CATV cable assemblies: screening requirements and performance

Cable assemblies using TV connectors are used widely for connecting a TV set both to its antenna outlet socket and video recorder, and for some time a German standard has existed which includes a specified limit for their screening attenuation. Cable assemblies are available in the retail market to this specification (DIN 45332) which is thought to require a screening attenuation of 80 dB at 300 MHz. In 1997 an IEC specification was published which also includes details of screening attenuation. The screening attenuation specified has a maximum value of 75 dB up to 1 GHz and 55 dB up to 3 GHz. There is some argument as to how this should be derived for a cable assembly, so a way round the problem is given in this paper, where the screening attenuation is obtained from individual measurements of the transfer impedance (Z_T ) on the connectors and cable of the cable assembly. The measurement of Z_T is much easier if an electrically short sample can be used. By restricting the sample length to 250 mm this is achieved if frequencies are kept below 100 MHz, with the effect of higher frequencies derived by extrapolation. Measuring methods for the full frequency range and one limited to electrically short test pieces are outlined. Their use is demonstrated with a variety of TV cable assemblies and connectors