phase and/or amplitude of two or more periodic signals through coherent digital summation techniques. In one embodiment of the invention, the digital filter forms part of an omega type radio navigation system in which a mobile receiver station receives a signal containing three preselected frequencies of interest transmitted from a plurality of fixed transmitting stations in timed sharing sequence. The mobile receiving station may contain band pass filters to receive the signal and separate and pass only the three preselected frequencies of interest. These three preselected frequencies of interest are then summed and introduced into a sample and hold circuit which operates in combination with an analog-to-digital converter.