Importance

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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. A cophase detection circuit (60, 62, 64, and 66) produces a high logic output when signals are passed by filters (56, 58), indicating that the frequency of the receiver signal differs from the desired tone detection frequency by less than f.sub.c1. To further enhance the accuracy of tone detection, a differentiating tone detection arrangement (68 through 84) is provided to determine whether .DELTA.f is less than some second cutoff frequency f.sub.c2 or greater than f.sub.c1. Two differentiating circuits (68, 70) receive the outputs of filters (56, 58) and produce bidirectional square waves having magnitudes corresponding to .DELTA.f. These square waves are then compared with some predetermined threshold to produce an output only when .DELTA.f is below f.sub.c2 or above f.sub.c1. The cophase and differentiating tone detection outputs are combined to produce an output when a tone corresponding to the detection frequency f.sub.1 or f.sub.2 has been detected.

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First Claim