Patent ID: 7439799

Claim:
A frequency shift keying demodulator robust for frequency offset, comprising: a frequency synthesizer, used for outputting a first frequency signal according to a frequency control signal; a first frequency mixer, used for receiving a frequency shift keying signal (FSKS) and the first frequency signal, frequency-mixing the two signals and then outputting a first mixed-frequency signal; a first low-pass filter, used for filtering the high-frequency component from the first mixed-frequency signal and outputting a baseband signal; a phase difference generator, used for receiving the low-frequency signal and according to the low-frequency signal generating a phase changed low-frequency signal with phase difference, wherein the low-frequency signal with phase difference has a phase-shift in comparison with the low-frequency signal; a second frequency mixer, used for receiving both the low-frequency signal and the phase changed low-frequency signal with phase difference, frequency-mixing the two received signals and outputting a second mixed-frequency signal; a second low-pass filter, used for filtering the high-frequency component from the second mixed-frequency signal and then outputting a low-frequency signal; a decider, used for receiving the low-frequency signal and according to the polarity of the low-frequency signal outputting a demodulated signal; and a microprocessor, used for, according to the duty cycle of the demodulated signal, outputting the frequency control signal to control the frequency synthesizer.