Digital Signal Processing Reference
In-Depth Information
3W
W
Figure 9.9
Side view of two traces showing the 3W rule of thumb.
Using this rule with 5-mil-wide traces would result in traces being routed as
close together as 10 mils. Judging from Figure 9.8, for the 65
trace the coupling
is quite high at that distance and it must be increased to over 25 mils before the
coupling is reduced to a reasonable value.
Ω
9.6 What Are the Circuit Effects When Switching Causes the
Impedance to Change?
Chapter 6 showed how voltage divider action between the impedances of the trace
and driver determines the voltage launched down the victim trace. Because switch-
ing activity of neighboring traces causes the victim impedance to change, neighbors
switching causes the launched voltage to change even if the power supply voltage
is held constant.
This is illustrated in Figure 9.10, which shows the voltage launched down the
transmission line circuit appearing in Figure 9.5, which was used to create Table
9.1. The drivers have a drive strength of 32 mA.
Figure 9.10 illustrates the two worst-case situations that bound the results.
Even though the power supply is 3.3V, the voltage launched by a 32-mA driver is
3.1V when the victim and neighbors are all switching in phase (corresponding to
a victim impedance of 69
in Table 9.1), but it falls to 2.9V when the neighbors
switch out of phase (the 42
Ω
impedance case). As explored in the Problems, a
weaker driver would launch a lower voltage and the difference in launched voltages
would be greater then the 200 mV shown in Figure 9.10.
Ω
9.7 How Is Receiver Timing Affected by In- and Out-of-Phase
Switching?
Table 9.1 shows how the switching activity of neighboring microstrips causes the
trace delay to vary from a low of 6.31 ns to a high of 6.92 ns, for a total variation
of 610 ps. However, as Figure 9.11 illustrates, the variation is 650 ps when the
measurement is made at the output of the victim receiver.
The added delay is caused by the difference in launched rise times between the
in-phase and out-of-phase cases. With an impedance of 69
, the in-phase signal-
ing case represents less capacitive loading to the driver and so has a sharper rise
time than the out-of-phase case, which has an impedance of 42
Ω
. The result is that
more time is required for the signal to reach the receiver switch point. This loading
Ω
