Selection and Testing of Electronic Components for LM

Fig. 8
Block diagram of the line terminating equipment
inZAD8-2
Electronic switch
Resonant circuit
Pulse regenerating circuit
Strapping field
Transformer
Line bullding-out network
DC-DC converter
Signal path
Fig. 9
Alarm functions in the line terminating equipment
Absence of pulses, send direction
Remote power feeding fault
Absence of pulses, receive direction
High error rate, receive direction
Strappable combinatory logic
Transmission path
Fig. 8 shows the block diagram for the
line terminating equipment in system
ZAD 8-2. In the send direction the bipolar
signal is converted from unbalanced
to balanced form in the send
transformer of the transformer unit. In
the receive direction the signal is regenerated
in the terminal repeater,
which is identical to a one-way dependent
repeater. If the repeater section
is short, LBO networks with an attenuation
of 30 dB at 4.224 MHz are
connected in.
The description above also applies for
ZAD 2-3. In this case no LBO network
is required, but a 6 dB flat attenuator
can be strapped in the send direction,
for example to reduce the cable crosstalk.
Leaving out the LBO network has
made it possible to incorporate the
alarm circuit in the transformer unit.
Remote power feeding
The dependent repeaters along the line
are power fed in series over the phantom
circuit from the remote power feeding
unit in the line terminating equipment.
The same type of unit is used in
both ZAD 8-2 and ZAD 2-3. The
power feeding takes place at a constant
direct current of 48 mA and a voltage of
up to ±106 V balanced to earth. The
distance between power feeding stations
varies from about 20 to 40 km in
the case of 8 Mbit/s and from about 40
to 60 km for 2 Mbit/s, depending on the
repeater spacing and the voltage drop
in the cable. This distance can almost
be doubled by using extended feeding
via separate pairs or by series connection
of power feeding units. In order to
ensure personal safety it is possible, by
strapping, to limit the maximum output
voltage to approximately 10 V if a break
should occur in the powerfeeding loop,
for example a cable break.
Alarm, automatic changeover
The line terminating equipments in ZAD
8-2 and ZAD 2-3 have the same alarm
functions, fig. 9. Plug-in U-links are
used to set up the desired connections
between primary and derived alarms.
The alarm state of the system is indicated
by light-emitting diodes, which can
be seen through the front plate of the
unit.
The line terminating equipment can be
provided with an auxiliary unit which,
in the case of an alarm in the receive direction,
provides automatic changeover
to another, predetermined system that
serves as a standby, fig. 10. The automatic
changeover, which is tied to
single-cable working, is particularly attractive
if the standby system goes via
another cable or a radio relay link.