Technical data

www.westermo.com Theoretical and general applications 43
Types of copper cables
The physical cable is often the weak link in data communication. It is the cable that
handles the interference sensitive analogue signal. It is the cable, through its design,
installation and length together with the surrounding electrical effects, which deter-
mines the rate and quality of communications.
Twisted pair wire
Twisted pair wire is the simplest, cheapest and most common cable. Usually as a twist-
ed pair 4-wire cable. It is a standard copper wire in a protective plastic sheath, with or
without a protective metal screen. There are different brands and types of cable offer-
ing different performance, which should be considered depending on the installation
requirement. And there are different isolation layers that suit different installation envi-
ronments. There are three important concepts that affect the transmission quality:
resistance, capacitance and attenuation.
Resistance states the cable’s electrical resistance. It is measured in ohm/km and
varies with the wire’s material and cross section. The resistance of the
cable is evident from the data sheet for each cable. Cable with a solid
conductor should not have a diameter of less than 0.26 mm
2
and for
multicore conductors 0.2 mm
2
. At low transmission rates it is the resist-
ance that sets the limitations.
Capacitance as the conductors in the cable are isolated from each other they will
generate a capacitive effect between each other. The twisted pair, con-
ductor material and any screen will also have an effect. The capacitance
attenuates the signals differently at different frequencies and the value is
usually stated at 800 Hz. Capacitance is measured in pF/m and a guide-
line value for a good data cable is approximately 50–70 pF/m.
At high transmission rates it is the capacitance that sets the limitations.
Attenuation states the cable’s overall attenuation of the signal from the transmitter to
the receiver. Cable attenuation is stated in dB/km and increases with
ascending frequency. An increase in attenuation of 3 dB represents a
halving of the output.
Attentuation (examples)
150 kHz 8 dB/km
1 MHz 20 dB/km
4 MHz 40 dB/km
10 MHz 65 dB/km
16 MHz 82 dB/km
25 MHz 105 dB/km
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