US Patent 7792511 Security system with dynamic range enhancement for FM demodulation

A system and method for communicating between a base and a remote device in a security system. The base receives an audio signal from a telephone network via a panel and then frequency modulates the audio signal at a carrier frequency to generate an FM signal. The remote device receives the FM signal from the base, determines a phase error signal representing the phase error between the received FM signal and an output signal of a voltage controlled oscillator, determines a difference between the carrier frequency and a center frequency of the voltage controlled oscillator, and, if there is a difference, then changes the center frequency of the voltage controlled oscillator to match the carrier frequency of the FM signal.