Patent ID: 7792511
Filing Date: 2010-09-07
Classification: H04L

Abstract:
1. A method of communicating between a base and a remote device in a security system, said method comprising: a) the base receiving an audio signal from a telephone network via a panel; b) the base frequency modulating the audio signal at a carrier frequency to generate an FM signal; c) the remote device receiving the FM signal from the base; d) at the remote device, determining a phase error signal representing the phase error between the received FM signal and an output signal of a voltage controlled oscillator; e) at the remote device, determining a difference between the carrier frequency and a center frequency of the voltage controlled oscillator; and f) if there is a difference, then changing, at the remote device, the center frequency of the voltage controlled oscillator to match the carrier frequency of the FM signal and wherein the step of changing, at the remote device, the center frequency of the voltage controlled oscillator to match the carrier frequency of the FM signal comprises: i) applying the phase error signal to a low pass filter, ii) if the output of the low pass filter is greater than a predetermined nominal voltage, then charging a capacitor; iii) if the output of the low pass filter is less than a predetermined nominal voltage, then discharging a capacitor; iv) adjusting a center frequency select signal based on the charging or discharging state of the capacitor; and v) changing the center frequency of the voltage controlled oscillator to match the carrier frequency of the FM signal based on the center frequency select signal.