3
Concepts
This section presents the basic concepts of characteristic impedance and propagation delay, and
reflections and crosstalk.
Characteristic impedance, Z0
The characteristic impedance of a transmission line is the ratio of the voltage to the current. For a uniform
line, it is invariant with respect to time and position on the line:
If R and G are zero, the characteristic impedance will not depend on frequency, and reduces to
Attenuation Constant
The attenuation constant is the real part of the propagation constant and is important when losses must
be considered.
Propagation Delay
The propagation delay is the reciprocal of the phase velocity multiplied by the length of the transmission
line:
where c is the speed of light, and
r
is the relative dielectric constant. For a uniform, lossless transmission
line.
Medium Delay (ps/in.) Dielectic Constant
Air 85 1.0
Coax cable (75% velocity) 113 1.8
Coax cable (66% velocity) 129 2.3
FR4 PCB, outer trace 140-180 2.8-4.5
FR4 PCB, inner trace 180 4.5
Alumina PCB, inner trace 240-270 8-10
Table 1. Delay And Dielectric Constants For Some Transmission Lines.
Reflections
When a voltage step is traveling down a uniform impedance transmission line, and then encounters an
abrupt change in impedance, a portion of the incident energy is "reflected" back. The amount of energy
C j G
L j R
Z
t y i
t y v
0
) , (
) , (
C
L
Z 0
c
length t
r
d
*
LC length td *
incident reflected V t coefficien reflection V * _
