Cables
87
A cable, like any electrically charged object, has capacitance. The capacitance between the
conductors of a cable is called mutual capacitance.
A cable’s mutual capacitance depends on many factors, including the length of the cable, the
distance between the conductors, and the type of dielectric (the insulation between the
conductors). Different brands of cables can have different capacitances, even if their gauge
and length are the same.
A capacitance higher than 35 nF can lead to signal distortion
If the capacitance of the cable is too high, signal distortion results at high frequencies,
including the frequencies used for communication between transponders. A cable with a
capacitance higher than 35 nF (nanofarads) can lead to signal distortion. Mircom has looked
at different brands of cables and calculated the length of each cable at 35 nF. This is the
maximum recommended length for each brand of cable. Table 23 lists these lengths.
Using capacitance to determine maximum recommended length
The manufacturer’s specifications for a cable usually lists the capacitance for a certain length.
From this information you can calculate the length of cable that has a capacitance of 35 nF.
For example, consider a cable that has a capacitance of 47 pF/ft (picofarads per foot). That is,
1 foot of this cable has a capacitance of 47 pF. You can calculate the length at which this cable
has a capacitance of 35 nF.
Calculate the maximum recommended length of a cable
1. Convert the capacitance to nanofarads. 1 pF equals 0.001 nF.
For example, 47 pF equal 0.047 nF.
2. Create an equation where one side is the known capacitance per foot, and the other side
is 35 nF per
x
feet.
3. Solve
for
x
.
The capacitance of this cable is 35 nF per 744.68 feet. Therefore, the maximum
recommended point-to-point length for this cable is 744.68 feet.
If you know the nanofarads per foot, you can divide 35 by this number to get the
maximum recommended length in feet.
12.2
Fiber Optic Cables
FleX-Net™ supports two kinds of fiber optic cables:
•
50/125
μ
m
•
62.5/125
μ
m
The numbers represent the diameter of the core/cladding in micrometers (microns).
0.047
1
-------------
35
x
------
=
0.047
(
)
x
( )
35
( )
1
( )
=
0.047
(
)
x
( )
0.047
--------------------------
35
( )
1
( )
0.047
-------------------
=
x
744.68
=
