Patent GB-1486986-A (GB)
Title: Regenerative repeater
IPC: H04L25/24
Filed: 1975-03-05
Abstract: 1486986 Regenerative equalizers WESTERN ELECTRIC CO Inc 5 March 1975 [11 March 1974] 9074/75 Heading H4R A regenerative repeater includes an equalizer 35, and a pulse regenerator 32 from whose output 42 a model pulse signal 49, corresponding to the effects of transmission and equalization, is derived 43 for comparison 46 with a delayed version 38 of the equalized pulse to produce an error signal e. The error may be used to automatically adjust the equalizer by a known correlation technique (Fig. 18) or may be fullwave rectified 47 and peak-detected 48 to provide an indication of the extent of eye-degradation. The difference between the minimum and nominal eye openings is a measure of both the degradation and the tolerance of the repeater to recognize whether or not a pulse exists. It is shown that the difference between the peaks of the equalized and nominal/model pulses represents both. By the combination of operating conditions and special tests the monitor output may indicate in which part of the repeater a fault has occurred. As described, the transmitted pulses are of either positive or negative voltage level. The decision circuit 39 of the regenerator has a zero voltage threshold by which to determine the presence and polarity of received pulses. Small variations in threshold level are not distinguished but large variations cause a sudden rise in monitor level indicating that such a fault has occurred. Interpulse interference manifests itself in terms of a probability density with sharp edges whereas Gaussian noise interference has blurred edges; the error rate probability being expressed as the eye degradation divided by rms. noise. The repeater can therefore be arranged to provide an alarm when the monitor output reaches a level corresponding to a predetermined error rate attributed to noise. Errors in sampling times cause the peak detector to read larger variations than usual owing to the shape of the eye. To distinguish this from the interpulse and Gaussian interference an out-ofservice test is performed. Test frequencies are transmitted and a plot made of the variation of monitor voltage with sampling pulse phase. The value of monitor output for which the phase shift is zero gives a base line in determining how much of the operational monitor output is attributed to sampling errors. In order to be able to distinguish between faults occurring simultaneously in the model and in the delayed pulses a switch 81 (Fig. 12, not shown) may be inserted between the delay 45 and comparator 46 and cyclically operated by a 300 Hz generator (82) so that the model and the difference pulses are alternately monitored. This necessitates that the peak detector be reset 84 to discharge its storage capacitor (91, Fig. 13, not shown) through reset capacitor (83). The monitor signal V is modulated on a 30 kHz generator (87) in PCM, FM or PWM formats and capacitor coupled (86) to the remainder of the equipment since the repeater casing is at 1100 v. above earth and the monitor signal contains intelligence down to D.C. The monitor circuit is detailed in Fig. 13 (not shown) and consists of a differential and full wave rectifying transistor amplifier (103, 104) receiving the delayed and model pulses. The emitters of the transistors are held at ground potential by a further transistor (105) but when the input difference is sufficiently large transistor (105) causes the appropriate transistor (103) or (104) to conduct thereby turning on detector diode (115) and negatively charging the storage capacitor (91). Diode (115) is normally clamped by a permanently-on transistor (114) to protect the diode (115) against reverse voltage and to enable it to be turned on quickly. The storage capacitor (91) is reset by a parallel transistor (94). The simulated equalized line 43 consists of a differential op-amp (121, Fig. 14, not shown) producing two outputs feeding respectively a reference bridged-T circuit and a chain of bridged-T circuits of progressively increasing delay time, the two paths being recombined before feeding to the monitor circuit.