Patent ID: 8199860

Claim:
A carrier offset detection circuit which is provided to a demodulation circuit which demodulates a received signal subjected to FSK (Frequency Shift Keying) modulation, and which detects a carrier offset frequency between a transmitting side and a receiving side, comprising: a zero-crossing detection unit which receives a base band signal in the form of a digital signal which indicates a level of the frequency deviation of the received signal from the carrier frequency on the receiving side, and detects a zero-crossing point at which the base band signal and a base band signal obtained by delaying the base band signal by one symbol cross each other; and an offset detection unit which determines a value of the base band signal at a timing of the zero-crossing point detected by the zero-crossing detection unit as the carrier offset frequency, wherein the offset detection unit performs smoothing processing on the base band signal, and determines a value of the base band signal smoothed by the smoothing processing at the timing of the zero-crossing point detected by the zero-crossing detection unit as the carrier offset frequency, and wherein the offset detection unit calculates a sum of the base band signal and the signal obtained by delaying the base band signal by one symbol, and performs smoothing processing on the summed signal.