Agilent Technologies 93000 SOC Series Training Manual page 537

N OT E
b
h
Characteristic Impedance:
Zo =
Strip Line Model
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:
Sharp Corner
Signal Input
Sharp Corner
In the above example, if the signal frequency is very high, the sharp corner
causes signal reflections. Use a blunt corner or rounded corner instead of
the sharp corner.
Lesson 2 – DUT Board Design Considerations
w
t
60 4*b
ln
π (
0.8*w + t )
0.67
Er
Blunt Corners
Rounded Corner
Ground Plane
Strip Line
Ground Plane
537