Digital Signal Processing Reference
In-Depth Information
For FR4 systems higher temperatures increase conductor resistance (and
therefore loss), while the temperature lowers loss tangent (and therefore di-
electric losses).
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8.7 Effects of Resin Content and Glass Weave on Delay Time and
Impedance
Researchers [8] have observed a variation in the dielectric constant of nearly 9%
occurring across a wide bus even when all its members are routed on the same layer.
This occurs because a trace running directly on top of a glass yarn bundle in
the laminate has a higher dielectric constant than one running along a resin filled
trough where there is no glass. Since the pitch of the glass yarn making up the mats
is typically between 16.7 and 23 mils (0.4-0.9 mm) [9], this effect is most likely to
be observed with wide (multitrace) buses, but it also can be significant for high-
speed serial signaling involving only two traces [10].
For instance, a 16-bit bus routed with 5-mil-wide traces and 5-mil space be-
tween traces will occupy 155 mils (0.4 cm) in width, which is several times the
pitch of the glass yarn. It is very likely that at least one of the traces will route over
a glass bundle while another in the same bus routes over an epoxy trough.
This effect can be ignored when timing skew between various signals does not
have to be tightly controlled. When signaling at high speeds, the two extremes are
addressed by creating two transmission line models: one when the trace is routed
over glass and the other when routed over epoxy.
8.8
Effects of Trace Shape
Figures 5.5 and 5.7 show that actual circuit board traces are trapezoidal rather than
perfectly rectangular in shape.
Because the surface area exposed to the return path will be less, a trapezoidal
trace will have lower capacitance and higher inductance than rectangular traces,
which increases the trace impedance. Trapezoidal traces also have less copper to
support current flow, which increases their resistance and the amount of conductor
loss.
Using a perfectly rectangular 5-mil-wide, 50
stripline as an example,
R
ac
in-
creases by some 22%, but the inductance only increases by about 4% and the
capacitance falls by about 2.5% when the top of the trace is reduced to 3.7 mils
(corresponding to a heavily overetched trace). Because the impedance changes as
the square root, this only results in the impedance increasing to 51
Ω
Ω
.
8.9 Main Points
Loss grows with frequency, causing a pulse to be distorted in shape and re-
duced in amplitude as each of its harmonics is attenuated differently.
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