Stubs - Compaq BL10e - HP ProLiant - 512 MB RAM Specification

Stubs

In PCB design, through-hole vias connect the signal path from one layer to another. As shown in
Figure 6, many instances of this path leave a portion of the via unused. As signal speed increases,
this unused portion of the via becomes a transmission line that can create significant reflections that
can seriously reduce signal quality. Any conductor that extends off the main conductor some length
and then stops is called a stub.
A through-hole via connecting the signal path from one layer to another that extends beyond the main
path is an example of a stub. As shown in Figure 6, traces changing layers frequently leave such an
unused portion of the via. A portion of the signal traveling on the trace will go down the stub and
reflect off the end and return to the trace after the delay of going down and back. This reflected signal
is added to the traveling signal. At high enough data rates this delay is long enough compared to the
bit length to adversely affect a portion of the next bit. This distorts the signal waveform.
Figure 6. Cross sectional view of a plated through hole, the portion of the via that is not used for the signal path
is known as a stub
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