Stubs are short trace segments that extend,
unterminated, from (an often controlled impedance or transmission line)
main trace. Most commonly they connect a pad, and therefore a device, to
a net. Figure 9-12 illustrates four stubs, S1 through S4. S1 and S3 are
normal stubs that we all use. Stub S2 is too long and should be avoided
in high-speed designs. Route the trace closer to the pad as shown for
are two reasons for avoiding long stubs. One relates to transmission
lines and is covered in Chapter 10. The other relates to EMI.
at your cell phone. There is probably a short antenna extending out
from the top. Now look at Stub S2 in Figure 9-12. This unterminated stub
looks a lot like the antenna that extends out of your cell phone. Stubs
like this can radiate just like antennas and should be avoided.
Figure 9-12: Various examples of stubs.
stub shown as S4 is particularly harmful. This might have been created
when a trace was rerouted and the designer forgot to remove a short
segment from the original route. This is an antenna. Its results might
not be harmful, depending on the circumstances, but they are never
beneficial. Never allow such stubs to exist on your board.
theory is a very complex area that is not well understood by most
people. There are a huge number of variables that affect whether a wire
or trace can function well as an antenna. Some of these include the
environment the wire or trace is in—the degree to which it is shielded
by the planes, its length, the surrounding material and its relative
dielectric coefficient, the specific shape of the wire or trace, and so
on. Another variable is the frequency of the signal (or its harmonics).
So it is difficult to look at a stub and say with certainty whether it
will create a problem. But the general rule is that stubs create
problems, so minimize them. Be sure to take the time to trim back any
trace that is left “hanging” and not connected to anything.