Patent ID: 7994933

Claim:
In a communication system, an apparatus for compensating for drift between a receiver and a transmitter local oscillator frequencies, the apparatus comprising: a receiver for receiving a first frequency modulated signal with frequency (F.sub.T+f.sub. 1 ) representing a first data value, and a second frequency modulated signal with frequency (F.sub.T−f.sub. 2 ) representing a second data value, where F.sub.T is a local oscillator frequency of the transmitter, and f.sub. 1 and f.sub. 2 are frequency deviations for modulating the first and the second data values, respectively; a mixer for down-converting or intermediate frequency (IF) converting the first and the second received signals by mixing the first and second received signals with the receiver's local oscillator frequency, wherein the receiver's local oscillator frequency is associated with conversion frequency; and calculating a drift between the transmitter's local oscillator frequency and the receiver's local oscillator frequency, F.sub.D, by: F.sub.D=F.sub.M 2 +f.sub. 2 (F.sub.M 1 −F.sub.M 2 )/(f.sub. 1 +f.sub. 2 ), where F.sub.M 1 and F.sub.M 2 are first and second down- or IF-converted frequencies, respectively.