Selection and Testing of Electronic Components for LM

Insulation: Solid
polyethylene
Conductor diameter: 0.5 mm
Cable length: 500 m
Insulation: Foamed
polyethylene
Conductor diameter: 0.5 mm
Cable length: 500 m
Insulation: Solid
polyethylene
Conductor diameter: 0.7 mm
Cable length: 500 m
Fig. 13
RMS values of capacitance unbalance distribution
for cross stranded cable
Each cross represents the RMS value of the 45 capacitance
unbalance values within a 10-pair group
Fig. 12
Distribution curves, showing the capacitance unbalance
for 25 and 10-pair groups
Insulation: Solid polyethylene
Conductor diameter: 0.5 mm
Cable length: 500 m
Curve A represents the capacitance unbalance values
within 25-pair groups In cross stranded cable
Curve B represents capacitance unbalance values within
10-pair groups in cross stranded cables
The characteristics of completed cables
are naturally also dependent on the
quality of the individual pairs as regards
the uniformity of conductors and insulation,
lay lengths etc.
As can be seen from fig. 11 the unbalances
between groups is much less
than the unbalances within groups.
Fig. 12 shows that the unbalances in
25-pair groups are lower than the corresponding
values in 10-pair groups.
The quality of a cable as regards capacity
unbalance is given as the RMS (root
mean square) value. The distribution
diagrams in fig. 13 represent the RMS
values obtained for different types of
cables.
As can be seen from the diagrams, the
spread is relatively large and thus a
reasonably large number of measured
values will be required in order to be
able to establish differences in the quality
of cables that have been manufactured
in different ways.
Mutual capacitance
In cross stranded cables there is no
systematic difference in mutual capacitance
between pairs, caused by their
positions in different layers. There are,
however, some small differences in
mutual capacitance because of the different
lay lengths and manufacturing
tolerances of the pairs. This is shown in
table 1.
Cross stranded, PE insulated 10-pair
group cables without jelly filling have a
lower mutual capacitance than the corresponding
10-group layer cables
(2 + 8). The reason for this is that the
cross stranded cables contain more air
because of the stranding method. A reduction
in mutual capacitance of about
3 % has been noted.
The mutual capacitance relationships
are different for cables with other types
of insulation material.
The cables are generally specified for a
fixed mutual capacitance and hence the
conductor insulation in cross stranded
cables can be reduced with a consequent
reduction in material consumption.
High frequency characteristics
The high frequency characteristics of
symmetrical cables are becoming increasingly
important. This applies
wherever the cables are situated in the
network and particularly when they are
to be used for PCM systems. Typical
crosstalk values for cross stranded cables,
given as the mean value m and the
standard deviations, are shown in table
2.
The standard deviation, o, for near-end
crosstalk is of particular interest. The