What can you do with a ferrite filter? Ferrite filter - what is it for?

Each of us has seen small cylinders on power cords or coordination cables for 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 with the keyboard, mouse, monitor, printer, scanner, etc. This element is called a “ferrite ring” (or ferrite filter). In this article we will look at the purpose for which manufacturers of computer and high-frequency equipment equip their cable products with the mentioned elements.

Physical properties

Ferrite is ferrimagnetic and non-conductive. electricity, that is, essentially it is a magnetic insulator. They are not created in this material and therefore it very quickly remagnetizes - in time with the frequency of external electromagnetic fields. This material property is the basis for effective protection of electronic devices. A ferrite ring placed on a cable can create a high active impedance for common-mode currents.

This material is formed from a chemical combination of iron oxides with oxides of other metals. It has unique magnetic characteristics and low electrical conductivity. Thanks to this, ferrites have practically no competitors among other magnetic materials in high-frequency technology. 2000nm ferrite rings significantly increase the cable inductance (several hundred or thousand times), which ensures suppression of high-frequency interference. This element is installed on the cord during its production or, cut into two semicircles, is put on the wire immediately after its manufacture. The ferrite filter is packaged in a plastic case. If you cut it open, you can see a piece of metal inside.

Do you need a ferrite filter? Or is this another deception?

Computers are very “noisy” (in electromagnetic terms) devices. So, motherboard inside system unit capable of oscillating at a frequency of one kilohertz. The keyboard has a microchip that also operates at high frequencies. All this leads to the so-called generation of radio noise near the system. In most cases, they are eliminated by shielding the board from electromagnetic fields metal body. However, another source of noise is the copper wires that connect various devices. In essence, 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. The 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 the right ferrite filter

To install a ferrite ring on a cable with your own hands, you need to understand the types of these products. After all, it depends on the type of wire and its thickness which filter (from what material) will need to be used. For example, a ring installed on a multi-core cable (power cord, data cable, video or USB interface) creates in this section a so-called in-phase transformer that passes antiphase signals carrying useful information, and also reflects common mode interference. In this case, one should not use absorbing ferrite to avoid disruption of information transmission, but a higher-frequency ferromaterial. But it is preferable to choose ferrite rings from a material that will dissipate high-frequency interference rather than reflecting it back into the wire. As you can see, an incorrectly selected product can worsen the performance of your device.

Ferrite cylinders

Thick ferrite cylinders cope most effectively with interference. However, it should be borne in mind that too bulky filters are very inconvenient to use, and the results of their work are unlikely to differ much in practice from slightly smaller ones. You should always use filters of optimal dimensions: the internal diameter should ideally match the wire, and its width should correspond to the width of the cable connector.

Don’t forget that it’s not only ferrite filters that help combat noise. For example, for better conductivity it is recommended to use cables with a larger cross-section. When choosing the length of the cord, you should not leave a large margin of length between the connected devices. In addition, poor quality of the connection between the wire and the connector can also be a source of interference.

Marking of ferrite rings

The most widespread type of record for marking ferrite rings is as follows: K D×d×N, where:

K is short for "ring";

D is the outer diameter of the product;

D - internal diameter of the ferrite ring;

H - filter height.

In addition to the overall dimensions of the product, the type of ferromagnetic material is encrypted in the marking. An example entry may look like this: M20VN-1 K 4x2.5x1.6. The second half corresponds to the overall dimensions of the ring, and the first half encodes the initial magnetic permeability (20 μ i). Except specified parameters, in the reference description, each manufacturer indicates the critical frequency, resistivity and Curie temperature parameters for a specific product.

How else are ferrite rings used?

In addition to the well-known use as high-frequency protection, they are used for the manufacture of transformers. They can often be seen in technology. It is well known that a ferrite ring transformer is very effective in balanced mixers. However, not everyone knows that it is possible to “stretch” balancing. This modification 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 are transformed. Thanks to the latter, this device can be used to change the range of capacitance tuning. "Stretch" transformers work well at frequencies below 10 MHz.

Conclusion

Those who are interested in how to wind a ferrite ring themselves should keep in mind that the series impedance introduced by a high-frequency ferrite core can easily be increased by making several turns of conductor on it. As electrical engineering theory 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 couple of turns on the core does not increase the impedance by four times as it should be, but a little less. As a result, in order for several turns to fit in a cable filter, you should choose a ring of a obviously larger standard size. If this is unacceptable and the wire must remain the same length, it is better to use several filters.

Even if a device is designed with noise in mind and layout, grounding, or circuit board filtering, it can still be a source of interference. high level interference or be susceptible to noise when connecting other devices with the interface cable. In particular, since cables have a high specific surface area due to their long length, they can emit or receive electromagnetic waves. In this regard, to suppress interference, it is advisable to use special devices, for example, a ferrite filter with a snap on the cable (see Figure 1).

