Selection and Testing of Electronic Components for LM

Selection and Testing of Electronic
Components for LM Ericsson's
Telephone Exchanges
Berndt Agneus and Ivan Borgstrom
Electronic components have formed part of automatic telephone exchanges for
a rather long time. The rapid development that these components, primarily micro
circuits, have experienced during recent years and which can be expected to continue
for a long time is of great importance for the design of new exchange systems.
In fact, the new systems are in the main based on electronic components. In the
design work it is essential to select components that not only have the desired
function but which are also stable during the estimated life of the exchange. The
article deals with various activities, which together are aimed at ensuring the correct
range of components for telephone exchange equipments. The illustrations
with captions provide information regarding various aids that are used in this connection.
The article also gives a summary of electronic components of current
interest and their probable development trends.
UDC 621.3.049.7 Component categories
Electronic components for LM Ericsson's
telephone exchange equipments
are to a certain extent manufactured
within the group. The remainder are
bought externally and usually belong to
the suppliers' standard ranges. In certain
cases, however, components are required
with characteristics that necessitate
either special selection from the
standard range or the introduction of
special "customer adapted" components.
The component quality corresponds
to the category "professional
components", which meet higher reliability
and long-term stability requirements
than so-called entertainment
components.
Electronic components are usually divided
into three main categories as regards
their function, namely passive
components (resistors, capacitors etc.),
discrete semiconductor components
(such as diodes, transistors and
thyristors) and various types of micro
circuits, see fig. 1.
The importance of the
components in the system
design
The design and characteristics of an exchange
system are to a great extent dependent
on the design of the components.
There is in fact mutual effect
since the system design influences the
design of the components.
LM Ericsson's "great" automatic
systems, the 500-line selector system
and the crossbar system, were both
based on electromechanical components.
However, electronic components
were also included in these systems at
an early stage. They were then mainly
used for secondary functions such as
series and parallel resistors, delay
capacitors, CR units for contact protection
etc. These components were often
soldered on to tags on the relays.
The electromechanical systems have
been modernised extensively as and
when the need has arisen. In connection
with this, discrete components (first
discharge valves, later on diodes and
transistors) were brought into use at
quite an early stage. They were then included
in such function units as test
circuits, MFC signalling systems and
charging units.
SPC technique (Stored Program Control)
was first used in LM Ericsson in the
transit exchange system AKE 13, and
was then based on diodes, transistors
and ferrite memories. Fast micro
circuits, including semiconductor
memories, became important for the
further development of the SPC technique.
Micro circuits are now primary elements
in modern exchange systems. They require
very little space in relation to the
large number of logic functions they are
able to perform. They are usually
mounted on printed circuit boards,
which makes for a compact construction
throughout. Reed switches, miniature
relays and certain other components
have also been adapted for
mounting on printed boards.
The development of components and
systems continues in close collaboration.
The need for such development
collaboration will increase as complex
"components" containing very large
numbers of functions are introduced.
An example of such "components" are
microcomputers, which in one or a few
micro circuit packages accommodate
the primary functions of a computer.
Principles for the selection
of electronic components
Components in telephone exchange
eauiartiants musLhe_abJe to perform the