Design Of A Twisted-Pair Line Simulator

The paper presents the design of a copper line simulator for communication equipment testing. Wireline simulators or artificial telephone lines are used as a replacement for signal transmission cables for the testing and measuring of wired communication equipment. Wireline simulators can be implemented as an expensive precision-measurement equipment used in laboratories or as simple passive instruments.The presented wireline simulator line is part of an automatic quality-control system for Single-pair High-speed Digital Subscriber Line (SHDSL) transmission equipment. Polyethylene-insulated 0.4 mm copper twisted pair (PE 04) connections are simulated by the presented low-cost solution. Frequency response and DC resistance can be adjusted separately by means of digital control inputs. Three lengths (1.5 km, 2.5 km and 3.5 km) and two DC resistances (600 Omega and 1000 Omega) are supported.The propagation constant gamma(omega) (3) and characteristic impedance Z(k)(omega) of the PEO4 twisted pair are calculated for the frequency-dependent cable parameters (Fig. 2). The input impedance Z, n of the twisted pair terminated by 135 Omega that is given by (5) is plotted in Fig. 3. The magnitude and phase of Z(in) vary with the length and frequency. In the important range of distances, i.e., from 1.5 km to 3.5 km, only the magnitude decreases with the frequency, while the phase varies only slightly.The topology of the wireline simulator is based on a centrally-symmetric two-port network (Fig.5a). The elements of the network are selected in order to approximate the magnitude vertical bar Z(in)vertical bar (Fig. 4) and the theoretical frequency response given by (6) (Fig. 5). The frequency response is approximated by a transfer function pole on the real axis implemented in the symmetrical two port network of the intermediate topology (Fig. 7). Frequency response and DC resistance between the ends of the simulated line are adjusted by mechanical relays (Fig. 8). Component values are found by matching the frequency response obtained by AC circuit analysis to the theoretical results. The optimal values of the capacitors are finally readjusted taking into account only commercially available values. The achieved frequency response match is shown in Fig. 9.In Fig.10, the measured frequency responses of the realized design are compared to the theoretical results and to responses of the Consultronics DLS 50 measuring line simulator. Practical use of the presented line simulator in automatic test equipment justifies the approximations used in the design.