Shielding Performance Tests for Twisted Pair Communications Cables

摘要

The HA 568 C.2 specification for Category cables has added a coupling attenuation test for shielded twisted pair cables. One of the methods of testing is the tri-axial method which provides an overall conductive enclosure around the cable under test to capture extraneous shield energy. The test procedure calls out testing over a range of 30 to 1000MHz. There is body of work that describes a number of test methods and options that were explored during the development of the current published test methods. There are also alternate test methods available such as the absorbing clamp procedure. This paper provides additional insight into the results obtained with the current tri-axial test method and procedures. The paper will provide test results using the triaxial method of the coupling attenuation of some of the different cable types and constructions currently available, and investigates the frequency dependent shielding performance of those cables. Also provided are test results for coupling attenuation using the absorbing clamp method and investigations in to the relationship between balance and coupling attenuation. The coupling attenuation value obtained is a result of the unbalance attenuation of the twisted pairs in the cable, as well as the screening attenuation of the shield around the twisted pair or pairs. The coupling attenuation procedure in the TIA 568-C.2 provides results of both screening and unbalance attenuation summed in the one test procedure. The paper will also investigate how the screening and unbalance attenuation contribute to the overall coupling attenuation performance. Coupling attenuation is the effect of the sum of the unbalance attenuation and the screening attenuation.