Estimate near-end and far-end crosstalk between two parallel traces on an outer or inner layer, from their geometry, coupled length and the rise time of the aggressor.
Coupling. The even- and odd-mode impedances of the two traces come from the same coupled-line models as the differential pair calculator (Kirschning–Jansen for microstrip, Cohn for stripline). The coupling coefficient is k = (Zeven − Zodd)/(Zeven + Zodd).
Near-end crosstalk. For weakly coupled, terminated lines the saturated backward coefficient is Kb = k/2. It is reached when the coupled length exceeds the saturation length Lsat = v·tr/2. Shorter couplings scale linearly: NEXT = Kb·L/Lsat.
Far-end crosstalk. It comes from the difference between even- and odd-mode velocities: FEXT = |td,even − td,odd| / (2tr). In a stripline with a homogeneous dielectric both modes travel at the same speed and FEXT is close to zero.
Limits. First-order estimates for two matched lines. Receivers that are not terminated, several aggressors and changes of reference plane can make the real noise larger.
Keeping the centre-to-centre spacing at least three times the trace width (a gap of twice the width) keeps near-end crosstalk between microstrips at a few percent.
Both coupling mechanisms, capacitive and inductive, cancel at the far end when the field is in a single homogeneous dielectric.
Bandwidth of a digital edge and the trace length above which it needs transmission line design.
Delay of a trace, maximum length mismatch for a skew budget, and wavelength on the board.
Width a trace needs for a current, or the current a trace can carry, with IPC-2152 and IPC-2221.
Impedance of an outer-layer trace over a plane, with solder mask or embedded, and width for a target impedance.
Impedance of an inner-layer trace between two planes, symmetric or offset, and width for a target.
Differential, odd, even and common-mode impedance of edge-coupled microstrip and stripline pairs.