[0001] This invention concerns intruder detection apparatus.
[0002] Various forms of intruder detection apparatus, particularly suitable for outdoor
detection using a microwave transmitter and receiver combination, with the transmitter
and receiver spaced some distance apart and aligned with each other, are known. The
receiver monitors the level of a signal received from the transmitter and should there
be a change in the level of the received signal, indicative of an intruder encroaching
on the area between the transmitter and receiver, an alarm is produced. A typical
detector of this kind, often known as a "microwave fence" is exemplified by United
Kingdom Patent No. 1409658.
[0003] However, on occasion it is a requirement that such a microwave fence be required
to function in close proximity to high-power pulse-modulated radars. Then, the received
signal level at the microwave fence receiver, from its own transmitter, can be quite
low, whereas the signal level, at the same receiver, resulting from the proximity
of the radar, can be extremely high; differences of the order of 120db can be experienced.
Further, the operating frequencies of the microwave fence and the radar can be of
the same order, so not surprisingly the operation of the microwave fence can be disturbed
as a result of breakthrough of the radar signal into the microwave fence receiver,
causing malfunction.
[0004] Typically, the radar pulses are a few microseconds wide and have a pulse-repetition
frequency of 1KHz.
[0005] An object of the present invention is to provide a microwave fence arrangement wherein
the above- discussed difficulty is obviated.
[0006] With this object in view, the present invention provides intruder detection apparatus
comprising a microwave transmitter, a corresponding receiver spaced from and aligned
with the transmitter to receive and monitor a signal from the transmitter and to provide
an alarm output upon detection of a change in the monitored signal indicative of an
intrusion into the area between the transmitter and the receiver, characterised by
the provision of eliminator means effective to eliminate any interfering pulses which
may be superimposed upon the detected signal (e.g. by adjacent radar or other apparatus)
comprising selector means operative to gate out the input to the receiver in response
to each interfering pulse, for a period exceeding the duration of the pulse, and delay
means effective to synchronise the respective gating-out condition with said pulse,
thereby to prevent said pulse being supplied to the receiver.
[0007] The invention will be described further, by way of example, with reference to the
accompanying drawing in which the single figure is a block circuit diagram of a preferred
embodiment of eliminator means for use in intruder detection apparatus of the invention,
and more particularly its manner of operation.
[0008] An embodiment of the.intruder detection apparatus of the invention comprises a microwave
transmitter (not shown) and a corresponding receiver which is spaced apart from the
transmitter so that the space therebetween is that to be protected. The receiver is
aligned with the transmitter, and serves basically to receive a signal therefrom and,
upon occurrence of change in level of this signal to provide an alarm output.
[0009] In the illustrated case, an incoming signal is picked up by an aerial 10 from which
it is fed to a detector diode 11, the output from which consists of the microwave
fence modulation, typically 1KHz. In the event of the apparatus being installed near
radar, this output is a modulated signal comprising the detected signal, plus narrow
pulses from the interfering radar. The combined signals are supplied to a highfrequency
selective amplifier 12 which passes the narrow pulses from the radar, but substantially
rejects the low-frequency 1KH
z modulation.
[0010] Each pulse output from the amplifier 12 is used to trigger a monostable 13 which
produces a corresponding respective pulse of some thirty-microseconds duration. Each
pulse is used to open a switch 14 (which will normally be closed) for the duration
of the pulse.
[0011] The detected signal is also fed to an amplifier 15. The output from this amplifier
15 is fed to a delay line 16 which delays the signal for a nominal five-hundred- nanoseconds
and the delay line output is amplified by wide-band amplifier 17 and applied to the
switch 14. As the switch 14 is under the control of the monostable 13, should an interfering
pulse occur, the switch 14 will open before the interfering pulse appears at the output
of the amplifier 17. As the duration of switch opening is in excess of any anticipated
interfering pulse, the pulse is effectively suppressed. Consequently, the signal across
a capacitor 18 is substantially that of the microwave fence modulation with the interfering
pulse removed.
[0012] The time constant of the capacitor 18 and a resistor 19 is such that whilst the switch
14 is open the voltage across the capacitor 18 will change little and hence the required
modulation waveform is preserved.
[0013] The wanted modulation signal is then subsequently filtered to remove the wide band
noise introduced by the wide band amplifiers 15 and 17 to improve the overall signal
to noise ratio.
[0014] The signal is then passed, as indicated at 20, to further signal-processing circuitry
which serves to generate an alarm output in the event of change occurring in the signal
indicative of an intrusion into the protected space.
1. Intruder detection apparatus comprising a microwave transmitter, a corresponding
receiver spaced from and aligned with the transmitter to receive and monitor a signal
from the transmitter and to provide an alarm output upon detection of a change in
the monitored signal indicative of an intrusion into the area between the transmitter
and the receiver, characterised by the provision of eliminator means effective to
eliminate any interfering pulses which may be superimposed upon the detected signal
(e.g. by adjacent radar or other apparatus) comprising selector means operative to
gate out the input to the receiver in response to each interfering pulse, for a period
exceeding the duration of the pulse, and delay means effective to synchronise the
respective gating-out condition with said pulse, thereby to prevent said pulse being
supplied to the receiver.
2. Intruder detection apparatus as claimed in claim 1 wherein the eliminator means
comprises a high frequency selective amplifier which will pass narrow interfering
pulses which serve to trigger a monostable which in turn serves to open a switch which
gates out the input to the receiver.
3. Intruder detection apparatus substantially as hereinbefore described with reference
to and as illustrated in the accompanying drawing.
