A 5Gb/s single-ended parallel receiver with adaptive FEXT cancellation

In high-speed communication with a data rate of multi-Gb/s, the crosstalk noise induced by electromagnetic coupling is becoming a significant noise source, requiring careful considerations in the design of transceiver circuits as well as physical dimension of the transmission lines. When an input voltage of V_i is applied to one of two parallel transmission lines, the induced voltages at the victim line by near-end crosstalk (NEXT) and far-end crosstalk (FEXT) can be expressed as V_NEXT(t)=1/4[C_m/C_S+L_m/L_S]·{Vi(t)-Vi(t-2t_f)}, V_NEXT(t)=t_f/2[C_m/C_S-L_m/L_S]·dVi(t-t_f)/dt (1) where t_f, C_m, C_S, L_m and L_S represent the time-of-flight, the mutual, self-capacitances, mutual, and self-inductances of the transmission line per unit length, respectively. Though NEXT has more energy due the inductive coupling added to the capacitive coupling, the wide distribution in time over 2t_f results in a small peak noise and is a concern only in high-speed low-voltage differential signaling. However, FEXT in parallel microstrip lines causes a significant peak noise due to smaller capacitive coupling by inhomogeneous structure with the upper side exposed to air. The effect of FEXT turns out to be non-zero signal delay at the receiver side, presenting a serious performance-limiting factor in single-ended high-density parallel links such as memory interface.