Abstract: A Doppler principle detection device includes means for transmitting signals into a field to be protected, means for receiving signals from said field, quadrature mixers for multiplying a reference signal and the received signal to discern between positive and negative Doppler shifts, an integrator for integrating the output of the detection device and an alarm adapted to be activated when the output of the integrator reaches a predetermined threshold level and incorporating a false alarm inhibitor for shifting by 90.degree. the difference in phase of the received signals with respect to the reference signal. In one form the false alarm inhibitor comprises analogue switches connected to a level detector and to an inverter connected to the level detector arranged to switch phase shifted signals to the quadrature mixers to shift the phase of the reference signal by 90.degree..