Digital Signal Processing Reference
In-Depth Information
Table 9.1
Impedance and propagation delay
of the victim 50
microstrip in Figure 9.5.
Ω
A4
A3
V
A2
A1
Z
tpd
−− +− −
42
6.31
+− +− +
43
6.43
SS
+
S
S
50
6.46
−− ++ +
52
6.54
−+ ++ −
63
6.65
++ ++ +
69
6.92
Transmission line impedance (
Z
) is in ohms and the delay (
tpd
)
is in nanoseconds/meter.
of the victim trace causing different voltages to be launched when neighboring
traces switch.
To summarize:
Stripline and microstrip victim traces have the nominal impedance when all
neighbors are static.
•
The lowest impedance for stripline or microstrip victims occurs when all
neighbors switch out of phase.
•
The highest impedance of stripline or microstrip victims occurs when all
neighbors switch in phase.
•
Stripline propagation delay is not affected by the switching activity of its
neighbors.
•
Microstrip has the nominal delay when all its neighbors are static.
•
Microstrip has the lowest propagation time when all its neighbors switch out
of phase and has the largest propagation time when all its neighbors switch
in phase.
•
9.5
What Are Odd and Even Modes?
In a two-trace system the switching mode is said to be even when the traces switch
in phase; it is odd when they are out of phase [5, 6].
Differential signaling (as discussed in Chapter 13, where one trace carries a
positive going signal and the other carries its complement) is an example of odd-
mode switching since the two signals are always intended to be out of phase.
From Figure 9.6 with even-mode switching, the inductive coupling moves in
the same direction and encompasses both traces, while the capacitive coupling does
not include the neighboring traces. From this we would expect the total inductance
to be equal to the self plus the mutual inductance and the total capacitance to be
only the capacitance to ground. Since by its definition the self-capacitance includes
all of the mutual capacitance, to obtain the total even-mode capacitance, the mu-
tual capacitance must be subtracted from the self-capacitance.
