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Receiver
Electromagnetic waves radiated by the alarm transmitter induce minute voltages in the remote receiver aerial. These voltages are in the region of millionths of a volt (uV), and the receiver's purpose is to amplify these to a level which the decoder can operate on.
A receiver' performance is primarily assessed by two factors, selectivity and sensitivity. Selectivity is a measure of its ability to discriminate between wanted signals and unwanted out of channel signals. Sensitivity is how capable it is of detecting weak signals in the presence of atmospheric and other noise. Several types of receivers are available. They range from low cost single stage imported devices, with poor selectivity and sensitivity, to dual conversion, multi-stage receivers offering high performance.
One of the most common types of receiver offering reasonable performance is the single conversion type. Signals picked up by its aerial are amplified by the radio frequency (RF) amplifier and then transferred to a mixer stage. The mixer combines the input signal with a local oscillator voltage producing a frequency component which is much lower than the original input signal, whilst still retaining the modulated wave form introduced by the transmitter encoder. This lower frequency product is more easily amplified by the intermediate amplifier. After amplification the signal is stripped of its carrier component by the detector circuit, leaving a waveform which has an identical binary word pattern as the transmitter encoder. A simple clipping amplifier then raises this waveform to a level sufficient for the decoder to identify.
Decoding
Decoders are now capable of recognising many thousands of different codes. Large schemes supporting many transmitters usually employ microprocessor based decoding techniques. This allows the storage and display of multiple alarms with the added sophistication of VDU and printer displays. Smaller domestic systems, which represent the majority in service, generally employ no more than ten transmitters, each individually coded for the room or area to be protected. Indication of alarm activity is usually presented in a similar manner to a conventional alarm system. Panel indicators show the current zone and systems states whilst initiating local or remote alarms. Control of the system can be via a conventional key switch, but an increasingly popular technique is to use a remote control key with integral transmitter. This device usually serves the dual role of control and personal attack. By entering the appropriate system codes, various system states are initiated. Typically, these include disabling of specific zones, adjustment of bell and entry/exit time periods, silent walk tests and system resets.
The transmitter units used with these systems come with varying degrees of sophistication. Popular enhancements are: sending of a low volts code when battery volts are depleted, transmit duration extender which holds on the unit for a period 15 - 20 seconds, used particularly where transmissions may be transitory such as personal attack alarms and magnetic door contacts. Because of the portable nature of radio alarms, their application in the field of personal attack is widespread. Uses vary from banks and supermarkets to nurses in hospital emergency departments. The units are normally of a self-contained compact design with a single button press operation.
Sensors
The majority of sensors used in modern hardwired alarm systems are suitable for use with radio alarms. Door and window contacts wired in conventional n/c loop system can be wired to a single transmitter, this hybrid configuration provides single room or area protection without the necessity of wiring back to the alarm controller. One particular device which lends itself to radio alarm applications is the PIR or passive infra red sensor. These can be completely self-contained, housing not only the transmitter and aerial, but also its power source in the form of an alkaline battery. Short term and temporary protection of particular areas is easily facilitated with these units. Their application however need not be restricted to the confines of the alarmed premises, garages, driveways and store house within the vicinity can be provided with intruder protection with none of the restrictions imposed by the cabling requirements of hard wired systems. Because of the constraint triggering which occurs in busy areas such as corridors and offices, better PIR's incorporate a timer circuit in order to conserve battery life. This ensures once the device is triggered, reactivation is inhibited for a period of 2 - 3 minutes after the alarm signal is transmitted. A similar technique to extend battery life is applied to transmitter units protecting frequently used doors.
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