Patent ID: 8660215

Claim:
A method for processing a frequency shift key signal comprising the steps of: mixing the frequency shift key signal with in-phase and quadrature components of a signal at a nominal center frequency corresponding to the frequency shift key signal to produce a mixed in-phase component and a mixed quadrature component; filtering mixed in-phase component and the mixed quadrature component to produce a difference in-phase component and a difference quadrature component; and ascertaining a logical symbol indicated by the frequency shift key signal at a point in time by determining whether the difference in-phase component leads or lags the difference quadrature component at the point in time, wherein the determining comprises: monitoring both the difference in-phase component and the difference quadrature component to detect a zero crossing of one of the difference in-phase component and the difference quadrature component; performing an exclusive-or operation on a sign of the difference in-phase component and a sign of the difference quadrature component at a point in time that is within the interval between the zero crossing detected in the detecting step and one fourth of a period of the component for which the zero crossing is detected in the detecting step; when the zero crossing is detected for the difference in-phase component, ascertaining the logical symbol directly and solely from the result of the exclusive-or operation on the sign of the difference in-phase component and the sign of the difference quadrature component without calculating a negative of the exclusive-or of the sign of the difference in-phase component and the difference quadrature component; and when the zero crossing is detected for the difference quadrature component, calculating a negative of the exclusive-or of the sign of the difference in-phase component and the difference quadrature component and ascertaining the logical symbol directly from the negative of the result of the exclusive-or operation on the sign of the difference in-phase component and the sign of the difference quadrature component.