DataCard CP60 Service Manual page 49

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).
CP60 and CP60 Plus Service Manual 2-37