Specifications

21
Schneider Electric
Choice of the Physical
Communication Medium (continued)
Equivalent diagram:
E = Transmitter
L = Series inductance
R = Series resistance
C = Capacitance between wires
G = Conductance (easier to calculate than parallel resistance) – represents loss in
insulation
Definition: The characteristic impedance (Zo) in Ω is the simplified model of the
representation of the cable. It is defined for a theoretical length of the infinite cable,
so that the termination of this cable need not be taken into account.
It depends of the physical and electrical characteristics of the conductors and varies
with frequency.
f: frequency
j: symbol of the phase (+90°).
G is negligible compared to 2*
π *f*C for commonly used insulating materials.
Likewise, at "low" frequencies (< 1MHz), R prevails over 2* π *f*L.
Thus the formula becomes:
At high frequencies (>1MHz) the formula becomes:
The following curve can thus be traced:
E52291E52292
E
I
L
R
C
G
Zo
(R+j*2*
Π *f*L)
(G+j*2* Π *f*C)
-------------------------------------
=
Zo
R
(j*2*
Π *f*C)
-----------------------------
=
Zo
(j*2*
Π *f*L)
(j*2*
Π *f*C)
---------------- therefore: Zo
L
C
----==
Transition regionLow frequency region
High frequency region
10,000
1000
100
50
20
10
10 K
100 K
1 M 1 G
10 M 100 M
10
100 1000
Zo = R
j 2 Π f C
Zo = L
C
Zo = R+j 2 Π f L
G+j 2 Π f C
Frequency (Hertz))
Impedance (ohms)
Cabling Guidelines