Analysis of an adjustable multipath network for use in building out transmission-line skin-effect losses

In wide-band repeatered coaxial transmission systems, the dominant loss due to skin effect of the cable varies with temperature and cable length. Line-build-out (LBO) networks, whose attenuation characteristics match various amounts of cable losses, are inserted to yield a fixed loss characteristic between repeaters. A multipath structure is proposed as a new means of realizing adjustable LBO networks. Variation of the proposed structure is accomplished through electronically controlled variable gain amplifiers, allowing precise setting of the build-out. The transmission characteristics of the paths can be realized with either lumped circuit elements or distributed networks, as the frequency of interest might dictate. The amplifier gain variation performs an interpolation among the transfer functions of the different paths. The theoretical upper limit of the usable adjustment range is determined by the tolerance on the inaccuracies of the interpolation process. The two-path (II-path) configuration appears to be the easiest to realize, while providing enough adjustable range and accuracy to be of practical interest to both digital and analog wide-band repeatered systems. In the baseband PCM application, simulation shows that a 7-dB adjustable range, at the half pulse rate, is feasible with only 0.9-percent pulse distortion.