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in the cable will therefore be of greatest significance for the reliability of the h. f. line. The long metallic circuits required by the d. c. supply render such protection even more necessary. The introduction of protector elements must, however, be done with the greatest caution. Repeater equipments are nowadays buried and must therefore operate with a minimum of maintenance. The use of carbon arresters or rare gas tubes is not compatible with this principle and should therefore be avoided. L M Ericsson have therefore based their overvoltage protector solely on semiconductors in combination with filters. It consists of a transverse protector, patented by LM Ericsson, on the inputs and outputs of the repeaters and a longitudinal protector in the power supply path. The overvoltage protector allows the equipment to operate with the outer conductors of the coaxial tubes earthed, which was highly desirable from the point of view of personal safety. All metallic objects with which staff may come into contact are in this way always at earth potential. Fault Tracing and Supervisory Equipment Any faults on long h. f. lines, primarily cable faults, must be immediately reported to the terminal stations and the staff there must be able accurately to locate the fault without comprehensive and time-consuming measurements, despite the many repeaters involved. It is important that the principle should be as simple as possible in order to arrive at the minimum fault rate for the fault-locating equipment. Since the fault-locating equipment must operate when there is a fault in the main system, it was considered advisable to make it as independent of the main system as possible. Interstitial pairs in the cable are therefore used for the purpose. Fig. 8 Simplified diagram of fault-location equipment P Pilot alarm relay PL Pilot alarm control Tk Temperature compensation By basing the fault-locating arrangements on the systematic use of pilot receivers, the latter have been further exploited so as to permit a simple faultlocating system. Fig. 8 shows a simplified diagram of the system. Every pilot receiver has a mercury-wetted relay which, on loss of the pilot, applies a short-circuit to one interstitial pair in the cable. Level alarm is thereby obtained at both terminal stations and, by measuring the loop resistance for Terminal repeater A Dependent repeater Terminal repeater B "1
the pair up to the short-circuit point, the location of the fault can he read on a station-graduated bridge. One pair is required for each direction of transmission. Up to 50 stations can be supervised in this way. The supervising section may therefore consist of two power-fed sections and a power-feeding intermediate repeater need not be attended. A circuit patented by L M Ericsson, by sensing the current-voltage condition on both sides of the fault, permits the correct issue of alarm even in the event of an open on a power feed loop. Otherwise accurate location would be impossible, since all repeater stations in the loop would be dead and give an alarm. The fault-location equipment incorporates an accurate level supervision device after the last amplifier in the receive section of the terminal repeater equipment. This pilot receiver has an adjustable alarm limit and can be furnished with recording instruments so as to check the stability of the line (block schematic, fig. 4). For customers who so desire, equipment for intermodulation supervision and noise blocking can be added in the terminal repeater. The function of the noise blocking equipment is to short-circuit the line output when the noise level per channel exceeds -40 dbmO (limit adjustable). This prevents the disturbance of traffic on other circuits by noise which—for example in the event of a cable fault—might be transmitted via group throughconnections. Even if the h. f. line has pilot-controlled level supervision, it may be desirable to supplement this by intermodulation measurement. With heavy feedback in the repeaters these may show the correct gain but nevertheless have too high a non-linear distortion. On the send side the measuring equipment injects two frequencies between the useful band and the regulating pilot, in the 1.3 Mc/s system 1309 kc/s and 1313 kc/s. The intermodulation product 2/,-/ 2 , which is 1317 kc/s, is obtained in each repeater. Since the frequencies lie within a narrow band, the product adds on a voltage basis, and its level can be checked in the receiving station. The principle is shown in the block schematic in fig. 4. Speaker Circuit A speaker circuit can be arranged, if desired, along the line. The main use of this circuit is when there is a fault in the system, and it must therefore be as independent of the system as possible. For this reason a four-wire circuit with terminal amplification has been adopted on ordinary interstitial pairs in the cable. The dimensions and loading of the pairs should be such that the attenuation at 800 c/s between the terminal points does not exceed 40 db. The amplifiers need be placed only in terminal and power-feeding intermediate repeater stations. A portable telephone set for plugging into the intermediate repeater stations enables conversations to be established between them or to the terminal stations. D. c. signalling is employed and provides a simple means of selective calling.
Page 1: ^ A
Page 4 and 5: L M Ericsson's H. F. Line Equipment
Page 6 and 7: Terminal repeater Cable Dependent r
Page 8 and 9: •ran •!!•••••••
Page 12 and 13: • The four-wire circuit permits a
Page 14 and 15: Fig. 12 Example of connection of ma
Page 16 and 17: AKD 735, Code Switch P.A.B.X. for 3
Page 18 and 19: Recall to operator. If an incoming
Page 20 and 21: Rack The rack has a light-grey enam
Page 22 and 23: The tones are generated by a transi
Page 24 and 25: Common night service. The operator
Page 26 and 27: Radio Alarming of Fire Brigade ANDE
Page 28 and 29: The system in its present form cons
Page 30 and 31: Aerial There is very little possibi
Page 32 and 33: Telephone Jubilee at Ericsson do Br
Page 34 and 35: G. H. Wieneke in Memoriam After a b
Page 36: The Ericsson Group Associated and c

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