Each of us saw small cylinders on the power cords or on the cables for matching electronic devices. They can be found on the most common computer systems, both in the office and at home, at the ends of the wires that connect the system unit to a keyboard, mouse, monitor, printer, scanner, etc. This element is called a ferrite ring (or ferrite filter). In this article, we will understand the purpose for which manufacturers of computer and high-frequency equipment equip their cable products with the mentioned elements.
Main purpose
A ferrite ring is able to reduce the effect of radio frequency and electromagnetic interference on the signal that is transmitted through the wire. Long signal and power cables of both computer and other power equipment have spurious properties, that is, they work like antennas. They very effectively emit various noises that are created inside the device into the external environment, thereby creating interference at radio stations when receiving a radio signal and other electronic equipment. Conversely, when receiving interference from the air from radio transmitting devices, a computer or other electronic device may malfunction. Here, to eliminate this phenomenon, they use a ferrite ring, worn on the supply or matching cable.
Physical properties
Ferrite is a ferrimagnet that does not conduct electric current, that is, in fact, it is a magnetic insulator. Eddy currents are not created in this material , and therefore it is very quickly magnetized - to the beat of the frequency of external electromagnetic fields. This property of the material is the basis for the effective protection of electronic devices. A ferrite ring worn on the cable can create a large active impedance for common-mode currents.
This material is formed from a chemical compound of iron oxides with oxides of other metals. It has unique magnetic characteristics and low electrical conductivity. Due to this, ferrites practically have no competitors among other magnetic materials in high-frequency technology. Ferrite rings of 2000 nm significantly increase the inductance of the cable (several hundred or thousand times), which ensures the suppression of high-frequency interference. This element is installed on the cord during its manufacture or, cut into two semicircles, is put on the wire immediately after its manufacture. The ferrite filter is packed in a plastic case. If you cut it, you can see a piece of metal inside.
Is a ferrite filter needed? Or is this another deception?
Computers are very "noisy" (in electromagnetic terms) devices. So, the motherboard inside the system unit is able to oscillate at a frequency of one kilohertz. The keyboard has a microchip, which also works at a high frequency. All this leads to the so-called generation of radio noise near the system. In most cases, they are eliminated by shielding the circuit board from electromagnetic fields with a metal case. However, another source of noise is copper wires that connect various devices. In fact, they act as long antennas that pick up signals from the cables of other radio and television equipment, and affect the operation of "their" device. A ferrite filter eliminates electromagnetic noise and broadcast signals. These elements convert electromagnetic high-frequency vibrations into thermal energy. That's why they are installed at the ends of most cables.
How to choose a ferrite filter
To install a ferrite ring on your cable yourself, you need to understand the types of these products. After all, it depends on the type of wire and its thickness which filter (from which material) will be required. For example, a ring mounted on a multicore cable (power cord, data transmission, video or USB interface) creates a so-called common-mode transformer in this section, which transmits out-of-phase signals that carry useful information, and also reflects common-mode interference. In this case, non-absorbing ferrite should be used to avoid disruption of information transfer, but a higher-frequency ferromaterial. But the ferrite rings on the antenna cable are preferable to choose from a material that will scatter high-frequency interference, rather than reflecting them back into the wire. As you can see, an incorrectly selected product can degrade the performance of your device.
Ferrite Cylinders
Thick ferrite cylinders most effectively deal with interference. However, it should be borne in mind that filters that are too bulky are very inconvenient to use, and the results of their work will hardly differ much in practice from slightly smaller ones. Filters of optimal dimensions should always be used: the inner diameter should ideally coincide with the wire, and its width should correspond to the width of the cable connector.
Do not forget also that not only ferrite filters help fight noise. For example, for better conductivity, it is recommended to use cables with a large cross section. When choosing a cord length, you should not make a large margin of length between connected devices. In addition, the poor quality of the connection of the wire and the connector can serve as a source of interference.
Labeling Ferrite Rings
The most widespread type of labeling marking ferrite rings is as follows: K D Γ d Γ N, where:
- K is an abbreviation for the word βringβ;
- D is the outer diameter of the product;
- d is the inner diameter of the ferrite ring;
- N - filter height.
In addition to the overall dimensions of the product, the type of ferromagnetic material is encrypted in the marking. An example of a recording may have the following form: M20VN-1 K 4x2.5x1.6. The second half corresponds to the overall dimensions of the ring, and the first magnetic permeability (20 ΞΌ i ) is encrypted in the first half. In addition to these parameters, in the reference description, each manufacturer indicates the critical frequency, parameters of the hysteresis loop, resistivity and Curie temperature for a particular product.
How else use ferrite rings
In addition to the well-known applications as high-frequency protection, ferromagnetic materials are used for the manufacture of transformers. They can often be seen in power supplies of computer equipment. It is well known that a ferrite ring transformer is very efficient in balanced mixers. However, not everyone knows that there is the possibility of "stretching" the balance. This modification of the transformer is able to perform the balancing operation more accurately. In addition, transformers on ferrite rings are widely used to match the output and input resistances of cascades of transistor devices. In this case, the active and reactive resistances are transformed . Thanks to the latter, this device can be used to change the ranges of capacity adjustment. Tensile transformers work well at frequencies below 10 MHz.
Conclusion
Those who are interested in how to wind a ferrite ring on their own should consider that the serial impedance introduced by a high-frequency ferrite core can easily be increased if several turns of the conductor are made on it. As the theory of electrical engineering suggests, the impedance of such a system will increase in proportion to the square of the number of turns. But this is in theory, but in practice the picture is somewhat different due to the nonlinearity of ferromagnetic materials and losses in them.
A pair of turns on the core increases the impedance not four times, as it should be, but a little less. As a result, in order for several turns to fit in the cable filter, a ring of obviously larger size should be selected. If this is unacceptable, and the wire should remain the same length, it is better to use several filters.