Digital Signal Processing Reference
In-Depth Information
13.6.6 Minimize Layer Hopping
Vias represent an impedance discontinuity unless their impedance is intentionally
designed to match the trace impedance (described in Chapter 14). This gives rise to
reflections and at high speeds can significantly impair the ability of a serial intercon-
nect from operating properly.
13.6.7 Use Differential Vias
A pair of vias can be designed to have differential impedance equal to the dif-
ferential impedance of the diff-pair traces. As described in Chapter 14, designing
these structures requires a 3D field solver to model the effects of via size, antipad
diameter, pad diameter, and laminate thickness on the differential impedance [29].
Such via structures greatly reduce (and may eliminate) the impedance discontinuity
presented by vias not as carefully designed, thereby reducing signal reflections and
allowing the signal to operate at very high speeds.
13.6.8 Match Overall Trace Lengths
Violations of this rule can be subtle and detecting them often requires a detailed
understanding of the routed path.
For instance, right-angle multirow connectors have different length pins in the
upper and lower rows. If these two rows are used to form a diff-pair, the length
skew occurs on each of the connectors forming a mated pair, doubling the differ-
ence. For this reason routing diff-pairs through these connectors should be done
with pins on the same row. This length mismatch can be present when routing
either edge-coupled or broadside traces.
This rule can be unintentionally broken when routing broadside diff-pairs. Be-
cause they are routed on different layers, vias or interlayer connections made by
connector pins moving signals to the board's outer surface naturally will be of dif-
ferent lengths [13]. When two connectors are used (as in a midplane or daughter
card situation) this inevitable length skew can sometimes be corrected by swapping
the layers on the two cards. If the two cards have the same stackup thickness the
via (or pin) lengths become matched, eliminating the skew.
A third subtle way in which this rule is violated is shown in Figure 13.16 where
a trace includes one or more bends as it changes direction.
Figure 13.16(a) shows how the bends cause a difference in trace length. In
high-performance signaling back-to-back bends can be used to readjust the overall
length.
13.6.9 Match Lengths on Each Layer
Besides ensuring that the total length of trace from the transmitter to receiver is
the same for the two traces forming a diff-pair, it is important that the lengths are
matched on each of the layers on which they are routed. For instance, a diff-pair
may exit a BGA pin field on the surface, using microstrip, and then transition with
a pair of vias to a stripline layer. The lengths of the two microstrip traces should be
