INFRARED DETECTOR.

摘要

The infrared detector is composed of a projector which produces a double-modulated infrared pulse beam by modulating a modulated beam of a high frequency with a low frequency and a light receiver which receives the double-modulated infrared pulse beam. The projector (1) comprises a divide-by-N frequency dividing circuit (12) which divides by N the frequency of basic-frequency signals having a basic frequency maintaining the freedom of switching and setting. A second frequency dividing circuit (13) generates signals of a low frequency by dividing a predetermined coefficient the frequency of the signals whose frequency has been divided by N. A frequency synthesising circuit (14) synthesises the signals whose frequency has been divided by N and the signals whose frequency is divided by the predetermined coefficient. A switch (17) switches the frequency-dividing factor of the divide-by-N frequency dividing circuit (12). The light receiver (2) comprises a first tuning circuit (23) which selects received light signals with a frequency that corresponds to the signals whose frequency has been divided by N and a second tuning circuit (24) which is tuned to the low-frequency signals in the tuning signals of the first tuning circuit (23). The second tuning circuit (24) has a PLL circuit and is further provided with a switch (26) interlocked to the two circuits to bring the tuning frequencies of the first and second tuning circuits into match with the frequency of the signals whose frequency has been divided by N and the frequency of the signals whose frequency has been divided by the predetermined coefficient.