Lesson 48 · Boards and embedded systems

Signal integrity: when wires become transmission lines

At high speed a copper trace is no longer a simple wire. Watch a pulse travel along a 50 ohm trace, bounce back from the receiver's open input and ring, then disappear cleanly into a matching termination resistor. Then meet the other troublemakers, crosstalk and ground bounce, and the fixes: controlled impedance, differential pairs and the eye diagram that checks the result.

14 min10 steps5 questionsAdvanced

Fig 48 · Interactive

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5 quick questions

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01Roughly how fast does a signal travel along a trace on an FR-4 circuit board?

Light covers about 12 inches (30 cm) per nanosecond in air. In FR-4, whose dielectric constant is about 4, it is slowed by the square root of 4, to about 6 inches (15 cm) per nanosecond.

02A 50 ohm trace ends at a load. Which load sends back no echo at all?

The reflection coefficient is (ZL - Z0) / (ZL + Z0). With ZL = Z0 = 50 ohms it is 0, so the line looks endless and nothing reflects.

03Why does the receiver in the figure see overshoot and ringing before the termination is fitted?

At the nearly open receiver input the reflection coefficient is close to +1, so the echo adds to the arriving pulse (overshoot). At the low-impedance driver it is negative, so the echo comes back flipped. The back-and-forth echoes are the ringing.

04What causes ground bounce?

The voltage across an inductance is L x dI/dt. When several outputs switch together, the total return current changes fast and the chip's own ground jumps for a moment.

05In an eye diagram, what does a wide-open eye tell you?

An eye diagram stacks many bits on top of each other. A large opening in the middle means every 1 stays well above every 0, with room to spare for noise and timing errors.

Key terms

Glossary

Signal integrity: How faithfully a signal arrives: whether the receiver still sees clean, correct 1s and 0s after the trip.
Transmission line: A pair of conductors (here a trace and the ground plane under it) along which a fast signal travels as a wave.
Characteristic impedance (Z0): The ratio of voltage to current in a wave travelling along a line, set by trace width, height and board material; often 50 ohms.
Reflection coefficient: Γ = (ZL − Z0)/(ZL + Z0): the share of a wave that bounces back where the line meets a load. 0 means no echo.
Termination: A resistor that matches the line's impedance so the wave is absorbed instead of reflected.
Crosstalk: Unwanted noise pushed onto one trace by fast edges on a neighbouring trace, through electric and magnetic coupling.
Ground bounce: A brief jump in a chip's ground voltage when many outputs switch at once, caused by the inductance of the ground path.
Differential pair: Two traces carrying opposite copies of a signal; the receiver reads the difference, so noise common to both cancels.
Eye diagram: Many bits of a signal overlaid on a scope; the size of the open 'eye' shows how much margin the link has.

Fine print

Notes and sources

The line is a teaching model: lossless, with a 10 Ω driver, a 1.0 V swing and an ideal open receiver input. Real chips clamp very large overshoots, and real lines also lose some signal along the way.
Signal speed: light travels about 12 inches per nanosecond in air; in FR-4 board material (Dk about 4) it is about 6 in/ns, roughly 15 cm/ns (E. Bogatin, Rule of Thumb #3).
Reflection coefficient Γ = (ZL − Z0)/(ZL + Z0): +1 for an open end, 0 for a matched load, negative for a load below Z0 (UC Berkeley EECS 117).
Crosstalk is drawn as forward (far-end) crosstalk, which follows the aggressor’s edges and grows with the coupled length; a backward (near-end) part also travels back toward the driver.
Layout numbers from TI SPRAAR7: USB 2.0 pairs 90 Ω differential (81 to 99 Ω), 50 Ω single-ended targets, 5W spacing between pairs, routing over a solid ground plane.

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