A ferrite clamp is a passive electronic component used to suppress high-frequency noise signals or Electromagnetic interference (EMI) signals on a power supply line/data line. This electronic component is commonly placed at the end of a power line or data cable that is incoming to a particular device, such as a laptop power cord, or USB cable. The ferrite clamp acts as an EMI/RFI filter that blocks high-frequency noises from entering or exiting a device, thereby safeguarding the connected device as well as other nearby electronic devices from potential interference/ EMI noises. Here, RFI stands for radio frequency interference. 

Ferrite Clamps

A ferrite clamp is also called a cable ferrite or ferrite EMI filter.  It looks a like short barrel at the end of power lines, data lines, USB cables, etc. (Figure 1). It can be opened and installed around a cable or wire without the need to disconnect any terminal. The Ferrite clamps are made of ferrite material (magnetic material). The ferrite material is typically comprised of iron oxide and oxide composite materials, such as nickel, zinc, manganese, and ceramic material. These clamps are available in various shapes/configurations such as hollow cylinder shape, split core ferrite shape, and flat cable core ferrite.

The primary function of the ferrite clamp is to absorb the conducted EMI (i.e., high-frequency noise signals traveling on power lines/cables) and dissipate the noise in the form of heat. It also prevents the power cable carrying a signal from acting as either a transmitting or receiving antenna (i.e., prevents radiated EMI emission from the cable as well as radiated EMI pickup in the cable).   In short, the ferrite clamp helps to reduce conducted and/or radiated emission, as well as suppress radiated EMI pickup in the cable. Thereby improving the performance of power supply cables and ensuring signal integrity. Moreover, it is effective in suppressing common mode noise, so can be implemented as a noise countermeasure without influencing signal quality.

The ferrite clamps are critical components in the field of Electromagnetic compatibility (EMC) to ensure electrical/electronic equipment's reliable operation. The use of Ferrite clamps is a low-cost solution to EMI/RFI issues and is ideal for use in electronic devices and systems used in various applications, including computer technology, terminals, digital telephones, audio equipment, electronic musical instruments, industrial controllers, medical electronics, office machines, and more.

Characteristics and Working principle of ferrite clamp:

Figure 3: Typical Impedance vs. frequency characteristic of Ferrite clamp

A ferrite clamp or ferrite EMI filter, by design, exhibits a specific characteristic that its impedance varies when frequency varies. It becomes highly resistive (i.e., lossy) at the designed frequency range (i.e., it provides high impedance (resistance) at the designed frequency range) and dissipates high-frequency noises in the form of heat. Below the designed frequency range, the ferrite clamp shows inductive behavior. Above the designed frequency range, it shows capacitive behavior (Figure). The ferrite clamp works like a low-pass filter that allows only desirable low-frequency signals to pass through the power line/cable and eliminates high-frequency noises on the power line/cable. 

The ferrite clamp is placed around a power line/cable to suppress EMI. When high-frequency noise currents within the target frequency range flow through the power line/data cable, creates a magnetic field around the power line/cable. This magnetic field magnetizes the ferrite core of the ferrite clamp. Because of the high permeability of ferrite material, the amount of energy stored magnetically in the ferrite clamp is very high. At the designed target frequency range, the ferrite clamp acts as a resistor (i.e., becomes lossy, low-Q), resulting in the energy stored magnetically in the ferrite core being dissipated as heat. In this way, the ferrite clamp absorbs the conducted EMI and/or radiated EMI on the power line/cable and dissipates it in the form of heat, suppressing EMI noises.

Figure 4: Ferrite clamp working principle

Moreover, the ferrite clamp will NOT impact the DC or low-frequency signals, meaning it attenuates the high-frequency noise signals without affecting the desirable lower-frequency components of the power line or cable. This operation protects electrical/electronic equipment from EMI/RFI and ensures the reliable operation of the products. 

Key specification parameters of ferrite clamp:

Type: Ferrite Clamp EMI filter

Material: Represents the ferrite material (e.g., NiZn or MnZn) that made the ferrite clamp.

Impedance range Vs frequency: Represents impedance Vs frequency characteristic of the ferrite clamp, usually presented in a graph in the manufacturer’s datasheet. Some manufacturer specifies impedance at one frequency, typically at 100 MHz or at a few frequencies. Select the ferrite clamp that offers high impedance (resistance) within the frequency range of interest that needs to be suppressed. 

Figure 5: Typical Impedance vs. frequency characteristics of ferrite clamp provided in a datasheet Aperture Diameter: It represents the aperture diameter of the ferrite clamp, expressed in mm. 

Length: Represents the length of the ferrite clamp in mm.

Height: Represents the height of the ferrite clamp in mm.

Operating temperature: It represents the operating temperature limit of the ferrite clamp, expressed in ^oC.