The system is composed of a transmitter, a synchronous receiver, a
microprocessor-controlled detector, and an alarm. The transmitter
produces two signals to excite the tag. One is a high frequency carrier
signal and the other is a much lower frequency electrostatic signal. In
North America the high frequency signal is hopped over a band of 902-906
MHz in distinct groups of 50 frequencies to avoid interference from
adjacent systems. In Europe the high frequency is between 2402 and 2486
MHz and is not hopped; each system has its own unique frequency
assigned.
The lower frequency is modulation signal is 111.5 kHz. It is a non
propagating electrostatic signal that limits the range of the high
frequency RF field to the desired surveillance zone. This prevents the
signals from traveling large distances and interfering with other
systems or causing alarms due to tags on articles just outside the
surveillance zone.
The tag is composed of a microwave diode and a combination antenna
for the receipt of the high and low frequency. When introduced into the
field of the transmitter, the tag combines or mixes the two fields and
re-radiates the combined signal to the high frequency receiver. The
resultant 111.5 kHz modulation of the high frequency signal is amplified
and compared to a reference in the detector to ensure it is at the
correct frequency and level. If correct, an alarm is initiated.
