Traces; Trace Geometry; Trace Characteristic Impedance Design - Xilinx Virtex-4 RocketIO User Manual

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For less dense applications, it might be possible to use a more traditional material such as
FR4, yet achieve lower loss by using wider lines. However, this is more often true in lower
data rate applications. At higher data rates, there is a crossover frequency above which the
linearly increasing dielectric loss dominates skin-effect loss, and only by choosing a lower-
loss dielectric material can overall losses be minimized. Conversely, skin-effect loss
dominates dielectric loss below the crossover frequency. Although not analytically correct,
it is accurate for typical applications to simply add the dielectric and conductor losses to
achieve the total transmission line loss.
The crossover frequency typically varies from several hundred MHz to several GHz for
line widths of .006" and loss tangents from .025 to .01. Narrower and wider lines and lower
loss tangents extend the crossover frequency extremes even further.
A "what-if" analysis can be done in HSPICE simulation to evaluate various substrate
materials by varying the line and dielectric geometries, dielectric constant, loss tangent,
and other parameters of the PCB substrate material. The impact on eye quality can be
simulated to justify the use of higher cost materials.

Traces

Trace Geometry

For any trace, its characteristic impedance is dependent on its stack-up geometry as well as
the trace geometry. In the case of differential traces, the inductive and capacitive coupling
between the tightly coupled pair also determines the characteristic impedance of the
traces. 2D field solvers are useful tools for estimating the characteristic impedance of
differential traces and for computing crosstalk between traces.
Wider traces create a larger cross-sectional area for current to flow and reduce resistive
losses. Use the widest traces that space constraints allow. Because trace width tolerances
are expressed in absolute terms, a wider trace also minimizes the percentage variation of
the manufactured trace, resulting in tighter impedance control along the length of the
transmission line.
Striplines are preferred over microstrips because the reference planes on both sides of the
trace provide radiation shielding. Microstrips are shielded on only one side (by the
reference plane) because they run on the topmost or bottommost layers, leaving the other
side exposed to the environment. However, many products are shipped with a substantial
amount of microstrip routing used for MGTs, because differential signaling cancels out
much of the radiation.

Trace Characteristic Impedance Design

Because the RocketIO MGTs use differential signaling, the most useful trace configurations
are differential edge-coupled center stripline and differential microstrip. While some
backplanes use the differential broadside-coupled stripline configuration, it is not
recommended for multi-gigahertz operation, because the P and N vias are asymmetrical
and introduce differential-to-common-mode signal conversion.
With few exceptions, 50Ω characteristic impedance (Z
channel. In general, when the width/spacing (W/S) ratio is greater than 0.4 (8 mil wide
traces with 20 mil separation), coupling between the P and N signals affects the trace
impedance. In this case, the differential traces must be designed to have an odd mode
impedance (Z
Z
Virtex-4 RocketIO MGT User Guide
UG076 (v4.1) November 2, 2008
) of 50Ω, resulting in a differential impedance (Z
0O
= 2 x Z .
DIFF 0O
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) is used for transmission lines in the
0
) of 100Ω, because
DIFF
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