Electronic security systems have been developed and used commercially for the purpose of detecting the presence of a marker within an interrogation zone. One type of such system is a radio frequency (RF) system that is used to detect the presence of a resonant tank circuit. An example of a resonant tank circuit is a tuned tank circuit that includes an inductor with a capacitor connected across the inductor terminals for the purpose of either modifying transmissions from an antenna, or retransmitting at its resonant frequency a signal which is received and amplified by the resonant tank circuit. The resonant tank circuit is tuned to a preselected frequency of the transmitter. The transmitter sweeps a range of frequencies centered about the expected marker resonant frequency. The tank circuit retransmits a signal which is detected by a receiver. Upon the signal being detected by the receiver, an alarm is set off to indicate the presence of the tank circuit in the interrogation zone.
In an ideal world, the interrogation zone would only have the electromagnetic field that has been generated by the antenna of the system. Unfortunately, in the real world, large numbers of devices transmit electromagnetic fields that overlap with the interrogation zone. As a consequence, the receiver of the detection system will receive these latter signals, which are referred to as white noise or background noise, and could inadvertently sound an alarm even though a tank circuit is not present within the interrogation zone. These false alarms can create serious problems in any implementation of a security system. There is the obvious difficulty with a customer being delayed and annoyed when a false alarm is tripped, and there is also the need to monitor the system after false alarms have been generated to prevent additional false alarms.
It clearly would be desirable to have an electronic surveillance system that has the capability of isolating, or sequestering, the background noise so that upon the entrance of a resonant tank circuit into the interrogation zone, it can be detected with a higher level of confidence.