Advanced Detection Methods for Perimeter Photo Beams

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					        OUTDOOR PROTECTION                          By James Babb

           Advanced Detection
      Methods for    Perimeter
                 Photo Beams
     P          e
                erimeter protection and intrusion detec-
                tion can be unwieldy, requiring special
                consideration to prevent the occurrence
                of false alarms, especially in light of poten-
                tially noisy environments.
                   Recent studies and programs have called
     attention to issues with regards to being able to have
     the confidence to dispatch personnel on alarm notifi-
                                                                 false alarm reduction programs of the FARA type but
                                                                 also for improved technology, which is what this ar-
                                                                 ticle addresses.
                                                                    Photo beam perimeters continue to be the first line
                                                                 of protection for an alarm system and detection at the
                                                                 protected premises. This article is designed to take a
                                                                 look at the technology behind detectors and several
                                                                 different beam approaches, which include:
     cations. There is an alarming (no pun intended) trend          Detection of light--There are detectors devised
                                                                 for virtually every wavelength of light known to ex-
     The trick of any detection scheme is                        ist, deep UV through far reaches of the infrared spec-
                                                                 trum. Detectors simply receive photons in the region
     to maximize the signal and minimize                         of interest (outside the region in the form of noise) and
     the noise (S/N ratio).                                      convert the photons to electrons. The electrons fill up
                                                                 a bucket which is based upon an R/C time constant
     toward a verified response approach—not to dispatch         (Resistance/Capacitance), which then dumps the
     emergency personnel until and unless a confirmation         bucket at a fixed interval.
     of the alarm condition is obtained. There are even or-         There are many sources of noise that can affect the
     ganizations such as the False Alarm Reduction Asso-         detected ratio of Signal to Noise (S/N), ranging from
     ciation (FARA) with programs designed to exchange           Shot and Johnson noise of the circuit itself, through
     information, provide a clearinghouse for agencies and       external noise sources such as ambient light (DC) 
Description: Phase Lock Loop is a technique where, when the phase matches there is a resultant 2X multiplier of the signal in phase, there is an additional benefit of effectively ignoring noise sources that are not in phase. It employs Dynamic Circuitry that constantly monitors and adjusts to assure no phase offset (see Illustration 1 Phase offset & 2 DMPLL). The aggregate benefit of Double Modulation further sets the technique apart in that it adds an additional second layer of modulation.
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