LMH2100 Datasheet, PDF (41/49 Pages) National Semiconductor (TI) – 50 MHz to 4 GHz 40 dB Logarithmic Power Detector for CDMA and WCDMA

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Layout Guidelines (continued)
METAL CONDUCTOR
W
FR4 PCB
LMH2100
SNWS020C – NOVEMBER 2007 – REVISED OCTOBER 2015
H
GROUND PLANE
Figure 89. Microstrip Configuration
A conductor (trace) is placed on the topside of a PCB. The bottom side of the PCB has a fully copper ground
plane. The characteristic impedance of the microstrip transmission line is a function of the width W, height H, and
the dielectric constant ε_r.
Characteristics such as height and the dielectric constant of the board have significant impact on transmission
line dimensions. A 50-Ω transmission line may result in impractically wide traces. A typical 1.6-mm thick FR4
board results in a trace width of 2.9 mm, for instance. This is impractical for the LMH2100 because the pad width
of the 6-Bump DSBGA package is 0.24 mm. The transmission line has to be tapered from 2.9 mm to 0.24 mm.
Significant reflections and resonances in the frequency transfer function of the board may occur due to this
tapering.
10.1.4 GCPW Configuration
A transmission line in a (grounded) coplanar waveguide (GCPW) configuration will give more flexibility in terms of
trace width. The GCPW configuration is constructed with a conductor surrounded by ground at a certain
distance, S, on the top side. Figure 90 shows a cross section of this configuration. The bottom side of the PCB is
a ground plane. The ground planes on both sides of the PCB should be firmly connected to each other by
multiple vias. The characteristic impedance of the transmission line is mainly determined by the width W and the
distance S. In order to minimize reflections, the width W of the center trace should match the size of the package
pad. The required value for the characteristic impedance can subsequently be realized by selection of the proper
gap width S.
METAL CONDUCTOR
S
W
S
FR4 PCB
H
GROUND PLANE
Figure 90. GCPW Configuration
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