1. Concept of shielded wire The shielded wire is a tran […]
1. Concept of shielded wire
The shielded wire is a transmission wire that uses a mesh braided wire to wrap the signal wire.
A wire wrapped with a conductor outside the wire is called a shielded wire, and the wrapped conductor is called a shielding layer, which is generally a braided copper mesh or copper mooring (aluminum mooring, etc.). The shielding layer needs to be grounded, and external interference signals can be introduced into the ground by this layer to avoid interference The signal enters the inner conductor to interfere while reducing the loss of the transmission signal. Common braided wires and twisted wires are shielded wires.
The production process of ordinary shielded wire is as follows:
Stranded copper-extrusion (inner core)-winding (inner core wire)-aluminum coating-braiding (or winding)-extrusion (outer quilt)-roll packaging
In the electronic connection line:
AL stands for aluminum pool. Such as: 4C+AL, which means 4-core wire covered with aluminum.
AB stands for braiding. For example: 4C+80/0.10AB, which means 4-core braided wire. The braided specification is 80 copper wires with an outer diameter of 0.10mm.
S stands for winding. For example: 4C+60/0.10S, which means 4C winding wire. The winding specification is 60 copper wires with an outer diameter of 0.10mm.
2. the role of shielding
Shielding is the isolation of metal between two spatial areas to control the induction and radiation of electric fields, magnetic fields and electromagnetic waves from one area to another. Specifically, shielding is used to surround the interference sources of components, circuits, assemblies, cables or the entire system to prevent the interference electromagnetic field from spreading; shielding to surround the receiving circuit, equipment or system to prevent them from being exposed to the outside world The influence of electromagnetic fields. Because the shielding body absorbs energy (eddy current loss), reflected energy (reflection of electromagnetic waves on the interface of the shielding body) and offsetting energy (electrical interference) from external interference electromagnetic waves and internal electromagnetic waves from wires, cables, components, circuits or systems. Magnetic induction produces a reverse electromagnetic field on the shielding layer, which can offset part of the interference electromagnetic wave), so the shield has the function of reducing interference.
(1) When the frequency of the interfering electromagnetic field is high, the eddy current generated in the low-resistivity metal material is used to form the cancellation effect of the external electromagnetic wave, so as to achieve the shielding effect.
(2) When the frequency of interfering electromagnetic waves is low, materials with high magnetic permeability should be used, so that the lines of magnetic force are limited to the inside of the shield and prevent spreading to the shielded space.
(3) In some cases, if a good shielding effect is required for both high frequency and low frequency electromagnetic fields, different metal materials are often used to form a multilayer shield. Many people do not understand the principle of electromagnetic shielding. They think that as long as a box is made of metal and then the box is grounded, it can play the role of electromagnetic shielding. Under the guidance of this concept, the result is failure.
Because the electromagnetic shielding has nothing to do with whether the shield is grounded or not. There are only two factors that really affect the shielding effectiveness of the shield: one is that the entire surface of the shield must be conductive and continuous, and the other is that there can be no conductors that directly penetrate the shield. There are many conductive discontinuities on the shield, the most important one is the non-conductive gap formed at the junction of different parts of the shield. These non-conductive gaps cause electromagnetic leakage, just as fluid leaks from a gap in a container. One way to solve this leakage is to fill the gap with conductive elastic material to eliminate non-conductive points. This is like filling rubber in the gaps of fluid containers. This elastic conductive filling material is an electromagnetic sealing gasket. In many documents, the electromagnetic shielding body is likened to a liquid-sealed container. It seems that only when the gap is sealed with a conductive elastic material to the extent that water does not leak, the leakage of electromagnetic waves can be prevented. Actually this is not exact. Because whether the gap or hole will leak electromagnetic waves depends on the size of the gap or hole relative to the wavelength of the electromagnetic wave. When the wavelength is much larger than the opening size, no obvious leakage will occur.
Therefore, when the frequency of interference is high, and the wavelength is short at this time, an electromagnetic sealing gasket is required. Specifically, when the frequency of interference exceeds 10MHz, it is necessary to consider the use of electromagnetic sealing gaskets. All elastic and conductive materials can be used as electromagnetic sealing gaskets. Electromagnetic sealing gaskets manufactured according to this principle include: Conductive rubber: Fill the silicone rubber with metal particles that account for 70 to 80% of the total weight, such as silver powder, copper powder, aluminum powder, silver-plated copper powder, silver-plated aluminum powder, Silver-plated glass balls, etc. This material retains a part of the good elastic properties of silicone rubber, while having good electrical conductivity. Metal braided mesh: Use beryllium copper wire, Monel wire or stainless steel wire woven into a long tubular strip, which looks like the shielding layer of a shielded cable. But its weaving method is different from the cable shielding layer.
The cable shielding layer is braided with multiple wires, and this shielding gasket is woven from one wire. Make an image metaphor, like the sleeve of a sweater. In order to enhance the elasticity of the metal mesh, a rubber core is sometimes added to the mesh tube. Finger-shaped reed: a reed made of beryllium copper, which has good elasticity and conductivity. Conductivity and elasticity. Multiple conductive rubber: composed of two layers of rubber, the inner layer is ordinary silicone rubber, and the outer layer is conductive rubber. This material overcomes the shortcomings of poor elasticity of traditional conductive rubber, and fully reflects the elasticity of rubber. Its principle is somewhat like a metal wire mesh strip with a rubber core. Four factors should be considered when choosing what type of electromagnetic sealing gasket to use: shielding efficiency requirements, environmental sealing requirements, installation structure requirements, and cost requirements. Comparing the characteristics of different gasket materials, shielding can be divided into electric field shielding, magnetic field shielding and electromagnetic field shielding according to the mechanism.