The appearance of the filter with a latch on the cable is shown in Figure 1.
Ferrite Cable Snap Filter consists of a ferrite core, which consists of two halves housed in a plastic flexible housing characterized by a long service life. This design allows you to attach it to the cable in one movement without cutting it. Since such a filter can be installed after the device is assembled, its use becomes especially relevant in cases where interference problems occur immediately before transportation. Figure 1 b shows a filter that is mounted on a cable inside the device.

The snap-on cable filter consists of a ferrite core, which consists of two halves housed in a plastic flexible housing characterized by a long service life. A large number of product types are available for order, produced in accordance with cable diameters.

Common mode filter type

Adjusting the winding size

As a complement to network adapters power supply (AC), various peripherals devices, such as digital cameras or mobile phones, are connected using various interface cables to terminals in the form of laptop PCs. Cable snap filters are installed on these interface cables and their effect on interference suppression is assessed.

Connecting the power cable alternating current

Noise emission spectrum from mobile phone Before and after connecting the ZCAT1518-0730 self-clamping filter to the power cable is shown in Figure 2. In this test, the cable had a double wrap around the filter. The measurement results are presented in Figure 3. Before installation, noise was recorded in the frequency range from 250 to 600 MHz, barely meeting the VCCI Class B standard. After installing a ferrite filter with a snap on the cable, the noise was reduced by approximately 5...10 dB.

Mobile phone connection

As shown in Figure 4, the portable terminal was connected to the phone using an exclusive type of cable, ZCAT1518-0730 filter was installed on the power cable. The measurement results are shown in Figure 5. Before installing the filter, noise was recorded in a wide frequency range from 100 to 600 MHz. As in previous testing, after double winding the exclusive cable around the filter, the noise level was reduced to 5..10 dB. In addition, it was discovered that interference at 600 MHz and above, which did not change after installing the filter, was caused by sources other than the cable.

Cable snap ferrite filters improve ESD resistance

By installing a snap-on filter on the cable, not only the noise level is reduced, but also the likelihood of errors caused by external sources interference such as power surges or static electricity. An ESD (electrostatic discharge) test based on the international standard IEC61000-4 for immunity testing was conducted to investigate the frequency or change in the number of errors before and after installing the filter.

Electrostatic discharge is a phenomenon that occurs when electric charge, accumulated on the surface of the body due to reasons such as friction with clothing, is discharged upon contact with the body electronic device. Noise immunity is resistance to noise from external sources.

Measurement method

As shown in Figure 6, under working conditions, the connection between the portable terminal and the printer was carried out. The portable terminal (PC) was experiencing static electricity discharge. The conditions under which errors occurred were recorded. The electrical shock was applied 10 times at one second intervals to the cable connector (where it connects to the cable) on the handheld terminal side. The discharge was applied using the contact discharge method in accordance with the international standard IEC61000-4-2. The pulse waveform for testing described in the IEC61000-4-2 standard is shown in Figure 7. The test voltages (discharge levels) were: 2 kV, 4 kV and 6 kV.

Test results

The test results are presented in Table 1. When the filter was not yet installed, errors such as stopping some printer operations were observed at a test voltage of 4 kV. At 6 kV the printer stopped working completely. When using the ZCAT2035-0930A (single winding) filter, there were no problems resulting from operations at 4 kV test voltage, but few operational errors were noted at 6 kV. When using a filter with a double winding, no errors were detected. The ESD waveforms before and after installing the filter are shown in Figure 8. A double winding was used. Electrostatic discharge was significantly reduced thanks to the filter. Signals were observed at a position near the filter on the cable between the filter and the printer.

ESD Noise Reduction on a Parallel Two-Wire Data Line

The ESD noise suppression effect of a snap-on cable ferrite filter was evaluated experimentally when installed on a parallel two-wire line. The comparison was carried out using the example of the filter discussed above.

Measurement setup

The measurement setup is shown in Figure 9. Two parallel wires of 1 m length were placed at a height of 0.1 m from the ground plane. A voltage of 6 kV, generated by an electrostatic generator, was applied to the input of the line using an electrostatic discharge generator. Contact has occurred between the electrostatic discharge and the line. The static electricity pulse shape generated by the electrostatic generator corresponded to a high-speed peak voltage with a rise time of 0.7 to 1 ns. Filters ZCAT2035-0930A (ZCAT) and board-mounted common mode choke ZJYS51R5-2P (ZJYS) were installed in the middle of the parallel wires. Next, a change in the shape of the electrostatic discharge signal at the output was observed. As shown in Figure 10, two types of boards were used on which ZJYS components were installed. The first board was 1 mm thick and there was no copper foil layer on the back side. The thickness of the second board was 0.3 mm, the entire surface of the reverse side was a ground plate.

