| 摘要 |
A narrow-band tone decoder has a controllable oscillator, connected in a fast-capture phase-locked loop, for generating a signal having a frequency determined by a control signal produced within the loop. A synchronous detector, which responds to an input signal of the tone decoder and to a signal from the controllable oscillator, produces a signal having a magnitude dependent upon the magnitudes of the input signal and of the signal from the controllable oscillator. A window comparator monitors the control signal within the loop for deciding when the generated signal is within a predetermined frequency range.
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