Issue link: https://iconnect007.uberflip.com/i/1457913
20 DESIGN007 MAGAZINE I MARCH 2022 trace to plane(s) separation, par- allel segment length, the trans- mission line load, and the technol- ogy employed. But crosstalk also varies depending on the physical stackup configuration. Crosstalk is caused by the cou- pling of electromagnetic fields. Electric fields cause signal volt- ages to capacitively couple into nearby traces. Capacitive cou- pling draws a surge of drive cur- rent which causes reflections on the transmission lines. Whereas magnetic fields cause signal cur- rents to be induced into nearby traces, inductive coupling pro- duces ground bounce and power supply noise. Crosstalk falls off rapidly with the square of the distance. e degree of impact is related to the aggressor signal voltage, available board real estate, and thus the proximity of signal traces. e easiest way to reduce crosstalk from a nearby aggressor signal is, of course, to increase the spacing between the signals in question. Doubling the spacing cuts the crosstalk to roughly a quarter of its original level. However, crosstalk is determined by the ratio of the trace separation and the height of the trace above the plane. By varying the trace height, one can also control the coupling—hence crosstalk. If real estate is limited, then this may be a better solution rather than increasing routing density. A tight coupling (less height) results in less crosstalk. ere is a sweet spot where the total energy stored in the electromagnetic field surround- ing the trace is optimized. Crosstalk between two or more conductors depends on their mutual inductance and mutual capacitance. e inductance plays a major role in this cou- pling. e signal return currents will generate EM fields. ose EM fields, in turn, induce voltages (crosstalk) into other signals. It can be seen in Figure 3 that the differen- tial impedance or the coupling of two par- allel traces levels off at 100Ω above 12 mils trace clearance (blue curve). is is simulated quickly by multiple passes of the field solver. All other factors being equal, the differential impedance will always be 100Ω regardless of increased spacing. is also represents the point at which crosstalk (coupling) begins. is curve provides a clear map of the design space and efficiently defines the stackup con- figuration for single-ended and coupled pairs. In this case, once the separation is less than 12 mils, the two traces begin to couple and trans- fer electromagnetic energy. Crosstalk is typically picked up on long par- allel trace segments. ese can be on the same layer but may also be broadside-coupled from the adjacent layer. It is for this reason that orthogonal routing is recommended on adja- cent layers (between planes) to minimize the coupling area. In conclusion, parasitic effects can be mini- mized by separating traces as much as possible, coupling signal traces close to the reference planes, reducing the loop area of return cur- Figure 3: Coupling levels off above 12 mils separation (simulated by the iCD Stackup Planner).