Apparatus for the surveillance of an electronic security element in an interrogation zone

An apparatus for the surveillance of an electronic security element in an interrogation zone, comprising:

• a transmitting device for emitting at least one cyclic interrogation signal into the interrogation zone, said interrogation signal causing the security element to emit a characteristic signal;
• a receiving device having at least one receiving channel for receiving said characteristic signal and generating a signal indicative thereof;
• and
• a computing/control unit for evaluating the signals received from said receiving device and producing an alarm when the presence of the security element is established, said evaluation comprising detecting spikes from the signals received from said receiving channels, creating a corresponding signal shape from said spikes in a self-learning process, and removing the determined spikes from the signals received from said receiving channels, wherein said computing/control unit includes spike determining means which determine a possible spike during an initialization phase by storing the received signals over several cycles and searching for possible spikes following activation of said transmitting device and said receiving device, creating a spike average over several cycles in the event that at least one spike is detected, and from the spike average a dc-average is subtracted.