Do you know the principle of shielded cables?

What is a shielded cable? Shielded cable is a kind of guided cable called shielded cable. The wrapped wire is called shielding layer, generally braided copper mesh or copper (aluminum), the shielding layer needs to be grounded, and external interference signals can be introduced into the ground. Its function is to prevent interference signals from entering the inner layer and reduce the loss of transmission signals. Shielded cables have common structure and advanced structure. The common structure is insulating layer + shielding layer + wire, and the advanced structure is insulating layer + shielding layer + signal wire + shielding layer grounding wire. The principle of shielded cable: The shielded wiring system originated in Europe. It adds a metal shield to the outside of the normal unshielded wiring system. It uses the reflection, absorption and skin tilting effects of the metal shielding layer to prevent electromagnetic interference and electromagnetic radiation. The shielding system comprehensively uses the balance principle of the twisted pair and the shielding function of the shielding layer, and has good electromagnetic compatibility (EMC) characteristics. Electromagnetic compatibility (EMC) means that electronic equipment or network systems have certain electromagnetic interference capabilities, and at the same time cannot produce excessive electromagnetic radiation. That is to say, the equipment or network system needs to work normally in a relatively poor electromagnetic environment, and at the same time, it cannot radiate too much electromagnetic waves to interfere with the normal operation of other surrounding equipment and networks. The balance characteristics of U/UTP (unshielded) cables are not only determined by the quality of the components themselves, but also by the surrounding environment. Metal around U/UTP (unshielded), hidden“land”During the construction process, tension, bending, etc. will destroy its balance characteristics, thereby reducing EMC performance. Therefore, to obtain long-lasting balanced characteristics, there is only one solution: add a layer of aluminum foil to ground on all cores. Aluminum foil adds protection to fragile twisted-pair U/UTP (unshielded) cables and artificially creates a balanced environment. Thus forming what we now call a shielded cable. The shielding principle of shielded cables is different from that of twisted-pair cables. The shielded cable is to add one or two layers of aluminum foil to the outside of the four pairs of twisted pairs, using the metal's reflection and absorption of electromagnetic waves and the principle of skin shrinkage effect (the so-called skin shrinkage effect refers to the distribution of current on the conductor cross-section. The higher the frequency, the smaller the skin contraction depth, that is, the higher the frequency, the weaker the penetration ability of electromagnetic waves), effectively preventing external electromagnetic interference from entering the cable, preventing internal signal radiation, and interfering with the work of other equipment . Experiments have shown that electromagnetic waves with a frequency exceeding 5MHz can only pass through 38μm thick aluminum foil. If the thickness of the shielding layer exceeds 38μm, the frequency of electromagnetic interference entering the cable through the shielding layer is mainly 5MHz, and the balance principle for 5MHz twisted pair can effectively offset the following low-frequency interference. According to the earliest wiring definition, it is divided into two types: unshielded cable-UTP and shielded cable-STP. Later, with the development of technology and different processes, many different shielding types were derived. F/UTPFoilScreenedCable single-layer aluminum foil shielding structure 2. FoilandBraidScreenedCable aluminum foil and copper braided double-layer shielding structure a) SF/UTP aluminum foil and copper braided mesh are wrapped in the outer layer of four pairs of wires at the same time b) S/FTP (PIMF) line-to-single PIMF=PairinMetalFoil for aluminum foil shielding and copper braid wrapped in the outer layer of four pairs of wires. The resistance of shielded cables to external interference is mainly reflected in: the integrity of signal transmission can be guaranteed through the shielding system. The shielded wiring system prevents external electromagnetic interference and radio frequency interference from affecting data transmission. Electromagnetic Interference (EMI) Low-frequency interference, motors, fluorescent lights, and power lines are common sources of EMI. Radio frequency interference (RFI) is high-frequency interference, mainly wireless frequency interference, including wireless communications such as radio, TV broadcast, and radar. For EMI, braided shielding is the most effective, i.e. metal mesh shielding, due to its lower critical resistance. For radio frequency interference, metal foil layer shielding is most effective because the gap created by the metal mesh shield allows high-frequency signals to pass in and out freely. For the interference field mixed with high frequency and low frequency, the combined shielding of metal foil layer and metal mesh should be used, that is, the form of S/FTP double-layer shielded cable, so that the metal mesh shielding is suitable for interference in the low frequency range, and the metal foil shielding is suitable for high interference in the frequency range. In IBMACS shielded cables, the single-layer thickness of the aluminum foil shielding layer reaches 50-62μm, which has a more complete shielding effect. At the same time, because only a single layer of shielding is used, the construction will be simpler, the installation is convenient, and it is not easy to cause artificial damage during the construction process, and the thickness of aluminum and silk can withstand greater destructive force. Thereby providing users with higher quality transmission performance. .

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