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MARCH 2023 I DESIGN007 MAGAZINE 21 • Forward crosstalk does not exist in the stripline configuration due to the fine balance between the capacitive and inductive coupling. • For a rise time of 200 ps, backward crosstalk reaches its maximum amplitude in 12 mm of parallelism between lines. • e amplitude of crosstalk is determined by the distance between the aggressor and victim lines relative to their height above the reference plane. It is extremely important to route all traces on a layer that is one dielectric distance above or below a ground plane. • A guard trace that is grounded every 1/12th wavelength (bottom) prevents radiation and drastically lowers cross-coupling. DESIGN007 Resources • Beyond Design columns by Barry Olney: "Return Path Discontinuities," "The Dark Side—Return of the Signal," "Stackup Configurations to Mitigate Crosstalk." • "Noise, EMI & SI and Single-ended Crosstalk," by Rick Hartley. • "Signal and Power Integrity Simplified," by Eric Bogatin. Rick Hartley is the principal engineer at R Hartley Enter- prises and has been in the industry for over 50 years. He is one of the primary consultants for PCB manufacturing and design companies. Rick has also conducted classes worldwide on EMI, signal integrity, and various other electrical topics for the last 30 years. Barry Olney is managing direc- tor of In-Circuit Design Pty Ltd (iCD), Australia, a PCB design service bureau that specializes in board-level simulation. The company developed the iCD Design Integrity software incor- porating the iCD Stackup, PDN, and CPW Planner. The software can be downloaded at au. To read past columns, click here. (impedance of free space) and will oen cause more problems than they solve. To minimize the detrimental interaction of electromagnetic fields, it is good design prac- tice to: • Separate asynchronous signals, unrelated signals, and signals of different potentials. • Avoid long parallel trace segments to minimize their coupling. • Route all critical signals embedded between the planes to reduce crosstalk. • Keep other signals away from I/O signals and connectors. Never allow anything to route into the I/O area of the board, except I/O lines. • Use low dielectric constant material and closely couple the signal trace to the reference plane to decrease the radiation. Key Points • EM fields do not merge with each other but rather add vectorially—distorting the signal. • Crosstalk usually comes from electric field-based capacitive coupling of trace segments running in parallel. • Mutual capacitance between the two lines couples energy in both directions on the victim trace. • A wide uniform plane as the return path is the configuration for the lowest radiation and crosstalk. • Mutual inductance between the lines couples energy onto the victim trace segment in the backward direction. • When multiple fields couple to a single reference line, in a connector or IC package, or into a single ground via, the return currents overlap, creating very high mutual inductance. • Electromagnetic fields have direction and polarity.

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