---
title: "Filtering for conductive noise coupling"
subject: "Instrumentation II"
chapter: 6
key: "semester_5/instrumentation_ii:conductive-noise-filtering"
source: https://www.bctengineer.com/q/semester_5/instrumentation_ii/conductive-noise-filtering
site: BCT Engineering
---

# Filtering for conductive noise coupling

Instrumentation II · Chapter 6: Grounding And Shielding

- Asked in: 2076 Chaitra, 6 marks; 2075 Asoj, 6 marks (asked 2 times)
- Video: [EMC Filter Design Part 1: Understanding Common Mode and Differential Mode Noise](https://www.youtube.com/watch?v=JQkNqY0I02Y) — Biricha
- Answer checked by Claude Sonnet 5.5 on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_5/instrumentation_ii/conductive-noise-filtering
- Whole chapter: https://www.bctengineer.com/md/semester_5/instrumentation_ii/chapter-6

## Question

Explain different types of filtering mechanisms (based on frequency, mode and amplitude) used to reduce conductive noise coupling.

## Short answer

Low‑pass LC filters block high‑frequency noise. Differential‑mode noise is reduced by X capacitors and series inductors, while common‑mode noise requires common‑mode chokes and Y capacitors to ground. Amplitude limiters such as TVS diodes, MOVs, and gas discharge tubes clamp spike levels.

## Answer

Conductive noise travels along wires (power, signal or ground). A **filter** passes the wanted signal and blocks the noise. Filters are classified in three ways.

#### Based on frequency
- **Low-pass:** passes low frequencies, blocks high-frequency noise. Used on power lines and sensor inputs (RC, LC, or ferrite bead plus capacitor).
- **High-pass:** blocks DC/low-frequency drift and hum, passes the signal (series capacitor, coupling).
- **Band-pass:** passes only a band around the signal frequency.
- **Band-reject (notch):** removes a single frequency such as 50 Hz mains hum.

```text
 Low-pass LC:   in --L--+-- out
                        C
                        |
                       GND
```

#### Based on mode
Noise appears in two modes:
- **Differential (normal) mode:** noise voltage between the two conductors (signal and its return), in opposite directions. Reduced by series inductors and a capacitor **across** the lines (X capacitor).
- **Common mode:** noise voltage that appears equally on both conductors with respect to ground, in the same direction. Reduced by a **common-mode choke** (both lines wound on one core) and capacitors from each line to ground (Y capacitors).

The two modes need different filters, so line filters normally contain both: X capacitor, common-mode choke and Y capacitors.

#### Based on amplitude
These limit the noise by its size, not frequency:
- **Clamping/clipping circuits** using zener, TVS or ordinary diodes limit spikes above a set level.
- **Varistors (MOV)** and **gas discharge tubes** absorb large surges.
- **Amplitude (slicing) filters/Schmitt triggers** pass only signals above a threshold, rejecting small noise.

In practice a combination (an LC low-pass for frequency, common-mode choke for mode and a TVS for amplitude) is used at the entry of the system.

## Check yourself

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

1. Which filter removes a single frequency such as 50 Hz hum?
   A. Low-pass
   B. Band-reject (notch)
   C. High-pass
   D. Band-pass
   Answer: B. A notch removes one frequency. (Key point: Notch)

2. Which filter parts reduce differential-mode noise?
   A. Y capacitors to ground
   B. A zener
   C. Series inductors and an X capacitor across the lines
   D. A varistor
   Answer: C. Differential noise is between the conductors. (Key point: Differential mode)

3. What reduces common-mode noise?
   A. A series resistor
   B. An X capacitor only
   C. A notch
   D. A common-mode choke and Y capacitors to ground
   Answer: D. Noise is the same direction on both lines. (Key point: Common mode)

4. Which devices filter by amplitude?
   A. Clamping diodes, TVS, varistors
   B. LC low-pass
   C. Ferrite bead
   D. Notch
   Answer: A. They limit spikes above a set level. (Key point: Amplitude)

## Related in Instrumentation II

- [Reducing conductive noise coupling](https://www.bctengineer.com/question.md?key=semester_5%2Finstrumentation_ii%3Aconductive-noise-reduce) — Instrumentation II, chapter 6
- [Types of noise coupling mechanisms](https://www.bctengineer.com/question.md?key=semester_5%2Finstrumentation_ii%3Anc-types) — Instrumentation II, chapter 6, asked 3×
- [Noise coupling mechanisms with pseudo impedance](https://www.bctengineer.com/question.md?key=semester_5%2Finstrumentation_ii%3Anc-pseudo-impedance) — Instrumentation II, chapter 6
- [Choke for line noise filtering](https://www.bctengineer.com/question.md?key=semester_5%2Finstrumentation_ii%3Achoke-line-noise) — Instrumentation II, chapter 6
- [Grounding and shielding to reduce PCB noise](https://www.bctengineer.com/question.md?key=semester_5%2Finstrumentation_ii%3Apcb-grounding-shielding-noise) — Instrumentation II, chapter 8
- [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×

## Related in other subjects

- [Ideal and RC filtering of white noise](https://www.bctengineer.com/question.md?key=semester_6%2Fcommunication_systems%3Arc-filtering-white-noise) — Communication Systems, chapter 9
- [Define cutoff frequency, dominant mode](https://www.bctengineer.com/question.md?key=semester_3%2Felectromagnetism%3Acutoff-frequency-dominant-mode-define) — Electromagnetism, chapter 6
- [Noise, interference, distortion; thermal and high frequency noise](https://www.bctengineer.com/question.md?key=semester_6%2Fcommunication_systems%3Anoise-effects-thermal-hf) — Communication Systems, chapter 1