Digital Signal Processing Reference
In-Depth Information
FEXT = Kf
××
(
l
dv/dt
)
(Capacitive coupling)
Victim
tpd
dt
Victim
FEXT = Kf
××
(
l
dv/dt
)
(Inductive coupling)
dv
Aggressor
dt
Voltage
Time
Figure 10.3
FEXT victim pulse width equals the aggressor rise time (
dt
). It has the same polarity as the ag-
gressor when capacitive coupling dominates but the opposite when inductive coupling dominates.
L
Z
=
s
(10.1c)
o
C
s
The sum of all the mutuals can be used in these equations to find
Kf
when more
than one trace aggresses a victim. The Problems include a worked example of how
to do this.
Because the mutual capacitance (
C
m
) term subtracts from the mutual induc-
tance term (
L
m
), it is apparent from (10.1a) how the inductive and capacitive ef-
fects can cancel one another as happens with stripline. This is explored in the
Problems. It is also apparent how either the mutual inductance or capacitance can
become dominant to determine the polarity of
Kf
.
The change in voltage (
dv
) and time (
dt
) emphasize that it is the difference be-
tween voltage levels (rather than the magnitude) that creates FEXT. For instance, a
signal with a 1-ns rise time switching between 0.6V and 1.6V will induce the same
