---
title: "Transmission line issues in high speed design"
subject: "Instrumentation II"
chapter: 7
key: "semester_5/instrumentation_ii:transmission-line-issues"
source: https://www.bctengineer.com/q/semester_5/instrumentation_ii/transmission-line-issues
site: BCT Engineering
---

# Transmission line issues in high speed design

Instrumentation II · Chapter 7: Circuit Design

- Asked in: 2079 Baisakh, 6 marks
- Answer checked by Claude Sonnet 5.5 on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_5/instrumentation_ii/transmission-line-issues
- Whole chapter: https://www.bctengineer.com/md/semester_5/instrumentation_ii/chapter-7

## Question

Explain the different types of transmission line issues that should be considered while designing the high speed circuit.

## Short answer

When trace length exceeds t_r/(2·t_pd), it behaves as a transmission line. Key issues: characteristic impedance matching to avoid reflections, propagation delay, skew between lines, crosstalk from adjacent traces, impedance discontinuities (stubs, vias, width changes), attenuation and dispersion, ground bounce/return path, and EMI. Use proper termination and controlled impedance routing.

## Answer

A trace behaves as a **transmission line** when the signal rise time is comparable to or shorter than twice the propagation delay of the trace (roughly length > t_r/(2·t_pd)). Then voltage and current exist as travelling waves, and the following issues must be considered.

1. **Characteristic impedance (Z₀) and impedance matching:** Z₀ = √(L/C) depends on trace width, dielectric thickness and εr. Source, line and load should be matched to avoid reflections.
2. **Reflections:** a mismatch (Γ = (Z_L - Z₀)/(Z_L + Z₀)) reflects part of the wave, producing ringing, overshoot and undershoot that can create false triggering. Use series or parallel termination.
3. **Propagation delay:** t_pd = √(μ·ε) per unit length (about 170 ps per inch on FR4); it affects timing, so delay-critical lines must be short.
4. **Skew:** unequal delay between lines of a bus or differential pair; match lengths.
5. **Crosstalk:** coupling between adjacent lines; control the spacing and use ground planes.
6. **Impedance discontinuities:** stubs, vias, bends, connectors, plane splits and width changes.
7. **Attenuation and dispersion:** conductor (skin effect) and dielectric loss at high frequency, which slows edges.
8. **Ground bounce/return path and EMI:** a continuous return path under the trace is required.
9. **Termination methods:** series (source) termination, parallel, Thevenin and AC termination; the choice affects power and speed.

## Check yourself

The answer's key points as multiple-choice questions, with the correct option and why.

1. When must a trace be treated as a transmission line?
   A. When the rise time is comparable to or shorter than twice the propagation delay of the trace
   B. When the signal is DC
   C. When the trace is shorter than 1 mm
   D. When the rise time is much longer than the propagation delay
   Answer: A. If edges are fast relative to the trace delay, voltage and current behave as travelling waves. (Key point: When a trace is a transmission line)

2. What does the characteristic impedance Z0 = sqrt(L/C) of a trace depend on?
   A. Trace length only
   B. Supply voltage
   C. Trace width, dielectric thickness and dielectric constant
   D. Signal frequency only
   Answer: C. Geometry and material set L and C per unit length, hence Z0. (Key point: Characteristic impedance)

3. What results from a mismatch between Z_L and Z0?
   A. Part of the wave reflects, causing ringing, overshoot and undershoot
   B. No effect on the signal
   C. Faster edges with no distortion
   D. Lower propagation delay
   Answer: A. Reflection coefficient (Z_L - Z0)/(Z_L + Z0) is nonzero, giving ringing that may falsely trigger inputs. (Key point: Reflection)

4. What causes attenuation and dispersion at high frequency?
   A. Decoupling capacitors
   B. Conductor skin effect and dielectric loss, which slow edges
   C. Use of 45-degree bends
   D. Orthogonal routing
   Answer: B. Skin effect and dielectric loss grow with frequency, degrading and slowing edges. (Key point: Losses at high frequency)

## Related in Instrumentation II

- [High speed circuit design guidelines](https://www.bctengineer.com/question.md?key=semester_5%2Finstrumentation_ii%3Ahigh-speed-guidelines) — Instrumentation II, chapter 7, asked 3×
- [Return current path in high-speed circuits](https://www.bctengineer.com/question.md?key=semester_5%2Finstrumentation_ii%3Ahigh-speed-ground-return) — Instrumentation II, chapter 6
- [Factors while routing signal traces on PCB](https://www.bctengineer.com/question.md?key=semester_5%2Finstrumentation_ii%3Aroute-factors) — Instrumentation II, chapter 8, asked 4×
- [Establishing requirements for circuit design](https://www.bctengineer.com/question.md?key=semester_5%2Finstrumentation_ii%3Areq-to-design) — Instrumentation II, chapter 7, asked 4×

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