CROSSTALK - Advanced Signal Integrity for High-Speed Digital Design - page 193

TABLE 4-4. Crosstalk Reduction Techniques and Trade-offs
Approach Trade-off

Increase the spacing, s , between the
PCB and package traces.

Can add cost by requiring additional layers in
PCBs and/or packages to route all traces.

Decrease the thickness of the
dielectric, h , between the ground
(return) layer and the signal layer
in PCBs and packages to couple
the transmission lines more
tightly to the return layer and
reduce the coupling to adjacent
signals.

Operating limits for characteristic impedance
may limit the minimum acceptable thickness
(recall that Z 0 decreases as h decreases).
The minimum manufacturable dielectric
thickness may also limit the effectiveness of
this approach.

Use differential signaling. Can add cost by requiring additional layers in

PCBs and/or packages to route all traces,
since each signal requires two traces. Can
also add cost to packages, sockets, and
connectors, due to increased pin counts.

Route traces on adjacent PCB signal
layers orthogonally to each other.

Can add cost by requiring additional layers in
PCBs and/or packages to route all traces,
since restricting signal routing direction may
decrease routing efficiency.

Route the signals in PCBs and
packages using striplines or
embedded microstrips to
eliminate velocity variations.

Striplines require at least six layers in the PCB.
Refer to Figure 4-31.

Minimize parallel run lengths
between signals in PCBs and
packages.

Can add cost by requiring additional layers in
PCBs and/or packages to route all traces,
since restricting signal routing direction may
decrease routing efficiency.

Reduce signal edge rates. May limit the maximum performance, since

rise and fall times typically must scale with
data rate.

Insert power/ground pins between
signal I/O pins in connectors,
sockets, and packages.

Cost increases with pin count.

defeating their purpose. As a result, the guard traces must be connected to the
ground layer at multiple points. The distance allowed between vias is inversely
proportional to the frequency content of the signals, making it impractical for
multigigabit per second data rates.

4.6 SUMMARY

In this chapter we described the coupling mechanisms that cause crosstalk in
digital systems. The SLEM modeling technique and the equations for coupled