2-36 Theory of Operation
Reader to Card Communication
The reader communicates with the card by sending an amplitude modulated
signal at 13.56 MHz. The modulation variant depends on the class of card. A
Type-A card uses 100% Amplitude Shift Keying (ASK100%). A Type-B card uses
ASK10% modulation. These two variants are shown in the previous graphics.
The benefit of ASK100 is that it is more immune to noise and errors because there
is a strong difference in amplitude at bit transitions. The drawback to this is that
the card doesn’t receive power during the low cycle of the modulation. The card
doesn’t turn off during these times, but the effect is that less overall (average)
power is being transmitted to the card.
ASK10 provides more power to the card because the modulated signal remains
within 10% of the maximum amplitude. The tradeoff is that the difference
between high and low amplitudes is minimal. This can lead to slightly more data
transmission errors.
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.
To Next Page
Loading...
Need help?
General
