Introduction
The MDU (Motion Detector Unit) is a miniature microwave Doppler radar sensor optimised for low power consumption, short range (<30 metres) and low cost. The circuit features a dielectric resonator stabilised FET oscillator, which provides stable operation over a broad temperature range in either CW or low duty cycle pulse mode and a balanced mixer for enhanced sensitivity and reliability.
For information on the variants currently available, see 'Products'
The MDU emits a low level X Band microwave signal which is reflected from all objects within its coverage area. If any of the objects that the signal has bounced off are moving towards or away from the sensor, the frequency of the reflected signal received back by the sensor will be increased or decreased from that of the transmitted signal by the Doppler effect.
The MDU compares the transmitted and received frequencies and produces an output signal, the frequency of which is proportional to the velocity of the object. The amplitude of this signal is a complex function of the size and reflectivity of the object and its distance from the MDU, as well as the characteristics of the MDU. See "Radar Range Equation" for a discussion of range.
Signal processing circuitry (not provided with the unit) amplifies this signal and analyses its frequency spectrum. If the signal strength is above a threshold level, and has the required frequency spectrum an output signal can be generated.
In order to conserve power it is usual for the MDU to be pulsed on and off rapidly so that it is only transmitting for approximately 5% of the time. As well as reducing power consumption, this also reduces the average power transmitted.
This does not reduce the ability of the MDU to detect moving objects.
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