--- title: "Stub discontinuity and crosstalk causes" subject: "Instrumentation II" chapter: 7 key: "semester_5/instrumentation_ii:stub-discontinuity-crosstalk" source: https://www.bctengineer.com/q/semester_5/instrumentation_ii/stub-discontinuity-crosstalk site: BCT Engineering --- # Stub discontinuity and crosstalk causes Instrumentation II · Chapter 7: Circuit Design - Asked in: 2068 Chaitra, 4+2 marks - Video: [Understanding Crosstalk - Altium Academy](https://www.youtube.com/watch?v=zOORgBmnDX8) — Altium Academy - Answer checked by Claude Sonnet 5.5 on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_5/instrumentation_ii/stub-discontinuity-crosstalk - Whole chapter: https://www.bctengineer.com/md/semester_5/instrumentation_ii/chapter-7 ## Question How stub discontinuity cause impedance mismatch. Also point out the causes of crosstalk. Explain in your own words with relevant figure. ## Short answer A stub adds parallel impedance, lowering effective Z0 and causing reflections; the open end reflects, adding ringing. Crosstalk is unwanted coupling via mutual capacitance (I = C·dV/dt) and inductance (V = M·dI/dt); caused by fast edges, small spacing, lack of ground plane; reduced by 3W spacing, shorter runs, ground plane. ## Answer #### How a stub causes impedance mismatch A **stub** is a short branch line connected to the main transmission line (for example a trace to a test point or a second receiver). At the junction, the signal travelling along the main line sees the continuing line and the stub in parallel. The effective impedance at the junction drops (for equal lines, Z₀ ∥ Z₀ = Z₀/2). This is a sudden change in impedance, so part of the wave is **reflected** (Γ = (Z₂ - Z₁)/(Z₂ + Z₁)). The stub itself is an open-ended line: the wave travels to its end, reflects back with the same polarity and returns after twice the stub delay, adding ringing and distortion to the main signal. The longer the stub, the larger the delay and effect; stub length should be less than about 1/10 of the rise-time length. ```text Source ======+=============== Load | | stub (open end) | Z drops at the junction -> reflection ``` Solution: route as a daisy chain (point to point), keep stubs very short, or use terminations. #### Causes of crosstalk 1. **Mutual (stray) capacitance** between adjacent traces: coupled current I = C·dV/dt. 2. **Mutual inductance** between adjacent loops: V = M·dI/dt. 3. Long parallel runs and **small spacing** between traces. 4. **Fast edges** (high dV/dt and dI/dt). 5. Shared or poor return paths (common ground impedance), high-impedance victim lines, and absence of a ground plane. 6. Higher signal amplitude, and routing on the same layer or adjacent layers over a long distance. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. Why does a stub cause an impedance mismatch at the junction? A. The stub adds series resistance to the line B. The signal sees the main line, continuing line and stub in parallel, so impedance drops suddenly C. The stub increases the characteristic impedance D. The stub removes the ground plane Answer: B. Parallel combination lowers impedance at the junction, and the sudden change reflects part of the wave. (Key point: Stub impedance drop) 2. What happens to the wave that travels into an open-ended stub? A. It is absorbed completely B. It reflects with inverted polarity at once C. It reflects back with the same polarity after twice the stub delay, adding ringing D. It passes into the ground plane Answer: C. An open end reflects with the same polarity, returning after twice the stub delay and distorting the main signal. (Key point: Stub open-end reflection) 3. Which is a recommended way to deal with stubs? A. Make stubs longer to absorb reflections B. Route as daisy chain, keep stubs very short or use terminations C. Remove the ground plane D. Increase edge speed Answer: B. Daisy chaining, short stubs and terminations avoid the discontinuity reflections. (Key point: Stub remedies) 4. Which combination increases crosstalk? A. Wide spacing and slow edges B. Ground plane close to traces and short runs C. Small spacing, long parallel runs and fast edges D. Differential routing with guard traces Answer: C. Mutual C and M coupling grow with fast dV/dt, dI/dt, close spacing and long parallel length. (Key point: Causes of crosstalk) ## Related in Instrumentation II - [Problems due to impedance mismatch](https://www.bctengineer.com/question.md?key=semester_5%2Finstrumentation_ii%3Aimpedance-mismatch-problems) — Instrumentation II, chapter 7 - [Routing rules for crosstalk and impedance matching](https://www.bctengineer.com/question.md?key=semester_5%2Finstrumentation_ii%3Arules-mismatch-crosstalk) — Instrumentation II, chapter 8, asked 3× - [Ground bounce, crosstalk, impedance matching, skew](https://www.bctengineer.com/question.md?key=semester_5%2Finstrumentation_ii%3Abounce-crosstalk-skew) — Instrumentation II, chapter 7 ## Related in other subjects - [Causes and crack patterns of out-of-plane failure](https://www.bctengineer.com/question.md?key=semester_5%2Fconcrete_technology_and_masonry_structure%3Aout-of-plane-failure-causes) — Concrete Technology and Masonry Structure, chapter 9 - [Crosstalk causes and hybrid transformer](https://www.bctengineer.com/question.md?key=semester_8%2Ftelecommunications%3Acrosstalk-hybrid-transformer) — Telecommunications, chapter 2 - [Driving point impedance and voltage ratio, two 0.5 F](https://www.bctengineer.com/question.md?key=semester_3%2Felectrical_circuit_theory%3Adriving-point-impedance-voltage-ratio-two) — Electrical Circuit Theory, chapter 5 - [Driving point impedance and voltage ratio, bridged RL network](https://www.bctengineer.com/question.md?key=semester_3%2Felectrical_circuit_theory%3Adriving-point-impedance-voltage-ratio-bridged) — Electrical Circuit Theory, chapter 5