---
title: "Impedance matching and discontinuities"
subject: "Instrumentation II"
chapter: 7
key: "semester_5/instrumentation_ii:impedance-matching-discontinuity"
source: https://www.bctengineer.com/q/semester_5/instrumentation_ii/impedance-matching-discontinuity
site: BCT Engineering
---

# Impedance matching and discontinuities

Instrumentation II · Chapter 7: Circuit Design

- Asked in: 2072 Kartik, 2 marks
- Video: [Impedance Matching Basics](https://www.youtube.com/watch?v=XCiBvhleDIM) — All Electronics Channel
- Answer checked by Claude Sonnet 5.5 on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_5/instrumentation_ii/impedance-matching-discontinuity
- Whole chapter: https://www.bctengineer.com/md/semester_5/instrumentation_ii/chapter-7

## Question

Define impedance matching. What is the impact of impedance discontinuities?

## Short answer

Impedance matching makes source, line, and load impedances equal (Z_s = Z_0 = Z_L) to prevent signal reflections. Impedance discontinuities (trace width changes, vias, stubs) cause reflections leading to ringing, overshoot, distortion, false triggering, timing errors, and EMI.

## Answer

**Impedance matching** is making the source impedance, the characteristic impedance (Z₀) of the transmission line and the load impedance equal (Z_s = Z₀ = Z_L), so that the signal is fully transferred to the load and no reflection occurs.

**Impact of discontinuities:** any sudden change in impedance (change in trace width, via, stub, connector, layer change, branch, gap in the ground plane) causes part of the signal to be **reflected**, with reflection coefficient Γ = (Z_L - Z₀)/(Z_L + Z₀). This results in ringing, overshoot/undershoot, signal distortion, false triggering, increased delay and timing error, higher EMI, and loss of signal power.

## Check yourself

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

1. What is impedance matching?
   A. ZL much larger than Z0
   B. Zs = 0 only
   C. Zs = Z0 = ZL, so full transfer and no reflection
   D. Zs = 2 Z0
   Answer: C. Equal impedances prevent reflections. (Key point: Definition)

2. What is the reflection coefficient?
   A. (ZL + Z0)/(ZL - Z0)
   B. (ZL - Z0)/(ZL + Z0)
   C. ZL Z0
   D. ZL / Z0
   Answer: B. It is zero when ZL = Z0. (Key point: Reflection coefficient)

3. What is an impact of discontinuities?
   A. Reflections causing ringing, overshoot, false triggering and EMI
   B. Cleaner edges
   C. Reduced EMI
   D. Faster signal with no distortion
   Answer: A. Part of the wave bounces back at each impedance change. (Key point: Impact)

## Related in Instrumentation II

- [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

- [Impedance matching techniques](https://www.bctengineer.com/question.md?key=semester_3%2Felectromagnetism%3Aimpedance-matching-quarter-wave-stub) — Electromagnetism, chapter 5, asked 3×
- [Double-stub matching 110+j110 Ω on 100 Ω line](https://www.bctengineer.com/question.md?key=semester_7%2Frf_and_microwave_engineering%3Adouble-stub-110-j110) — RF and Microwave Engineering, chapter 2, asked 2×
- [Matching techniques and double-stub S-matrix](https://www.bctengineer.com/question.md?key=semester_7%2Frf_and_microwave_engineering%3Amatching-techniques-double-stub-smatrix) — RF and Microwave Engineering, chapter 2
- [Single stub matching for ΓL = 0.5∠51°](https://www.bctengineer.com/question.md?key=semester_7%2Frf_and_microwave_engineering%3Asingle-stub-reflection-0-5-51) — RF and Microwave Engineering, chapter 2