Characteristic
impedance of
Cable
The Engineering Department of Belden CDT Electronics
Division sheds light on the characteristic impedance of
cables at high and low frequencies
The increasing use of
electrical pulses in
transmission of data by cable
has resulted in a need for a better
understanding of the electrical
characteristics of a cable.
Many system specifications
state that the cables used shall
have a characteristic impedance
specified in ohms. Any cablemaker's
catalogue will list the
characteristic impedance values of
most coaxial cables, which usually
range from 50 to 95 ohms. This
catalogue may also refer to values
of 100 to 200 ohms for certain
shielded pairs, which appear to be
designed for special applications.
But, impedance information on the
more common types of shielded
pairs is not readily available to the
cable user. Why? Because there are
too many variations of applications
involved with these cables.
Impedance defined
Let us examine characteristic
impedance - what it is and what it
is not - so that a better
understanding of what the
numbers mean and how they
apply to our application can be
derived.
Magnitude of impedance
Ohm's Law states that if a voltage
(E) is applied to a pair of terminals
and a current (I) is measured in this
circuit, the following equation can
be used to determine the
magnitude of the impedance (Z).
F / I=Z
This relationship holds true for
both direct current (DC) and
alternating current (AC).
Phase
In the case of alternating current,
impedance has two components:
magnitude in ohms (discussed
earlier) and phase. Phase refers to
the instantaneous relationship
between voltage and current.
....CONTD
