Ublox LEA-M8F, Hardware Integration Manual

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LEA-M8F - Hardware Integration Manual
UBX-14000034 - R03 Early Production
Information
Design
Page 14 of 30
As seen in Figure 7 , an isolated ground area is created around and below the RF connection. This part of the
circuit MUST be kept as far from potential noise sources as possible. Make certain that no signal lines cross, and
that no signal trace vias appear at the PCB surface within the area of the red rectangle. The ground plane should
also be free of digital supply return currents in this area. On a multi layer board, the whole layer stack below the
RF connection should be kept free of digital lines. This is because even solid ground planes provide only limited
isolation.
The impedance of the antenna connection has to match the 50
Ω
impedance of the receiver. To achieve an
impedance of 50
Ω
, the width W of the micro strip has to be chosen depending on the dielectric thickness H,
the dielectric constant
ε
r
of the dielectric material of the PCB, and on the build-up of the PCB (see section 2.1.3).
Figure 8 shows two different builds: A 2-layer PCB and a 4-layer PCB. The reference ground plane is on layer 2 in
both designs. Therefore, the effective thickness of the dielectric is different.
Module
micro strip line
Ground plane
Module
micro strip line
Ground plane
PCB
PCB
Either don't use these layers or fill with ground planes
H
H
Figure 8: PCB build-up for micro strip line. Left: 2-layer PCB, right: 4-layer PCB
General design recommendations:
•
The length of the micro strip line should be kept as short as possible. Lengths over 2.5 cm (1 inch) should be
avoided on standard PCB material and without additional shielding.
•
For multi-layer boards, the distance between micro strip line and ground area on the top layer should at
least be as large as the dielectric thickness.
•
Routing the RF connection close to digital sections of the design should be avoided.
•
To reduce signal reflections, sharp angles in the routing of the micro strip line should be avoided. Chamfers
or fillets are preferred for rectangular routing; 45-degree routing is preferred over Manhattan style
90-degree routing.
A
nte
nn
a
A
nte
nn
a
A
nte
nn
a
PCB PCB PCB
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Wrong better best
Figure 9: Recommended micro strip routing to RF pin (for exact pin orientation see LEA-M8F Data Sheet [1])
•
Do not route the RF-connection underneath the receiver. The distance of the micro strip line to the ground
plane on the bottom side of the receiver is very small (some 100 µm) and has huge tolerances (up to 100%).
Therefore, the impedance of this part of the trace cannot be controlled.
•
Use as many vias as possible to connect the ground planes.