Ublox SARA-N2 Series, Руководство по системной интеграции

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SARA-N2 / N3 series - System integration manual
UBX-17005143 - R13 Design-in Page 47 of 95
C1-Public
Guidelines for RF transmission line design
The transmission line from the ANT pad up to antenna connector or up to the internal antenna pad
must be designed so that the characteristic impedance is as close as possible to 50  .
The transmission line can be designed as a micro strip (consists of a conducting strip separated from
a ground plane by a dielectric material) or a strip line (consists of a flat strip of metal which is
sandwiched between two parallel ground planes within a dielectric material). The micro strip,
implemented as a coplanar waveguide, is the most common configuration for printed circuit board.
Figure 25 and Figure 26 provide two examples of proper 50  coplanar waveguide designs. The first
transmission line can be implemented in case of 4-layer PCB stack-up herein described, the second
transmission line can be implemented in case of 2-layer PCB stack-up herein described.
35
µ
m
35
µ
m
35
µ
m
35
µ
m
270
µ
m
270
µ
m
760
µ
m
L1 Copper
L3 Copper
L2 Copper
L4 Copper
FR-4 dielectric
FR-4 dielectric
FR-4 dielectric
380
µ
m 500
µ
m 500
µ
m
Figure 25: Example of 50

coplanar waveguide transmission line design for the described 4-layer board layup
35
µ
m
35
µ
m
1510
µ
m
L2 Copper
L1 Copper
FR-4 dielectric
1200
µ
m 400
µ
m 400
µ
m
Figure 26: Example of 50  coplanar waveguide transmission line design for the described 2-layer board layup
If the two examples do not match the application PCB layup, the 50  characteristic impedance
calculation can be made using the HFSS commercial finite element method solver for
electromagnetic structures from Ansys Corporation, or using freeware tools like Avago / Broadcom
AppCAD (
https://www.broadcom.com/appcad ), taking care of the approximation formulas used by
the tools for the impedance computation.
To achieve a 50  characteristic impedance, the width of the transmission line must be chosen
depending on:
•
the thickness of the transmission line itself (e.g. 35
µ
m in the example of Figure 25 / Figure 26 )
• the thickness of the dielectric material between the top layer (where the transmission line is
routed) and the inner closer layer implementing the ground plane (e.g. 270
µ
m in Figure 25 , 1510
µ
m
in Figure 26 )
• the dielectric constant of the dielectric material (e.g. dielectric constant of the FR-4 dielectric
material in
Figure 25 and Figure 26 )
•
the gap from the transmission line to the adjacent ground plane on the same layer of the
transmission line (e.g. 500
µ
m in Figure 25 , 400
µ
m in Figure 26 )