PCB Design
What causes crosstalk between PCB traces?
Crosstalk is a signal leaking from one PCB trace onto its neighbour. Two traces running side by side are the plates of a small capacitor, with the board between them, and at high frequencies even a few picofarads pass signal easily. How much arrives depends on three things: how big that capacitance is, how fast the signal is, and how low an impedance the quiet trace, the victim, ends in.
A capacitor you didn't draw
A long run with a small gap might be 10 pF. Its reactance falls with frequency:
At 10 MHz that's 1.6 kΩ. At 100 MHz it's only 160 Ω, and signals go through it.
A divider onto the victim
The gap's reactance and the victim's termination form a divider. The two are 90° apart, so they add like the sides of a right triangle:
With 50 Ω at each end of the victim, 25 Ω in all, a 10 pF gap at 100 MHz lets through 25 ÷ 162, about 15 %. Move the traces apart until the gap is 1 pF, 1.6 kΩ, and it drops to about 1.6 %.
Live experiment
Open the gap
A fast trace and its quiet neighbour, with the gap between them as a capacitor of 10, 3 or 1 pF. Pick the gap, the frequency and how the victim ends. At 100 MHz, 10 pF into 25 Ω leaks about 15 %; 1 pF leaks under 2 %. End the victim in a 1 kΩ input and the same 1 pF lets about half through.
“Neighbours talk” is lesson 5 of 6 in Copper Cay, PCB Design World 1. Free with an account. Lessons unlock in order, so you’ll do the 4 before it first.
High impedance catches more
A victim that ends in a 1 kΩ input has nowhere low for the coupled current to go, so it makes a big voltage. The same 1 pF gap then lets through 1000 ÷ √(1000² + 1600²), about 53 %. Even a 0.5 pF gap, 3.2 kΩ, gives about 30 %. Put 50 Ω across that input and the victim's impedance drops to 47.6 Ω: about 1.5 %.
How to reduce crosstalk
- Space them: a bigger gap is a smaller capacitor. Keep fast traces about three widths apart.
- Shorten side-by-side runs: capacitance grows with the length of the shared edge.
- Put ground between: a ground plane underneath and a grounded trace between keep fields to themselves.
- Cross at right angles on neighbouring layers instead of running parallel.
- Keep edges no faster than needed.
This model covers only capacitive coupling. Traces also couple magnetically through their currents' fields, and on an outer layer over a ground plane the two are often comparable, but the same layout fixes cure both.
Free lesson
Now build it yourself
Each lesson comes in three short parts: see the idea, try it on a live board, then build it yourself, with quick checks as you go.
“Neighbours talk” is lesson 5 of 6 in Copper Cay, PCB Design World 1. Free with an account. Lessons unlock in order, so you’ll do the 4 before it first.
Or start from the very first lesson, no account neededPractice questions
Three quick questions from the lessons. Pick an answer to check it.
Show the answer
Answer: 15 %: 25 ÷ √(25² + 160²). The gap and the termination are a divider. The two are 90° apart, so they add like a right-angled triangle: 25 ÷ 162 of the driver reaches the victim, about a sixth.
- Why not “85 %”? That would need the gap's reactance to be small next to 25 Ω. At 160 Ω it's the bigger of the two.
- Why not “50 %”? Half would need a termination comparable to the gap. 25 Ω against 160 Ω gives about a sixth.
- Why not “1 %”? 1 % would need a gap of about 2.5 kΩ. 160 Ω lets far more through.
Show the answer
Answer: Roughly doubles the capacitance between them, and the crosstalk. A longer shared edge is a bigger capacitor. Keep fast traces apart, and keep their side-by-side runs short.
- Why not “Changes nothing”? The capacitance grows with the length the traces run side by side.
- Why not “Halves it”? A longer shared edge makes a bigger capacitor, so more couples across, not less.
- Why not “Removes it”? Running further together can only add coupling. Spacing and short runs are what reduce it.
Show the answer
Answer: 30 %: 1000 ÷ √(1000² + 3200²). A 1 kΩ load against a 3.2 kΩ gap catches nearly a third, even with a wide gap.
- Why not “1 %”? About 1 % is what a low 25 Ω termination would catch. A 1 kΩ input catches far more.
- Why not “0”? Any capacitance passes some signal at 100 MHz, and 1 kΩ is high enough to catch plenty.
- Why not “90 %”? The gap's 3.2 kΩ is still bigger than 1 kΩ, so well under half gets through.
“Neighbours talk” is lesson 5 of 6 in Copper Cay, PCB Design World 1. Free with an account. Lessons unlock in order, so you’ll do the 4 before it first.
Questions people ask
What is the 3W rule for PCB traces?
A rule of thumb for fast signals: space traces at least three trace widths apart, centre to centre. The wider gap shrinks the capacitance and the magnetic coupling between them. For very sensitive signals, designers go wider still or put a ground trace between.
Does a ground plane reduce crosstalk?
Yes. A trace close above a solid ground plane keeps its fields tight between itself and the plane, so less reaches its neighbours. The closer the trace is to the plane, the less it couples.
Why is crosstalk worse with faster signals?
The coupling capacitor's reactance, 1 ÷ (2π f C), falls as frequency rises, so more signal gets across. For digital signals it's the edges that count: a sharp edge contains high frequencies even on a slow clock.
Related topics
- Capacitive and inductive reactance explainedWhy a capacitor passes treble and a coil passes bass: X_C, X_L and Ohm's law for AC.
- What is a ground plane on a PCB?A whole layer of copper for ground: milliohms for steady current, about a nanohenry for fast current.
- Star grounding explainedGive each load its own return to one point, so a big current never flows through a sensitive circuit's ground.
- Impedance matching explainedA load takes the most power when its resistance equals the source's: why radio is 50 Ω, and how to match.