6.1 Full duplex system
An FDX transponder receiving the activation field shall transmit its code during the activation period. The FDX transponder uses a modified DBP encoded sub�carrier, which is amplitude modulated upon the radio frequency carrier. Because the slope of a low�high transition is not infinitely steep, every low�high transition is advanced in time to a maximum of eight cycles to obtain optimal performance (see figure 1). The transponder shall send its message back using the frequency bands 129,0 kHz to 133,2 kHz and 135,2 kHz to 139,4 kHz. The duration of one bit is 32 activation field cycles. This corresponds to a bit rate of 4 194 bits/s.
NOTE � The basic frequency of the sub�carrier, containing the phase encoded data signal, is not influenced by the advancement in time of the low�high transition and remains 4 194 Hz (binary 1: 180º phase shift; binary 0: no phase shift).