Issue link: https://iconnect007.uberflip.com/i/1494346
MARCH 2023 I DESIGN007 MAGAZINE 19 ally inductive coupling, driving the FEXT co- efficient negative (Figure 3a). However, for- ward crosstalk does not exist in the stripline configuration. e fine balance between capac- itive and inductive coupled crosstalk produces no observable forward crosstalk (Figure 3b). And, the NEXT component is halved. So, all other factors being equal, here is just another good reason why one should always route criti- cal high-speed signals on the inner layers of a multilayer PCB. Figure 4 illustrates the "backward crosstalk critical length." is is a measure of how far an aggressor line and a victim line can route in parallel before the amplitude of backward crosstalk (NEXT) reaches its maximum level. Parallelism beyond this length will widen the coupled pulse in the victim line, but will not increase its amplitude. As rise time gets faster, "critical length" becomes shorter. In today's systems, with extremely fast IC out- put switching times, almost any parallelism Figure 3: a) Forward FEXT; and b) backward NEXT crosstalk for microstrip and stripline configurations. Figure 4: Backward crosstalk critical length. a b