MAX-7 / NEO-7 - Hardware Integration Manual
UBX-13003704
-
R09
Production Information
Design
Page
21 of 52
Figure 12: Recommended layout
As seen in Figure 12
an isolated ground area exists 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 must match the 50
impedance of the receiver. To achieve an
impedance of 50
, the width W of the micro strip has to be chosen depending on the dielectric thickness H,
the dielectric constant
r
of the dielectric material of the PCB and on the build-up of the PCB (see
section 3.3.5
Figure 13 shows two different builds: A 2 Layer PCB and a 4 Layer PCB. The reference ground plane is in both
designs on layer 2 (red). 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 13: PCB build-up for micro strip line. Left: 2-layer PCB, right: 4-layer PCB
3.3.4
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.
