| ABSTRACT:
A field disturbance sensor operates with relatively low power, provides
an adjustable operating range, is not hypersensitive at close range, allows
co-location of multiple sensors, and is inexpensive to manufacture. The
sensor includes a transmitter that transmits a sequence of transmitted
bursts of electromagnetic energy. The transmitter frequency is modulated
at an intermediate frequency. The sequence of bursts has a burst repetition
rate, and each burst has a burst width and comprises a number of cycles
at a transmitter frequency. The sensor includes a receiver which receives
electromagnetic energy at the transmitter frequency, and includes a mixer
which mixes a transmitted burst with reflections of the same transmitted
burst to produce an intermediate frequency signal. Circuitry, responsive
to the intermediate frequency signal indicates disturbances in the sensor
field. Because the mixer mixes the transmitted burst with reflections of
the transmitted burst, the burst width defines the sensor range. The burst
repetition rate is randomly or pseudorandomly modulated so that bursts
in the sequence of bursts have a phase which varies. |