The Systematic Behavior of Insertion Loss Deviation in Data Grade Channels, with Longitudinally Distributed Impedance Variations in the Cables, its Partial Measurement and Characterization as a Specification Requirement

摘要

The importance of insertion loss deviation (ILD) has been highlighted in a previous paper [2]. ILD has two components one which represents an offset to the summed up operational attenuation of the components, and a second component, which is oscillatory in nature over frequency. Both components of the ILD are the result of signal reflections, arriving overlaid to the signal at the receiver end. Therefore ILD is a noise, arriving with the signal at the receiver (ghosts). ILD is from a noise point of at least as restrictive on channel or link performance as NEXT. Both within ISO/IEC and TIA/EIA prime importance has been so far attached to the channel performance based on the attenuation to near-end/far-end cross talk ratios of links and channels. Within IEEE importance has been attached so far mainly to the alien crosstalk, though this crosstalk noise is far less restrictive than the echo forward loss resulting from the ILD. It is shown that the ILD-noise is at least equally limiting as the cross talk noise of links and channels with respect to their signal to noise ratio, and it will have to be specified accordingly. In case of the 10GbBase-T protocol, for instance, it is strongly suggested that it should be compensated for. Otherwise the promised extended bandwidth of the links and channels, recently specified, cannot be used by system designers, due to lack of critical information. This would be counteractive to the deployment of high performance channels. The ILD is not only caused by impedance mismatches at the cable connector interfaces, but also by the impedance roughness of the cables, which is subject of prime consideration here. Hence special attention is given to the influence of the cable impedance roughness on the ILD. The results obtained are used to derive recommended specification limits. Some results are given and compared to the partial measurement (oscillatory component) of the ILD. A challenge is posed to develop methods to bring a more complete measurement of ILD into the realm of attainability.