Importance

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Abstract

Disclosed is a tone decoder (20) that identifies one or more tones encoded on an RF carrier signal for use in controlling various systems and devices located at a remote site. The tone decoder includes a bandpass filter (22), an A/D converter (24), and a tone detection component (26). An input signal having a nominal frequency f.sub.in is received by the tone decoder, filtered, and then converted to digital form for subsequent digital processing by the tone detection component. The tone detection component compares the input signal to a reference signal having a frequency f.sub.r. The tone detection component mixes the input signal with cophase versions of the reference signal to produce a cophase difference signal. The cophase difference signal is used to calculate the instantaneous magnitude and instantaneous phase of the mixed input and reference signal pairing. An indication of the frequency of the difference signal is determined by differentiating the instantaneous phase. The instantaneous magnitude and frequency calculations are then compared against threshold values to determine (a) the presence of a valid input signal and (b) whether the actual frequency f.sub.in of the input signal sufficiently corresponds to the frequency f.sub.r of the reference signal to indicate the presence of a tone.

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