--- title: "Noise coupling mechanisms with pseudo impedance" subject: "Instrumentation II" chapter: 6 key: "semester_5/instrumentation_ii:nc-pseudo-impedance" source: https://www.bctengineer.com/q/semester_5/instrumentation_ii/nc-pseudo-impedance site: BCT Engineering --- # Noise coupling mechanisms with pseudo impedance Instrumentation II · Chapter 6: Grounding And Shielding - Asked in: 2069 Chaitra, 6 marks - Video: [Noise Coupling (General)](https://www.youtube.com/watch?v=6uMjC8J3CLM) — Blake Jacquot - Answer checked by Claude Sonnet 5.5 on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_5/instrumentation_ii/nc-pseudo-impedance - Whole chapter: https://www.bctengineer.com/md/semester_5/instrumentation_ii/chapter-6 ## Question Explain different types of Noise coupling Mechanism with concept of Pseudo impedance. ## Short answer Pseudo impedance Z = V/I identifies coupling: high Z (>377 Ω) denotes capacitive (E-field), low Z (<377 Ω) inductive (H-field), and ~377 Ω radiated (far-field). Conductive coupling uses shared conductors. This guides shield selection. ## Answer **Pseudo (source) impedance** is the ratio of voltage to current of the noise source, Z = V/I. It tells whether the noise energy is mostly electric field (high V, low I) or magnetic field (high I, low V). It is compared with the intrinsic impedance of free space (377 ohm). #### Types of coupling - **Capacitive (electric field):** high-voltage, low-current source, so Z is high (Z ≫ 377 ohm). The stray capacitance C passes I = C·dV/dt. Examples: high-voltage lines, unshielded fast switching nodes. - **Inductive (magnetic field):** high-current, low-voltage source, so Z is low (Z ≪ 377 ohm). Induced voltage V = M·dI/dt. Examples: motors, transformers, power cables. - **Radiated (electromagnetic):** in the far field the electric and magnetic fields are in a fixed ratio; wave impedance Z = 377 ohm (E/H = 377). Examples: radio transmitters, long antennas. - **Conductive:** noise flows through common wires; here Z is the impedance of the shared conductor, and noise voltage = I × Z_common. ```text Z (pseudo impedance) high >>377 ohm : capacitive (E field) 377 ohm : radiated (plane wave) low <<377 ohm : inductive (H field) ``` Near fields change with distance. For a high-impedance (electric) source, the electric field falls off as 1/r³ and the magnetic field as 1/r²; for a low-impedance (magnetic) source it is the other way round. In both cases the wave impedance moves towards 377 ohm as the distance exceeds λ/2π. So by calculating Z of the source, the dominant coupling can be identified and the correct shield chosen (electrostatic for E, magnetic for H). ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. What is pseudo (source) impedance? A. The resistance of the victim wire only B. The ratio V/I of the noise source, compared with 377 ohm C. The ratio of frequency to wavelength D. The impedance of ground Answer: B. It shows whether the field is mainly electric or magnetic. (Key point: Definition) 2. What does a source impedance much greater than 377 ohm indicate? A. Capacitive (electric field) coupling B. Inductive coupling C. Far-field radiated wave D. Conductive coupling only Answer: A. High V and low I means an electric-field dominated source. (Key point: Capacitive case) 3. Which source gives inductive coupling? A. High voltage and low current B. Z exactly 377 ohm C. Zero current D. High current and low voltage, Z much less than 377 ohm Answer: D. Large current produces a strong magnetic field and induced V = M dI/dt. (Key point: Inductive case) 4. What is the wave impedance in the far field? A. 0 ohm B. Infinite C. 377 ohm, with E/H in a fixed ratio D. It varies with source current Answer: C. In the far field electric and magnetic fields travel together at 377 ohm. (Key point: Radiated case) ## Related in Instrumentation II - [Types of noise coupling mechanisms](https://www.bctengineer.com/question.md?key=semester_5%2Finstrumentation_ii%3Anc-types) — Instrumentation II, chapter 6, asked 3× - [Identify noise coupling mechanism by calculation](https://www.bctengineer.com/question.md?key=semester_5%2Finstrumentation_ii%3Aswitching-noise-calc) — Instrumentation II, chapter 6 - [Filtering for conductive noise coupling](https://www.bctengineer.com/question.md?key=semester_5%2Finstrumentation_ii%3Aconductive-noise-filtering) — Instrumentation II, chapter 6, asked 2× - [Noise coupling types and checking predominance](https://www.bctengineer.com/question.md?key=semester_5%2Finstrumentation_ii%3Anc-predominance) — Instrumentation II, chapter 6, asked 2× - [Inductive noise and capacitive coupling shielding](https://www.bctengineer.com/question.md?key=semester_5%2Finstrumentation_ii%3Ainductive-noise-capacitive-shield) — Instrumentation II, chapter 6 - [Arc welder noise coupling in robotic arm controller](https://www.bctengineer.com/question.md?key=semester_5%2Finstrumentation_ii%3Aarc-welder-noise) — Instrumentation II, chapter 6 ## Related in other subjects - [Sequence impedance of load with mutual coupling](https://www.bctengineer.com/question.md?key=semester_5%2Fpower_system_analysis_ii%3Amutually-coupled-load-sequence-impedance) — Power System Analysis II, chapter 4