4.6. EFFECT OF TOPOLOGY
So far in this book we have covered many issues that deal with an interconnect connecting
two components. However, this is not always the case. Often, it is required that a single
driver be connected to two or more receivers. In these cases the topology of the
interconnects can affect the system performance dramatically. For example, consider
Figure
4.20
, which is a case where one driver is connected to two receivers. In one case, the
impedance of the base (Z
o1
) is equal to the impedance of the two legs (Z
o2
). When the signal
propagates to the junction, it will see an effective impedance of Z
o2
/2, resulting in the
waveform in the figure that steps up toward the final value. These reflections can be
calculated using a lattice diagram similar to that of
Figure 2.17
.
Figure 4.20: Signal integrity produced by a balanced T topology.
When the impedance of the legs are twice the impedance of the base, the effective
impedance the signal will see at the junction will be equal to the base; subsequently, there
will be no reflections generated. This case, when Z
o2
= 2Z
o1
, is shown in
Figure 4.20
. When
the structure is unbalanced, as in the case where one leg is longer than the other, the signal
integrity will deteriorate dramatically because the reflections will arrive at the junction at
different times. This is shown in
Figure 4.21
. The signal integrity can be
Summary :
In one case, the impedance of the base (Z o1 ) is equal to the impedance of the two legs (Z o2 ). This case, when Z o2 = 2Z o1 , is shown in Figure 4.20 .
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figure,impedance,case,signal,two,junction,420,base,integrity,reflections,topology,drier,legs