Security system with dynamic range enhancement for FM demodulation

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.

2. 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;

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;

g) the remote receiving a second audio signal from a microphone;

h) the remote frequency modulating the second audio signal at a carrier frequency to generate a second FM signal;

i) the base receiving the second FM signal from the remote;

j) at the base, determining a phase error signal representing the phase error between the received second FM signal and an output signal of a voltage controlled oscillator;

k) at the base, determining a difference between the carrier frequency and a center frequency of the voltage controlled oscillator; and

l) if there is a difference, then changing, at the base, the center frequency of the voltage controlled oscillator to match the carrier frequency of the second FM signal and wherein the step of changing, at the base, the center frequency of the voltage controlled oscillator to match the carrier frequency of the second 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 second FM signal based on the center frequency select signal.

3. A security system comprising:

a) a panel adapted to communicate over a telephone network;

b) a base interconnected to the panel to send and receive audio signals from the telephone network via the panel, the base comprising an FM modulator for frequency modulating an audio signal at a carrier frequency to generate an FM signal; and

c) a remote device in wireless communication with the base, the remote device comprising:

i) a receiver for receiving the FM signal from the base;

ii) a voltage controlled oscillator;

iii) a phase lock loop circuit for determining a phase error signal representing the phase error between the received FM signal and an output signal of the voltage controlled oscillator;

iv) a center frequency adjusting circuit for:

determining a difference between the carrier frequency and a center frequency of the voltage controlled oscillator; and, if there is a difference;

changing the center frequency of the voltage controlled oscillator to match the carrier frequency of the FM signal and wherein the center frequency adjusting circuit comprises a low pass filter to which the phase error signal is applied, the low pass filter comprising a capacitor that is charged if the output of the low pass filter is greater than a predetermined nominal voltage or discharged if the output of the low pass filter is less than a predetermined nominal voltage.

4. A security system comprising:

a) a panel adapted to communicate over a telephone network;

b) a base interconnected to the panel to send and receive audio signals from the telephone network via the panel, the base comprising an FM modulator for frequency modulating an audio signal at a carrier frequency to generate an FM signal; and

c) a remote device in wireless communication with the base, the remote device comprising:

i) a receiver for receiving the FM signal from the base;

ii) a voltage controlled oscillator;

iii) a phase lock loop circuit for determining a phase error signal representing the phase error between the received FM signal and an output signal of the voltage controlled oscillator;

iv) a center frequency adjusting circuit for:

determining a difference between the carrier frequency and a center frequency of the voltage controlled oscillator; and, if there is a difference;

changing the center frequency of the voltage controlled oscillator to match the carrier frequency of the FM signal and

further wherein

the remote device further comprises an FM modulator for frequency modulating a second audio signal at a carrier frequency to generate a second FM signal; and

the base further comprises:

i) a receiver for receiving the second FM signal from the remote device;

ii) a voltage controlled oscillator;

iii) a phase lock loop circuit for determining a phase error signal representing the phase error between the received second FM signal and an output signal of the voltage controlled oscillator;

iv) a center frequency adjusting circuit for:

determining a difference between the carrier frequency and a center frequency of the voltage controlled oscillator; and, if there is a difference;

changing the center frequency of the voltage controlled oscillator to match the carrier frequency of the second FM signal and wherein the center frequency adjusting circuit comprises a low pass filter to which the phase error signal is applied, the low pass filter comprising a capacitor that is charged if the output of the low pass filter is greater than a predetermined nominal voltage or discharged if the output of the low pass filter is less than a predetermined nominal voltage.