Issue link: https://iconnect007.uberflip.com/i/981887
MAY 2018 I DESIGN007 MAGAZINE 29 G. Minimizing signal crosstalk Crosstalk causes noise induction in victim signals which in turn increases the bit error at receiving IC. Therefore, in addition to pro- viding continuous reference plane mentioned above, non-interleaved routing shall also be applied on stripline, due to a lower far end crosstalk (FEXT) compared to near end cross- talk (NEXT). Whereas interleaved routing be applied on microstrip due to a lower NEXT compared to FEXT. Channel analysis [17] com- pares the NEXT versus FEXT on microstrip and stripline respectively in time domain. Besides that, inter-pair spacing shall be at least triple the trace width [18] . H. Intra-pair skew Intra-pair skew in PCB traces introduces higher insertion loss, which in turn increases the bit error of the physical channel, due to the fact that when inverting and non-inverting sig- nals are not 180 degrees out of phase, the eye height in differential mode becomes smaller. The impact of intra-pair skew on signal integ- rity is shown in differential insertion loss plots in Figure 9, where larger skew leads to higher insertion loss. Therefore, each physical chan- nel's intra-pair skew shall be kept within 5mil to mitigate the transmission loss. The skew can be minimized using serpentine routing technique [19] . However, the differential imped- ance changes once the intra-pair spacing is increased in serpentine segment. Hence, intra- pair spacing for serpentine and non-serpentine segments shall not differ too much, to mini- mize the full path differential impedance toler- ance or discontinuity within ±5%. PAM-4 channel simulation Figure 10 depicts the channel simulation topology using Keysight ADS at BER 1e-6, where Figure 9: Differential insertion loss plots for eight-inch PCB trace with varying intra-pair skew simulated in Hyperlynx. Figure 10: PAM-4 channel simulation topology at 28GBaud using Keysight ADS.