High-level impulse noise suppression effect

Wide range of manufactured components

In conclusion, the selection chart for TDK's ZCAT series filter line is shown in Table 2. TDK provides various series of components covering a wide range of applications from cable use to general purpose to flat cables.

Application	Type	Cable diameter, (mm)	Order code	Image
Cables	Self-clamping mechanism	3...5	ZCAT1325-0530A (-BK)
		4...7	ZCAT1730-0730A (-BK)
		6...9	ZCAT2035-0930A (-BK)
		8...10	ZCAT2235-1030A (-BK)
		10...13	ZCAT2436-1330A (-BK)
	The cable is attached to the body with a nylon strap	7 max.	ZCAT1518-0730 (-BK)
		9 max.	ZCAT2017-0930 (-BK)
		9 max.	ZCAT2032-0930 (-BK)
		11 max.	ZCAT2132-1130 (-BK)
		13 max.	ZCAT3035-1330 (-BK)
Flat cables	20 core flat cables	12 max.	ZCAT3618-2630D (-BK)
	26-core flat cables	13 max.	ZCAT4625-3430D (-BK)
	40 core flat cables	17 max.	ZCAT6819-5230D (-BK)

In our everyday life, a huge variety of computer technology has appeared that operates on high-frequency currents. After all, the higher the frequency, the higher the speed of information processing.

However, high-frequency currents impose a number of technical limitations on connecting cables for transmitting such signals. This is primarily due to side electromagnetic radiation and interference (PEMIN).

The simplest way to combat PEMIN is to increase the inductance.

Inductance is an indicator of the relationship between the amount of current passing through a circuit and the magnetic flux it creates. If we are talking about straight wires, then by inductance we mean a quantity that characterizes the energy of the magnetic field (here the current is considered a constant value).

The inductance can be increased by using a special ferrite ring. You can see what ferrite filters look like on cables in the photo below.

Ferrite rings– these are the components electrical circuit, which are used as passive elements to filter high-frequency interference by increasing the inductance of the conductor and absorbing interference above a given threshold.

Such properties of a ferrite filter are given by the material from which it is made – ferrite.

Ferrite is the general name for compounds based on iron oxide and oxides of other metals. Ferrites combine the properties of ferromagnets and semiconductors (sometimes dielectrics) and therefore are used as coil cores, permanent magnets, act as absorbers of high-frequency electromagnetic waves, etc.

Snap-on ferrite cable filters - operating principle

The performance of a ferrite filter directly depends on the characteristics of the material from which it is made. Due to special additions of oxides of various metals, the properties of ferrite change.

There are fundamentally several ways to use ferrite rings:

1. On single-core (single-phase) wires, it can, on the contrary, absorb radiation in a certain range, converting interference into thermal energy. In this way, negative frequencies can be absorbed (cut off) by the ferrite ring.
2. On single-core wires, where it works as a kind of amplifier, as it returns part of the high-frequency magnetic field back into the cable, which leads to amplification of the signal in a given range.
3. On stranded wires, the ferrite acts as an in-phase transformer that passes unbalanced signals in the cable (current pulses, for example, in data cables or DC power circuits) and suppresses symmetrical signals (which can potentially be caused in such cables only by electromagnetic interference).

Where to use and how to choose a ferrite filter

If we talk about the practice of application, then on power cables, ferrite rings are used to reduce interference that the cables themselves can create, and on signal (transmitting data) ferrites dampen possible external interference and interference.

Ferrite cable filters can be built-in (the cable is sold already with a ferrite ring) or separate (most often these are models that snap around the wire), which do not require any modifications to the cable itself.

The wire can be inserted into the center of the ferrite filter (a single-turn coil is obtained), or it can form several turns around the ring (toroidal winding). The latter method significantly increases the efficiency of the filter.

To select a ferrite ring to meet the specified requirements, you need to know the characteristics of the material from which it is made and the dimensions of the product.

As an example, the table below shows the main characteristics of ferrite filters offered on the market.

Frequency versus impedance graph

Impedance is the total internal resistance of an electrical circuit element to alternating (harmonic) current (signal). It is measured, like regular resistance, in ohms.

Another important parameter of ferrite filters is their magnetic permeability.

Magnetic permeability is a coefficient that characterizes the relationship between magnetic induction and magnetic field strength in a substance.

Based on the above, in order to indicate the main properties of ferrite filters, manufacturers resort to the following markings:

3000HH D * d * h, where:

1. 3000 is an indicator of the initial magnetic permeability of ferrite,
2. HH is a grade of ferrite (most often these are HH - general purpose ferrites, or HM - for weak magnetic fields),
3. D – largest (external) diameter,
4. d – smaller (internal) diameter,
5. h is the height of the toroid.

