
5 DUT Board Performance Considerations DUT Board Design for Mixed-Signal Tests
130 System Reference, January 2001
where
Er
means the relative dielectric constant of the
board material.
The following figure shows the model of a strip line:
Figure 52 Strip Line Model
NOTE As shown in the above figures, the microstrip line and
strip line consist of a set of a trace and ground or power
plane(s). Thus, the ground or power plane(s) must not
have any discontinuity under the trace.
If the number of PCB layers and the board material are
determined, that is, Er, h, b, and t are determined, the only
trace width, w, determines the characteristic impedance
(Zo) of the transmission line.
Therefore, to design a trace correctly to be a specified
impedance, pay attention to the width of the trace on the
PCB. However, even if the width of a trace is exactly
designed to match the termination resistor value, the
following factors cause impedance mismatching:
Strip Line
w
h
t
Ground Plane
b
Ground Plane
Characteristic Impedance:
Zo =
Er
60
0.67 0.8*w + t
4*b
ln
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