Shielded twisted-pair cable overview

More than 5MHz electromagnetic wave frequency only through 38 muon m thick aluminum foil. If shielding layer thickness more than 38 muon m, such as U.S. parkson FTP cable for two layers of 25 muon m thick aluminum foil shielding, made by entering cable inside the shield the frequency of electromagnetic interference, and mainly in 5MHz below for the low frequency interference 5MHz following the principle of usable double skein of effective offset。

Transmission medium

Wiring system using transmission medium main have twisted-pair cable and optical cable. Twisted-pair cable insulation cover by two root of copper wire composition, divided into the shielding twisted-pair cable (UTP), shielding twisted-pair cable  STP, while shielding twisted-pair cable and divided into aluminum foil shielded twisted-pair cable FTP, independent shielded twisted-pair cable (STP)。

In the system, the shielding in wiring still USES the shielded twisted-pair twisted-pair, in the industry remains in controversy. Stick with the shielding view think that one: shielding system is refers to the entire system, its itself is the whole screen, can raise a good idea, but the rate of the signal transmission standards high, investment installation; Although shielding can resist noise interference, improve the transmission rate, but if carelessly a bit in wiring process, it will affect the whole system of shielding effect, it will reduce the system performance; The whole shielding wiring, below the same transmission bandwidth poly-rowed fiber of cost。

From performance to price, it will still be level routing sub-systems shielded twisted-pair cable and optical fiber is the world. And hold the people think by shielding views: canned system can improve stable performance and high quality transmission signals, can provide higher bandwidth and can support future high-speed network system, and improve the transmission distance further. The construction requirements of high is professional installation company strictly according to the wiring, as long as things, will operate specification requirements for users provide shielding wiring system。

Wiring system adopts the shielded twisted-pair cable or shielded twisted-pair cable, from the construction quality, time limit and investment to see a clear differences. The shielding system adopts the shielded twisted-pair cable, construction is simple, low quality standards, construction period shorter, low investment. While shielding system USES shielding twisted-pair cable, the processing of shielding layer demanding, but ask the shielding layer cannot have the link, also requires shielding the breakpoint, the whole process of access should be complete shielding。

Will tell from the current construction condition of the whole system, it is difficult to achieve process shielding. So objectively, design requirement for personnel in voice communication, data communication and image transmission medium when the choice, want to not shielded twisted-pair cable, shielding twisted-pair cable and optical fiber technology index understand somewhat. Namely the voice communication: 3 class or higher 100 Ω the shielding twisted-pair cable; General data communication: 5 kinds of 100 Ω not shielded twisted-pair cable, 150 Ω shielding twisted-pair cable; High-speed data communication: 2 core 625,000/125 muon m fiber; Visual communication: 5 kinds of 100 Ω not shielded twisted-pair cable, 2 core 625,000/125 muon m fiber etc。

Balance transfer

What does UTP cable through the double ground to achieve HPVV  which means EMI EMC performance received was first UTP cable  then be offset. But as frequency, the EMC performance increase UTP will decrease. After measuring found that cable 30MHz wringing only to satisfy to double the EMC performance  for higher electromagnetic interference will be powerless. While the majority of the actual network application is below the working frequency of 30MHz, and, the ideal balance transmission system is not exist. What does UTP cable balance characteristics not only depend on the quality of parts itself (as to), and will be stranded by surroundings. Because the metal, concealed UTP around the "land", construction of pull, bending, etc can destroy the balance characteristics, which can reduce the EMC performance。

In fact, we installed cable usually will wear it in the into metal conduit, plastic conduit or other have different in protection of ground impedance. So, to get the right performance enduring, and only one solution, in all, HPVV plus much layer aluminum foil were grounded. Aluminum foil as fragile double ground added protection, also suitable for UTP cable artificial creates a balanced environment. This means that based on the FTP cable shielding solution is independent of the environment with context-free  namely. FTP is shirt-sleeve UTP balance characteristics and construction flexibility and STP shielding effect, namely balance and shielding principle perfect combination。

FTP, the attenuation UTP

FTP cable attenuation index completely meet ISO/iec11801and and related standards TIA568B EIA /. If will UTP cable Pierce metal conduit, the impedance characteristics will decline and cause attenuation increases. FTP cable during the production process considering the influence of aluminum foil around in manufacturing process, have to compensate。

FTP, UTP the transmission distance

Cable transmission distance is the attenuation and propagation delay of cable, and decided whether shielding no relationship. Affect the transmission distance is the key factor of propagation delay by cable, it is the decision: ordinary NVP value of UTP cable NVP about 0.66, American parkson UTP, FTP, STP cable in NVP about 0.68。

Radiation and grounded

System by the shield performance the performance of the worst part decision, shielding the weakest part is system of information socket and cabinet modular jump line through the following dish, can raise its EMC methods of performance: far from the interference, such as elevators, air conditioning, power equipment, fluorescent lamp, mobile base, etc., and adopt the with EMC shields information socket and jumper dish; Cable section and information socket and jumper cables widely distributed, compared to dish in the whole construction, the surrounding electromagnetic environment more complex  unable to predict and control, so cable part is the whole wiring system is the most need to be part of the electromagnetic protection。

Through the FTP cable shielding principle can learn, shielding layer also has a shielding effect, grounding of aluminum foil the shield shield effect and grounding is irrelevant. But if grounding is bad, FTP shielding layer will become interference source. Integrated wiring system as passive product, itself won't generate electromagnetic radiation, but if grounding is bad, the cable shielding layer can absorb external electromagnetic interference, transmission to the radiation. Of course outward radiation also need certain conditions, namely the energy required must exist radiation and antenna size and waves in the same order of magnitude. Only satisfies the conditions and cable shielding layer will be more likely to become "potential antenna". Integrated wiring system in the whole system development, the development process have fully considering the high frequency grounding problem realized cable shielding layer around the ground, to avoid the area of so-called "antenna effect"。

Measurement shielding

The advantage of FTP cable shielding system provide a UTP cable is better EMC performance, based on the system insulated from external electromagnetic environment, because the external existing electromagnetic environment will affect the whole wiring system data transmission. So far  haven't performance index or testing method to express or compare EMC performance. But European standardization commission already start the job CENELEC  Coupling Attenuation  Coupling Attenuation is defined as the index measuring EMC performance, the index was written into the international wiring standards.