DataCard CP60 Service Manual page 48

Card to Reader Communication
The communication method back to the reader is elegantly simple, but a little
more complicated to understand. It is a passive communication because the card
doesn't really send a signal back to the reader—it modifies the signal emanating
from the reader. This is called Load Modulation, and is based on a loosely coupled
transformer (two coils separated by distance and without an iron core).
A normal transformer is frequently thought of as a method for changing one
voltage to another, such as a computer power supply that takes 120 volts and
reduces it to 12 volts. While the voltage decreases across the transformer shown
below, the current increases proportionally. The value of the current in the
secondary winding is dependant on the load being powered. Consequently, the
current in the primary winding is inversely proportional to the load on the
secondary winding. As a result, if the load is varying with time, the circuitry on
the primary side of the transformer can detect the change in current draw.
When the card engages and disengages (switches) resistance in its own antenna,
the reader detects this change by examining the current draw within its antenna.
Type-A Card Modulation
The type of return modulation from the card to the reader differs with the two
classes of cards (Type-A and Type-B). Type-A cards use a simple On-Off-Keying
modulation (OOK). This is the binary equivalent of amplitude modulation. The
card switches the resistance seen by the card reader at a frequency of 1/16
frequency of the carrier signal. This means that during a modulation period, the
carrier signal is attenuated every eight cycles. The data stream is even slower
than this (1/128
transitions from modulation to non-modulation of the carrier.
2-36
of the carrier frequency) so each data bit will have several
th
the
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Theory of Operation