Physical layer
Data communication is provided only in one direction
from the receiver to the transmitter. It is realized by load
modulation. The receiver changes its impedance
periodically by switching e.g. a resistor or capacitor. This
leads to a modulation of the coil current or coil voltage in
the transmitter, which is detected and demodulated by a
suitable circuit. The implementation in the receiver is not
standardized. However, a minimum modulation depth of the
coil current of 15 mA or coil voltage of 200 mV in the
standard transmitter is specified in the standard. Figure 16
shows possible realizations. It is recommended to realize the
modulator by switching a capacitor between the receiver
coil and the rectifier. Experiments showed problems when
switching a (resistive) load connected at the output of the
rectifier due to the low internal resistance of the battery.