Selection and Testing of Electronic Components for LM

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Digital Line Equipments for 8 Mbit/s and 2 Mbit/s Juho Arras and Örjan Mattsson This article presents the digital line equipments included in LM Ericsson's new family of PCM systems in the M5 construction practice. PCM multiplex and signalling conversion equipment has been described earlier\ The two line equipments are intended for transmitting 8.448 Mbit/s and 2.048 Mbit/s over pair and quad cables. A unique strapping network has been introduced in the repeater equalizers which makes it possible to use existing paper-insulated cables for transmitting 8.448 Mbit/s. When designing the equipments the latest CCITT and CEPT recommendations have been taken into consideration and also the experience gained from the earlier generation of 2 Mbit/s line equipment. The equipments are characterized by high reliability with generously dimensioned lightning protection, good transmission characteristics and a high degree of flexibility in combination with a design that makes installation and maintenance easy. The two systems are closely related as regards their design. UDC 621.395 343 621.3152: 621.391.31 Fig. 1 Block diagram and interfaces for digital line equipment MUX PCM multiplex equipment tor 30 circuits LTE Line terminating equipment lor a PCM system In the terminal station Two-way intermediate repeater in a dependent repeater station D Digital line interface 75 Q S Line Interface RS Repeater section PFS Power feeding section FLS Fault location section for repeaters CS Cable section 1) Interface cable (coax. 75 Q) 2) Line (pair or quad cable) ~ ~ Signal path System aspects The new family of digital line equipments in the M5 construction practice for transmission over pair and quad cables consists of - ZAD 8-2, with a bit rate of 8.448 Mbit/s corresponding to PCM transmission of 120 telephony channels - ZAD 2-3, with a bit rate of 2.048 Mbit/s corresponding to PCM transmission of 30 telephony channels. There are great similarities between the two systems as regards design and equipment. The article will deal mainly with ZAD 8-2. The description of ZAD 2 — 3 is restricted to the parts that differ from the previous generation 2 or are common for ZAD 2 - 3 and ZAD 8-2. Fig. 1 shows the structure of a digital line equipment. The basic principles of its function have been described in detail earlier 2 . The connection to the line is via the internationally standardized coaxial D interface. The matching between the interface and the cable takes place in the bay-mounted line terminating equipment. The bipolar line signal is regenerated in dependent twoway regenerative repeaters placed in re peater housings along the cable. Two line terminating equipments and the intermediate line repeaters form a digital line section, which is designed as an independent functional block with its own power and alarm systems. Fault location equipment, which is usually common for several digital line sections, is normally included so that any faulty repeaters can be located. The transmission takes place over symmetrical pairs, one pair for each direction of transmission. The two pairs can be in the same cable — single-cable operation — or in separate cables —two-cable operation. Single-cable operation means simpler system design. Twocable operation has transmission advantages since near-end crosstalk, which is often the predominant source of interference, is eliminated. 8 Mbit/s on existing cables First-order PCM systems were introduced in the telecommunication network mainly because of the increase in the capacity of existing cables that was thereby obtained. Experience has shown that these systems have provided good technical performance and good economy. In the case of second-
JUHO ARRAS ÖRJAN MATTSSON Transmission Division Telefonaktiebolaget LM Ericsson Fig. 2 Comparison of the attenuation characteristics of paper-insulated star quad cable (1.2 mm, 25 nF/km) and polythene-insulated cable based on the assumption that the attenuation of both cables at 4.224 MHz is 60 dB. The frequency dependence of the paper-insulated cable corresponds to \ 7+0.6 f and that of the polytheneinsulated cable to \ T. ^"" Paper-insulated ^ Polythene-insulated Fig. 3 a) Overvoltage protection for the ZAD 2 — 3 line repeaters Longitudinal overvoltages build up a voltage across R1 and T so that the gas tube G strikes. During this process the repeater equipment and feeding diode D are protected by an effective current division between the power diode T and R2. b) A test pulse used for overvoltage testing. The shortcircuit current of the surge voltage generator has been varied over the range 200 A to 1300 A. order PCM systems it has been assumed that special cables with polythene insulation will be used. These often contain screened groups in order to keep the near-end crosstalk at an acceptable level with single-cable transmission. The 8 Mbit/s system makes much greater demands than the 2 Mbit/s system as regards the separation between the transmission directions. Considerable financial advantages