Technical Manual

Technical Manual AM Systems
AM operating principle v4.4 Page 12 of 72
5 AM operating principle
The AM system operates on the pulse-listening principle. Using the zero crossing of the
50Hz mains frequency as a trigger, a short burst of 58kHz signal is transmitted and a
receiver window is ‘opened’ after the transmission burst has stopped.
If a tag was present within detection range during the transmission burst, the resonance
of this tag will be detected in the receiver window and an alarm will be triggered.
A single 50Hz mains frequency cycle is dived into three phases; phase A, B and C. Each
phase covers 1/3 of the 50Hz cycle duration.
A noise reference window is opened to detect the environmental noise. This signal is
compared with the signal received in the receiver window and allows the software to
properly distinguish a tag signal from a noise signal.
Figure 1 shows the AM operating principle (at 50Hz, with a default delay of 200μs) during
one 50Hz cycle.
Figure 1: AM operating principle
Transmission burst
Receiver window
Reference window
Phase C
Phase B
Phase A