Acousto-magnetic EAS systems use a transmitter to create a
surveillance area where tags and labels are detected. The transmitter
sends a radio frequency signal at a frequency of 58 kHz (thousands of
cycles per second), but the frequency is sent in pulses. The transmit
signal energizes a tag in the surveillance zone. When the transmit
signal pulse ends, the tag responds, emitting a single frequency signal
like a tuning fork. The tag signal is at about the same frequency as the
transmitter signal.
While the transmitter is off between pulses, the tag signal is
detected by a receiver. A microcomputer checks the tag signal detected
by the receiver to ensure it is at the right frequency, occurs in time
synchronized to the transmitter, at the proper level, and at the correct
repetition rate. If the criteria is met, an alarm occurs.
