User's Guide

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104 105
Pad Layout
The pad layout diagrams below are designed to facilitate both hand and
automated assembly. Figure 112 shows the footprint for the smaller version
and Figure 113 shows the footprint for the pre-certified version.
0.420"
0.015"
0.028"
0.050"
0.060"
0.070"
0.015"
0.136"
0.100"
0.101"
0.060"
0.065"
0.090"
0.015"
0.420"
0.015"
0.028"
0.050"
0.520"
0.070"
0.015"
Figure 112: HUM-***-PRO Recommended PCB Layout
Figure 113: HUM-***-PRO-UFL/CAS Recommended PCB Layout
Microstrip Details
A transmission line is a medium whereby RF energy is transferred from
one place to another with minimal loss. This is a critical factor, especially in
high-frequency products like Linx RF modules, because the trace leading
to the module’s antenna can effectively contribute to the length of the
antenna, changing its resonant bandwidth. In order to minimize loss and
detuning, some form of transmission line between the antenna and the
module should be used unless the antenna can be placed very close (<
1
⁄8in)
to the module. One common form of transmission line is a coax cable and
another is the microstrip. This term refers to a PCB trace running over a
ground plane that is designed to serve as a transmission line between the
module and the antenna. The width is based on the desired characteristic
impedance of the line, the thickness of the PCB and the dielectric constant
of the board material. For standard 0.062in thick FR-4 board material, the
trace width would be 111 mils. The correct trace width can be calculated
for other widths and materials using the information in Figure 114 and
examples are provided in Figure 115. Software for calculating microstrip
lines is also available on the Linx website.
Trace
Board
Ground plane
Figure 114: Microstrip Formulas
Example Microstrip Calculations
Dielectric Constant
Width / Height
Ratio (W / d)
Effective Dielectric
Constant
Characteristic
Impedance (Ω)
4.80 1.8 3.59 50.0
4.00 2.0 3.07 51.0
2.55 3.0 2.12 48.8
Figure 115: Example Microstrip Calculations