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Using Ferrite Rings as Signal Attenuators

Key Takeaways

  • Ferrite rings are shields made of magnetic material for attenuating high-frequency signals from cables and circuits.

  • The simplest ferrite ring can be formed by inserting a conducting wire through a hollow piece of ferrite. 

  • At the resonant frequency, the ferrite ring acts as a resistor, creating power loss in the form of heat. 

 Ferrite ring

Axial ferrite rings included in the cables for EMI shielding

You might have noticed a cylindrical lump in your laptop charger or power cord; these lumps are the axial ferrite rings included in cables for EMI shielding. A laptop charger cable carries electric current and introduces high-frequency energy, especially radio frequency signals, in power lines. The cable acts as an antenna, radiating and receiving radiofrequency signals from surrounding electrical and electronic devices. The inclusion of a ferrite ring in a charging cable blocks emissions, thereby reducing the electromagnetic interference associated with the cable. Let's take a closer look at the purpose of ferrite rings in this article.   

What Are Ferrite Rings?

Ferrite rings are shields made of magnetic material for attenuating high-frequency signals from cables and circuits. In ferrite rings, attenuated high-frequency energy gets dissipated as heat energy. A fundamental working principle of a ferrite ring is that it behaves like a resistor at high frequency and dissipates the high-frequency energy as heating power loss. 

For high-frequency signal attenuation, cables are drawn through a ring made of ferrite material, forming axial ferrite rings. The axial ferrite rings are designed for cable mounting. In PCBs, surface-mounted device-type ferrite rings are used for noise suppression. They are also called chip ferrite beads and are available in packages such as 1206 or 0603.  

The Functions of Ferrite Rings

Ferrite rings are employed in circuits or inserted in cables to:

  1. Suppress EMI: Ferrite beads block the high-frequency signal emissions and absorption associated with cables, helping to suppress electromagnetic interference.

  2. Control parasitic oscillation: In RF, audio, and other electronic amplifiers, unfavorable oscillations called parasitic oscillations are observed due to the feedback employed. By inserting suitable ferrite rings along with the leads of active amplifying devices, it is possible to limit the effect of parasitic oscillations from the amplifier output. 

  3. Reduce RF coupling: Commonly, RF signals get coupled with audio signals when audio amplifiers and RF transmitters are integrated. RF coupling is often seen in audio amplifiers, boosting the microphone output and supplying the same to RF transmitters. Incorporating ferrite rings in such circuits can block RF signals from coupling with audio signals. 

  4. Improve power supply decoupling: When IC power pins are connected to a power supply or power plane, it is advantageous to insert ferrite rings. Usually, a decoupling capacitor is introduced between the power pin of an IC and the ground to bypass high-frequency signals. The ferrite rings impart high impedance for the high-frequency signals from ICs and direct them to the ground via a decoupling capacitor (low impedance path). There is a tendency for the high-frequency energy generated in a chip to interfere with the power supply. The placement of ferrite beads can also prevent the passage of high-frequency signals from the power supply to ICs and vice versa.   

Ferrite Rings and Frequency Dependency

The simplest ferrite ring can be formed by inserting a conducting wire through a hollow piece of ferrite. Depending on the ferrite material type and ferrite core shape, the characteristics of the ferrite ring change.

The impedance offered by ferrite rings to signals varies with their frequency. The impedance of ferrite rings is a function of the frequency; therefore, the frequency of the application is an important parameter in the design of ferrite rings. The nature of the ferrite ring impedance can be summarized as:

  • For frequencies less than the resonant frequency (fr), the ferrite ring behaves like an inductor.
  • At the resonant frequency, the ferrite ring acts as a resistor, creating power loss in the form of heat. 
  • For frequencies greater than the resonant frequency, the ferrite ring exhibits capacitive impedance due to the influence of parasitic elements. 

To function as a noise suppressor or EMI suppressor, it is necessary to design the ferrite ring with resonant frequency closer to the frequency band of the signals in use. Such designs make ferrite rings similar to resistors and thus aid the removal of noises or EMI in the form of heat energy. 

Ferrite Rings for Cable Shielding

Ferrite rings are commonly used to resolve EMI issues in cables. For a ferrite ring to function effectively as a cable shield, there are certain guidelines to follow during installation. Some key points to remember during cable ferrite ring installation are:

  1. Size of the cable: The diameter of the cable running through the ferrite ring influences its performance as an attenuator. Multiple cables may get inserted through a single ferrite ring. Introducing a snug attachment that increases the magnetic path length can boost the signal attenuation in cable ferrite rings. 
  2. Placement: It is more accurate to place the ferrite ring closer to the cable termination where it leaves the electronic enclosure. If the cable connects two separate enclosures, there is nothing wrong with installing ferrite rings at both ends. The aim is to place the ferrite ring closer to the RF source. 
  3. Tight fit: Providing a tight fit between the outer dimension of the cable and the inner dimension of the ferrite ring can help improve the signal attenuation ratio and magnetic path. 

Placing a ferrite ring on a power cable and inserting it on the leads of an active device or power pin of an IC can suppress EMI only when the design takes into consideration the application frequency and follows the installation guidelines. Consider Cadence’s PCB design and analysis software to help you do this.

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