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

# Choke for line noise filtering

Instrumentation II · Chapter 6: Grounding And Shielding

- Asked in: 2074 Asoj, 8 marks
- Video: [EMC Filter Design Part 6:  Common Mode Choke Operation](https://www.youtube.com/watch?v=cxPAGWTiNSc) — Biricha
- Answer checked by Claude Sonnet 5.5 on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_5/instrumentation_ii/choke-line-noise
- Whole chapter: https://www.bctengineer.com/md/semester_5/instrumentation_ii/chapter-6

## Question

Explain the mechanism of filtering line noise with the aid of chokes. How does a choke differentiate between the signal that it needs to pass and the noise that it needs to suppress? Describe the circumstances where chokes are preferred over other noise filtering approaches.

## Short answer

A series choke uses inductive reactance to block high‑frequency noise while passing DC/low‑frequency signals. A common‑mode choke cancels flux for differential current but adds impedance for common‑mode noise. Chokes are preferred for high‑current lines where a resistor would cause unacceptable voltage drop or power loss.

## Answer

A **choke** is an inductor (usually wound on a ferrite or iron-powder core) placed in series with a line. It offers a high impedance to high-frequency noise and low impedance to the wanted signal or DC, since inductive reactance X_L = 2πfL rises with frequency.

#### Mechanism
- The choke is in series with the line, and a capacitor to ground usually follows it, forming an LC low-pass filter.
- High-frequency noise current meets high X_L and is blocked/reflected, and the leftover is diverted to ground by the capacitor.
- At high frequency the ferrite core also loses energy as heat (core loss), so it absorbs noise instead of only reflecting it.

```text
 line in ---[ L choke ]---+--- to load
                          |
                         [C]
                          |
 return ------------------+--- GND
```

**Common-mode choke:** both the signal and return wires are wound on the same core in the same sense.
- Wanted (differential) current flows in opposite directions in the two windings, so their magnetic fluxes cancel and the choke shows almost no inductance. The signal passes freely, and the core does not saturate.
- Common-mode noise current flows in the same direction in both wires, so the fluxes add and the impedance is high, blocking the noise.

#### How it separates signal from noise
The separation is by **frequency** (the signal is DC or low frequency, the noise is high frequency; X_L = 2πfL differs by a big factor) and, in the common-mode choke, by **mode** (direction of current).

#### Where chokes are preferred
1. **Power lines** carrying large currents: a resistor would waste power and drop voltage, but a choke has only small DC resistance.
2. **Low-voltage DC supplies** (e.g. 5 V logic) where series resistance would cause voltage drop.
3. **Common-mode noise** on cables and mains (USB, Ethernet, SMPS inputs) where only a CM choke works without affecting the signal.
4. **High-frequency noise** from switching supplies and digital clocks, where wide-band attenuation is needed (ferrite beads).
5. When a large capacitor alone would be unsuitable (a large capacitor draws high inrush current or slows the signal).

## Check yourself

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

1. Why does a choke block high-frequency noise?
   A. It has high dc resistance
   B. X_L = 2 pi f L rises with frequency
   C. Its capacitance rises
   D. It reflects only dc
   Answer: B. Low impedance for dc and low frequencies. (Key point: Mechanism)

2. What does a ferrite core do at high frequency?
   A. Reflects it all
   B. Saturates
   C. Absorbs noise as heat
   D. Amplifies
   Answer: C. Core loss dissipates noise energy. (Key point: Core loss)

3. Why does the CM choke pass the signal?
   A. Signal current is high frequency
   B. The core is air
   C. Fluxes add
   D. Differential currents flow oppositely, fluxes cancel, little inductance
   Answer: D. Common-mode noise fluxes add, giving high impedance. (Key point: CM choke)

4. Why prefer a choke on a power line with large current?
   A. Small DC resistance, so little power waste or voltage drop
   B. It is a resistor
   C. It drops voltage
   D. It's cheap
   Answer: A. A series resistor would waste power. (Key point: Preference)

## Related in Instrumentation II

- [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×
- [Why serial transfer is preferred over parallel](https://www.bctengineer.com/question.md?key=semester_5%2Finstrumentation_ii%3Aserial-vs-parallel-preferred) — Instrumentation II, chapter 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
- [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
- [Noise coupling mechanisms with pseudo impedance](https://www.bctengineer.com/question.md?key=semester_5%2Finstrumentation_ii%3Anc-pseudo-impedance) — Instrumentation II, chapter 6

## 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
- [Advantages of electrical energy over other forms](https://www.bctengineer.com/question.md?key=semester_7%2Futilization_of_electrical_energy%3Aadvantages-electrical-energy-other-forms) — Utilization of Electrical Energy, chapter 1, asked 2×
- [Why NOS preferred over DOS](https://www.bctengineer.com/question.md?key=semester_7%2Fdistributed_system%3Anos-vs-dos-realization) — Distributed System, chapter 3, asked 2×
- [T/F: ACSR preferred over AAAC](https://www.bctengineer.com/question.md?key=semester_8%2Ftransmission_and_distribution_system_design%3Atf-acsr-vs-aaac) — Transmission and Distribution System Design, chapter 4