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Abstract

An infrared detector device comprising a light projector adapted to output double-modulated infrared pulse light by modulating the amplitude of high frequency modulated light by means of low frequency waves, and a light receiver adapted to receive the double-modulated infrared pulse light. The light projector (1) comprises a 1/N-frequency dividing circuit (12) for dividing by N the frequency of the basic frequency signal from a basic frequency oscillator in such a manner that it can be set for switching, a second frequency dividing circuit (13) for dividing the frequency of the 1/N frequency-divided signal by a given factor to produce a low frequency signal, a frequency combining circuit (14) for combining the 1/N frequency-divided signal and the signal frequency-divided by a given factor, and a switch (17) for switching the frequency dividing factor for the 1/N-frequency dividing circuit (12). The light receiver (2) comprises a first tuning circuit (23) for selecting the optical signal on the basis of a frequency which corresponds to the 1/N-frequency-divided signal, a second tuning circuit (24) adapted to be tuned to the low frequency signal in the tuned signal from the first tuning circuit (23), the second tuning circuit (24) being provided with a PLL circuit, and a two-circuit ganged type changeover switch (26) for registering the first and second tuning frequencies with the respective frequencies of the set 1/N-frequency-divided signal and the signal frequency-divided by a given factor.

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