DataCard CP80 Plus Service Manual page 51

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.
Type-B Card Modulation
A Type-B card uses a combination of amplitude modulation and phase
modulation, called Binary Phase Shift Keying (BPSK). This method sends a
constant subcarrier frequency of 847 kHz using amplitude modulation, but shifts
the phase of the subcarrier by 180 degrees (inverted) at each data transition. In
other words, the carrier is always modulated with the subcarrier, but during data
transitions, the subcarrier signal is altered (inverted). The subcarrier is permitted
to invert only at its own transition points (which is why the frequencies are even
multiples of each other).
CP80 and CP80 Plus Service Manual
of the carrier frequency) so each data bit will have several
th
the
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