Here are typical examples of the use of ferrites:

• Grade 100NN can be used for cables with frequencies up to 30 MHz,
• 400NN - with frequencies not higher than 3.5 MHz,
• 600NN - with frequencies up to 1.5 MHz
• 1000NN - up to 400 kHz.

That is, for example, the antenna ferrite filter should be of the HH brand.

And here is a ferrite filter for USB cable It is best to choose the HM grade (for cables with a weak magnetic field).

The ratio of brands and frequencies is as follows:

• 1000NM - used with cables operating with a frequency of no more than 1 MHz,
• 1500NM - no more than 600 kHz,
• 2000NM and 3000NM - no more than 450 kHz.

In most cases, it is enough to select the correct ferrite filter and snap it onto the cable closer to the connection point to the device.

However, in some cases, to increase the impedance, you can make the cable several turns around the ferrite ring and then the impedance will increase as a multiple of the square of the number of turns. That is, from two turns it is 4 times, and from 3 turns it is already 9 times.

In practice, of course, the actual increase is slightly less than the theoretical one.

In order for the ferrite ring to snap into place after winding, it is necessary to determine in advance the number of turns of the wire and calculate the internal diameter of the filter so that it closes without crushing the cable.

You have probably noticed more than once that on the wires from a laptop, monitor and other electronic equipment there are strange bulges in the form of a cylinder. This was done for a reason or for beauty. The fact is that the plastic cylinder is a special ferrite filter. People often call it a filter for suppressing high-frequency interference, or more simply, a “noise” filter. Why and what is it needed for?

The fact is that any device connected to electrical network, is a source of electromagnetic waves, which, in turn, are high-frequency interference that affects the operation of other devices located nearby. Long external power and interface cables act as a kind of antennas, which quite strongly emit interference into the external environment that is created by the equipment during operation. This can have a big impact on performance. wireless networks WiFi, radio equipment and precision instruments. To prevent this from happening, the cable must be shielded. But then its price will rise significantly! A ferrite ring and filters made of this material came to the rescue.

How does a ferrite filter work?

Ferrite is a special material consisting of a compound of iron oxide and a number of other metals that does not conduct current and effectively absorbs electromagnetic waves. The ferrite ring is an excellent magnetic insulator and thus filters out high-frequency interference and electromagnetic noise. It absorbs the electromagnetic waves coming out of electronic equipment before they are amplified in the cable, as in an antenna.

A ferrite filter is a cylinder-shaped core made of this material, which is put on the cable either immediately in production or later. At self-installation it must be located as close as possible to the source of interference. Only this will prevent the transmission of interference through other elements of the device’s design, where it is much more difficult to filter it out.

Monitors, printers, video cameras and other computer equipment, a ferrite cylinder in a plastic shell.

What is it for?

A ferrite cylinder is a shield that protects against electromagnetic interference and interference: it prevents distortion of the signal transmitted via the cable from exposure to an external electromagnetic field, and also prevents the radiation of the electromagnetic field (interference) from the cable into the external environment.

What is the principle of protection based on?

Internal and external computer equipment can act as miniature antennas as they convert so-called voltage and current noise into electromagnetic radiation. Unshielded ones emit noise due to common-mode noise flowing through their copper conductors, that is, high-frequency current flowing in the same direction through all conductors. This current creates a magnetic field of a certain magnitude and direction.

Ferrite is a ferromagnet that does not conduct electric current (that is, in fact, ferrite is a magnetic insulator). Eddy currents are not created in ferrites, and therefore they are very quickly remagnetized - in time with the frequency of the external electromagnetic field (the effectiveness of their protective properties is based on this).

Ferrite rings without a shell can also be found inside the block.

How to Increase the Noise Reduction Effectiveness of Ferrite

1. Increase the length of the part covered by the ferrite core.

2. Increase the cross section of the ferrite core.

3. The inner diameter of the ferrite should be as close as possible (ideally equal) to the outer diameter.

4. If the design features of the cable-ferrite pair allow, you can make several turns (usually one or two) around the ferrite core.

To summarize the above, we can say that the best ferrite core is the longest and thickest that can be placed on a particular one. In this case, the inner diameter of the ferrite should, if possible, coincide with the outer diameter.

How to use ferrite

Sometimes on sale you can find detachable ferrites in a plastic shell (heat-shrink tube) with two latches. How to use them?

The open ferrite cylinder is placed on the cable, which must be protected from electromagnetic interference and interference, approximately 3 cm from the tip. A loop is made around the cylinder shell. After this, the shell snaps into place. For reliability, you can equip the other end with a ferrite cylinder.

Goodbye interference, hello undistorted signal!..