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.

Abstract: The disclosed decoder (14) produces an output signal when a receiver (12) signal contains frequencies that are sequentially within a predetermined passband surrounding detection frequencies f.sub.1 and f.sub.2. The receiver signal is mixed at exclusive-OR logic gates (52, 54) with cophase reference signals R1 and R2, which are produced by a reference tone generator (18) and have a frequency f.sub.r that corresponds to the desired tone detection frequency f.sub.1 and f.sub.2 and a phase difference of 90.degree.. The outputs of gates (52, 54) are applied to low pass filters (56, 58), which produce outputs having a frequency .DELTA.f equal to the absolute value of the difference between the input and reference frequencies f.sub.i and f.sub.r, provided that .DELTA.f is below the filter's cutoff frequency f.sub.c1.