Communications: electrical – Condition responsive indicating system – Specific condition
Patent
1983-04-18
1985-07-16
Rowland, James L.
Communications: electrical
Condition responsive indicating system
Specific condition
G08B 1318, G08B 1324
Patent
active
045299722
ABSTRACT:
The invention relates to intruder detection systems of the microwave radiation type in which a number of microwave transmitter/receiver units are placed around an area under surveillance. If a moving intruder is present, the microwave radiation emitted by one or more of the units will undergo a slight change when reflected back to the same unit. Mixing the received radiation within the transmitted radiation will therefore produce a low frequency output which can be detected by a Doppler amplifier. The Doppler amplifier will also produce an output if microwave radiation inadvertently received by that unit from another unit has a frequency which is such that the mixing produces a signal within the Doppler amplifier bandwidth. In order to prevent the latter signal causing a false alarm, any Doppler amplifier output is used to shift the frequency of a modulator controlling the pulse repetition frequency of the transmitted radiation. This shift in frequency will cause substantially no change in the Doppler amplifier output if the frequency shift is a shift caused by a moving intruder. However, if the signal is caused by mixing radiation inadvertently picked up from another unit, the Doppler amplifier output will be nulled when the change in transmitted radiation causes the signal to fall outside the Doppler amplifier bandwidth. In this way, false alarms can be prevented.
REFERENCES:
patent: 3828336 (1974-08-01), Massa
patent: 4107684 (1978-08-01), Watson
Morrison Alan D.
Paterson Neil G.
Hofsass Jeffery A.
Racal Security Limited
Rowland James L.
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