would be attained if existing paper-insulated cables could also be used for second-order systems. In many cases two-cable operation can be arranged or a large cable can be utilized so that it is possible to achieve the necessary separation. As has been mentioned in a previous article 3 , at the frequencies of interest when transmitting at 8 Mbit/s, paperinsulated cables have an attenuation curve that deviates from that of polythene-insulated cables. There are also considerable differences between different types of paper-insulated cables. The article mentioned above gives the theoretical background for the method whereby the 8 Mbit/s line repeater can be adjusted optimally to suit different types of cables, by means of straps in the equalizer. ZAD 8 —2 therefore offers the attractive possibility of working with existing paper-insulated cables and also polythene-insulated ones. The use of only one type of repeater is advantageous from the point of view of maintenance and the stocking of spares. On the island of Funen in Denmark some fifty two-way repeaters have successfully been installed and put into operation on paper-insulated cable, conductor diameter 1.2 mm, capacitance 25 nF/km. The repeaters work with twocable operation, with a repeater spacing corresponding to 60 dB attenuation at 4.2 MHz. In order to give an idea of the difference in the attenuation characteristics, the attenuation curves of paper and polythene-insulated cables are compared in fig. 2. The difference in attenuation is as much as ±10 dB within the frequency range of the repeater. Reliability, overvoltage protection, maintenance -High reliability is a prerequisite for a 115 line equipment. This applies particularly for the cascade-coupled line repeaters, which are often located in inaccessible places. In the design stage great efforts have therefore been made to choose the most suitable components, and to dimension the circuits with ample safety margins. In addition the burn-in method is used during manufacture in order to eliminate unreliable components. In rural areas PCM transmission is often used in cables where the repeaters can be subjected to very severe stress in connection with lightning and short circuit to earth in nearby power lines. Extensive work has been devoted to equipping repeaters and other equipment with efficient overvoltage protection. Fig. 3 shows how such protection is arranged in the ZAD 2 — 3 line repeaters. The dimensioning, which can cope with pulses far in excess of the CCITT requirements, has been tested in field trials in Norway with both aerial and buried cable. The repeater protection occupies about 20% of the area of the printed wiring board. The volume of the components concerned must be large, among other reasons because they have to be able to withstand the high powers that arise with induced currents of some tens of amperes and of long duration. The overvoltages are as far as possible leaked away via overvoltage tubes before they reach the printed board. Exposed conductors are made wide, with a large distance to adjacent conductors. Efficient methods for locating repeater and cable faults are essential from the point of view of maintenance. A new fault locating system for repeaters has been developed for ZAD 8 — 2. It is based on remotely controlled fault detectors in each repeater housing and permits measurements during traffic. The line terminating equipment includes alarm circuits for supervising transmitted and received signals and the remote power feeding, so that the type of fault can easily be determined. The equipment at the terminal can be provided with a unit that carries out automatic changeover to a standby system if a fault occurs in the working system.
Page 1: ERICSSON REVIEW 3 1977 SELECTION AN
Page 4 and 5: Selection and Testing of Electronic
Page 6 and 7: Fig. 2 Curve tracer The special typ
Page 8 and 9: Fig. 6 Humidity test High air humid
Page 10 and 11: 100 Type of check Type of component
Page 12 and 13: Fig. 12 Computer-controlled test sy
Page 14 and 15: 104 Fig. 16 Comparison of fault tra
Page 16 and 17: Fig. 2 Cable groups with the conven
Page 18 and 19: 108 Fig. 9 Take-up stand lor connec
Page 20 and 21: Insulation: Solid polyethylene Cond
Page 22 and 23: New Telephone Set Arne Boeryd and G
Page 26 and 27: Fig. 5 ALBO network for automatic e
Page 28 and 29: Fig. 8 Block diagram of the line te
Page 30 and 31: Fig. 13 Small repeater housing of t
Page 32 and 33: 122 Fig. 16 Fault detector shelf an
Page 34 and 35: 124 References 1. Lindquist, S. and
Page 36 and 37: LARS G ERICSSON AKE PERSSON Telepho
Page 38 and 39: 128 Fig. 3 Standardization of progr
Page 40 and 41: 130 Fig. 6 The control room in an A
Page 42 and 43: Table 3 Number of component faults
Page 44 and 45: 134 ence this as congestion or a "s
Page 46 and 47: ABJ 101-the Modern Public Magneto S
Page 48 and 49: Fig. 3 Line unit for 10 magneto lin
Page 50 and 51: " The maximum number of lines that
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Selection and Testing of Electronic Components for LM

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