A microwave intrusion detector is provided with a periodic signal detector that identifies periodic signals not typical of an intrusion and adjusts the alarm threshold in response such identification. The intrusion detector includes an adjustable threshold and a noise compensating circuit that actively...http://www.google.co.uk/patents/US5581237?utm_source=gb-gplus-sharePatent US5581237 - Microwave intrusion detector with threshold adjustment in response to periodic signals
