Lesson · Problem 12
Route a Differential Pair
Two traces that travel together
You should be able to
- Match a pair’s length and gap over a continuous ground reference.
- Separate those layout limits from a calculated impedance this course does not grade.
High-Speed Fundamentals · Routing
What it is
Match length so the skew stays inside the limit. Keep the edge gap inside the window the rules state. Couple them over a continuous ground reference. This challenge does not calculate a target impedance. Width and gap are the published numbers, not a field solver result.
A pair is one signal seen as the voltage between two traces. Length mismatch turns that into a time mismatch, which this course calls skew and measures in millimetres of copper, including via barrels when the rule says so. Impedance is the electrical look of the pair. You will hear 90 Ω in industry. This challenge does not solve for it. It grades the geometry it published. Scope: routing geometry and reference continuity, not a field solver.
Why this matters
A differential pair carries a signal as the voltage between two traces. If one trace is longer, or leaves the ground under it, the pair stops behaving as a pair.
What you are building
Two traces of a stated width, a stated gap, and a length difference inside the skew limit, over ground.
Prerequisites
This lesson uses trace, trace width, return path, reference plane, via, routed length. It introduces differential pair, common mode, skew, pair gap, pair coupling, impedance caveat.
Component guide
What the parts are. The requirements panel is still the list that is graded.
Differential pair
- What it does
- Two traces used as one signal, compared with each other.
- Why it is here
- Length mismatch and a missing ground reference stop them behaving as a pair.
- Symbol
- Two nets, often labeled P and N or D+ and D−.
- Constraints
- Width, gap, and skew are the published geometry. This challenge does not solve 90 Ω.
- Beginner mistake
- Adding a via on one polarity to “match” a picture and leaving the ground reference.
- What an engineer checks
- The width, the gap window, and the skew limit in the requirements. Not a field-solver target.
Interview lens
If someone says “make it 90 Ω,” say this challenge does not solve impedance. It grades width, gap, skew, and the reference.
What PCBGrade measures
Those geometric limits, including via barrels when the rule says so. No field solver.
Where this shows up
USB and other pairs are routed this way. Matching by eye on the wrong layer still fails.
Terms
- Differential pair
- Two traces used as one signal, compared with each other.
- Skew
- The difference in length between the two polarities, including via barrels when the rule says so.
- Impedance
- How the pair looks electrically. Width and gap are not a solved impedance. This challenge does not grade one.
- Common mode
- The part of the two traces that moves together. The receiver is looking at the difference instead.
What is happening electrically
A differential pair carries information as the voltage between two traces. The two currents are meant to be equal and opposite. That difference is the signal. What the two traces do together — both rising, both falling — is the common-mode part. A receiver that looks at the difference rejects a lot of what hit both wires the same way.
The fields between the two traces and the fields from each trace to the reference plane are both real. Pair coupling is the part between the traces. The reference still matters: the return for any common-mode current, and for energy at a discontinuity, lives in the plane. If one trace is longer, the difference arrives skewed. If the pair leaves the plane, the fields rearrange.
Why the geometry matters
Keep the two traces together, at the width and gap the rules state, over continuous ground. Match length so the skew stays inside the budget. A bend on only one polarity adds length. A via changes layer, adds barrel length, and can leave the L2 reference. A reference-plane change is a discontinuity even when the two traces stay the same distance apart. Width and gap set the geometry this course can measure. They are not, by themselves, a solved impedance.
How an engineer reasons
Treat the pair as one signal with two wires. After every obstacle, ask three questions: are the lengths still matched, are they still over the same ground, and did a via or a gap change the reference? Industry will also ask what impedance the geometry makes. This challenge deliberately does not. Guessing “0.20 mm and 0.20 mm gap is 90 Ω” is the mistake.
Worked example
Both polarities are 0.20 mm on L1 over L2 ground. Skew must be at most 0.50 mm. Uncoupled length and the gap window are in the requirements. One uncompensated 90° bend is already on the order of the skew budget.
Good and bad
Both polarities on L1, same width, gap inside the window, skew under 0.50 mm, ground unbroken underneath: the pair is still a pair. One polarity detouring around a part, “fixed” by a via and a loop on the bottom, can look matched in a picture and still leave the reference and spend the skew budget. A 90° jog on one wire alone is already on the order of the skew this board allows.
Common mistake
Routing one polarity around an obstacle and “fixing” the other with a via to the bottom, where the reference is not ground.
Where this rule stops
PCBGrade does not pretend that a fixed width and gap equal a target impedance. Impedance depends on stackup, dielectric, and copper thickness, and this course does not field-solve them. The published width, gap, skew, uncoupled length, and reference are the grade. Outside this challenge, you still calculate or simulate impedance when the standard demands it.
A bend, a via, and a reference change are discontinuities even when the gap number is still inside the window. Match the length on the layer that faces ground before you spend a via to make a picture look even.
Before the challenge
The challenge publishes width, gap, skew, and the reference layer. Match the pair there. It does not ask you to compute an impedance.
Ready for the challenge
You can keep both polarities on the reference layer and explain why a via is not a free length fix.
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