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handbook of modern sensors

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5.3 Excitation Circuits 173<br />

(A)<br />

(B)<br />

Fig. 5.20. LC sine-wave oscillators.<br />

Fig. 5.21. LC radio-frequency oscillator as a capacitive occupancy detector.<br />

tance, which somewhat reduces the frequency <strong>of</strong> the oscillator. When a person moves<br />

into the vicinity <strong>of</strong> the antenna, he/she brings in an additional capacitance which lowers<br />

the oscillator frequency even further. The output <strong>of</strong> the oscillator is coupled to a<br />

resonant tank (typically, an LC network) tuned to a baseline frequency (near 30 MHz).<br />

A human intrusion lowers the frequency, thus substantially reducing the amplitude<br />

<strong>of</strong> the output voltage from the tank. The high-frequency signal is rectified by a peak<br />

detector and a low-frequency voltage is compared with a predetermined threshold<br />

by a comparator. This circuit employs an oscillator whose frequency is modulated<br />

by a sensing antenna. However, for other applications, the same circuit with a small<br />

inductor instead <strong>of</strong> an antenna can produce stable sinusoidal oscillations.

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