Microstrip Crosstalk Calculator converts trace spacing, substrate height, dielectric constant, rise time, and drive voltage into coupled backward crosstalk voltage between traces.
Calculate Backward Crosstalk Between Microstrip PCB Traces
This calculator estimates the near-end (backward) crosstalk voltage coupled onto a victim microstrip trace from an adjacent aggressor trace, based on rise time, drive voltage, trace geometry, and substrate properties. PCB layout and signal integrity engineers use it to check whether trace spacing on a board is tight enough to risk false triggering or timing jitter on nearby signals.
Entering Trace Geometry and Reading the Coupling Results
Enter the aggressor’s rise time (ns) and drive voltage (V), the parallel-routed length (inch), substrate height (mil) and trace spacing (mil), and the substrate’s dielectric constant (εr). The calculator returns the coupled voltage on the victim trace, total attenuation in dB, the saturation length past which crosstalk stops growing, and whether the routing is in the saturated or unsaturated regime.
The Backward Crosstalk Saturation Model
Per Howard Johnson and Martin Graham’s High-Speed Digital Design: A Handbook of Black Magic (Prentice Hall, 1993), the standard reference for this saturation model, near-end (backward) crosstalk grows with coupled length only up to a saturation length, Lsat:
$$L_{sat} = \frac{RT \times c}{2\sqrt{Dk_{eff}}}$$
where RT is the aggressor’s linear rise time, c is the speed of light (11.8 in/ns), and Dkeff is the effective dielectric constant surrounding the microstrip trace — not the bulk substrate εr, but a lower value produced by the air above the trace and the substrate below it (Dkeff ≈ (εr+1)/2 as a first-order microstrip estimate).
Below Lsat, the coupled voltage scales linearly with the ratio of actual parallel length to saturation length:
$$V_{cross} = K \times V \times \min\left(1, \frac{L}{L_{sat}}\right)$$
Here K is the geometric coupling coefficient set by trace spacing (S) and substrate height (H), and V is the aggressor’s drive voltage; K×V is the saturated (maximum possible) coupled voltage that Vcross approaches as L grows past Lsat.
What we couldn’t verify: we could not trace the exact closed-form expression this calculator uses to convert the S/H ratio directly into K to a specific citable equation. Established references derive K from even/odd-mode impedance (Zeven, Zodd) or from mutual-to-self capacitance and inductance ratios, both of which normally require a field solver or trace-width input rather than S and H alone. Treat the S/H-based K value as a fast design-stage estimate, not a substitute for a 2D field solver when a design is close to a noise-margin limit.
Common mistake: substituting the bulk substrate dielectric constant εr directly for Dkeff. Because microstrip traces sit exposed on top of the board with air above them, Dkeff is meaningfully lower than εr — using εr directly overstates Lsat and can shift a layout across the unsaturated/saturated boundary in the wrong direction.
Aggressor and Victim Trace Cross-Section
Microstrip Crosstalk: Common Questions
Why does crosstalk stop increasing after a certain trace length?
Backward crosstalk builds from coupling that arrives back at the near end within one rise time. Once the parallel length is long enough that round-trip delay exceeds the rise time, additional length just adds overlapping, not additional, coupling — so the voltage saturates.
Is this the same as far-end crosstalk (FEXT)?
No. This calculator computes near-end (backward) crosstalk only. Far-end crosstalk behaves differently — it grows roughly linearly with length rather than saturating — and depends on the difference between capacitive and inductive coupling, so it needs a separate calculation.
Why does the calculator ask for substrate height but not trace width?
The Dkeff approximation used for Lsat here depends mainly on substrate height and dielectric constant. Trace width also affects the true field distribution and coupling coefficient in a full field-solver model, so results here are a fast estimate, not a field-solver replacement.
What does “saturated” vs “unsaturated” mean for my layout?
Unsaturated means your parallel run is shorter than the saturation length, so shortening it further would reduce crosstalk. Saturated means you’ve already reached the maximum coupled voltage for this geometry — only wider spacing or a stackup change will reduce it further.
How much can I trust the exact coupled voltage number?
As a first-pass estimate for comparing layout options, it’s useful. For signals close to a noise-margin limit, verify with a 2D field solver or signal integrity simulation, since simplified S/H-based coupling models can diverge from full-wave results by a meaningful margin.