handbook of modern sensors

6.2 Microwave Motion Detectors 229
(A)
(B)
Fig. 6.1. Microwave occupancy detector: (A) a circuit for measuring Doppler frequency; (B)
a circuit with a threshold detector.
moisture, and so forth. The operating principle of the microwave detector is based
on radiation of electromagnetic radio-frequency (RF) waves toward a protected area.
The most common frequencies are 10.525 GHz (X band) and 24.125 GHz (K band). 2
These wavelengths are long enough (λ = 3 cm at X band) to pass freely through most
contaminants, such as airborne dust, and short enough for being reflected by larger
objects.
The microwave part of the detector consists of a Gunn oscillator, an antenna, and
a mixer diode. The Gunn oscillator is a diode mounted in a small precision cavity
which, upon application of power, oscillates at microwave frequencies. The oscillator
produces electromagnetic waves (frequency f 0 ), part of which is directed through an
iris into a waveguide and focusing antenna which directs the radiation toward the
object. Focusing characteristics of the antenna are determined by the application. As
a general rule, the narrower the directional diagram of the antenna, the more sensitive
it is (the antenna has a higher gain). Another general rule is that a narrow-beam
antenna is much larger, whereas a wide-angle antenna can be quite small. The typical
radiated power of the transmitter is 10–20 mW. A Gunn oscillator is sensitive to the
stability of applied dc voltage and, therefore, must be powered by a good quality
voltage regulator. The oscillator may run continuously, or it can be pulsed, which
reduces the power consumption from the power supply.
The smaller part of the microwave oscillations is coupled to the Schottky mixing
diode and serves as a reference signal (Fig. 6.1A). In many cases, the transmitter
and the receiver are contained in one module called a transceiver. The target reflects
some waves back toward the antenna, which directs the received radiation toward the
mixing diode whose current contains a harmonic with a phase differential between
the transmitted and reflected waves. The phase difference is in a direct relationship to
the distance to the target. The phase-sensitive detector is useful mostly for detecting
the distance to an object. However, movement, not distance, should be detected.
Thus, for the occupancy and motion detector, the Doppler effect is the basis for the
operation of microwave and ultrasonic detectors. It should be noted that the Dopplereffect
device is a true motion detector because it is responsive only to moving targets.
Here is how it works.
2 The power of radiation must be sufficiently low not to present any health hazards.