Ublox LENA-R8 Series, Руководство по системной интеграции

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LENA-R8 series - System integration manual
UBX-22015376 - R02 Design-in Page 52 of 116
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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 examples of the Figure 33 and the
Figure 34 )
• 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 33 ,
1510
µ
m in Figure 34 )
• the dielectric constant of the dielectric material (e.g. dielectric constant of the FR-4 dielectric
material in
Figure 33 and Figure 34 )
• 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 33 and Figure 34 )
If the distance between the transmission line and the adjacent GND area (on the same layer) does not
exceed 5 times the track width of the micro strip, use the “Coplanar Waveguide” model for the 50 
calculation.
Additionally, to the 50  impedance, the following guidelines are recommended for the transmission
line design:
• Minimize the transmission line length: the insertion loss should be minimized as much as possible,
in the order of a few tenths of a dB.
• Add GND keep-out (i.e. clearance, a void area) on buried metal layers below any pad of component
present on the RF transmission line, if top-layer to buried layer dielectric thickness is below
200
µ
m, to reduce parasitic capacitance to ground.
• The transmission line width and spacing to GND must be uniform and routed as smoothly as
possible: avoid abrupt changes of width and spacing to GND.
• Add GND vias around transmission line, as described in Figure 35 .
• Ensure solid metal connection of the adjacent metal layer on the PCB stack-up to the main ground
layer, providing enough on the adjacent metal layer, as described in
Figure 35 .
• Route RF transmission lines far from any noise source (as switching supplies and digital lines) and
from any sensitive circuit (as analog audio lines).
• Avoid stubs on the transmission line.
• Avoid signal routing in parallel to the transmission line or crossing the transmission line on buried
metal layer.
• Do not route the microstrip line below discrete components or other mechanics placed on the top
layer.
Two examples of a suitable RF circuit design for ANT pin are illustrated in Figure 35 , where the cellular
antenna detection circuit is not implemented (if the cellular antenna detection function is required by
the application, follow the guidelines for circuit and layout implementation detailed in section
2.4.5 ):
• In the first example shown on the left, the ANT pin is directly connected to an SMA connector by
means of a suitable 50

transmission line, designed with the appropriate layout.
• In the second example shown on the right, the ANT pin is connected to an SMA connector by means
of a suitable 50  transmission line, designed with the appropriate layout, with an additional high
pass filter to improve the ESD immunity at the antenna port. (The filter consists of a suitable
series capacitor and shunt inductor, for example the Murata GRM1555C1H150JB01 15 pF
capacitor and the Murata LQG15HN39NJ02 39 nH inductor with SRF ~1 GHz.).