User's Manual

HE910 Hardware User Guide
1vv0300925 Rev.28 – 2015-06-24
Reproduction forbidden without Telit Communications S.p.A. written authorization - All Rights Reserved page 46 of 46
6.6 PCB Guidelines in case of FCC
certification
In the case FCC certification is required for an application using HE910, HE910-D, HE910-
NAx, according to FCC KDB 996369 for modular approval requirements, the transmission line
has to be similar to that implemented on module’s interface board and described in the following
chapter.
6.6.1 Transmission line design
During the design of the interface board, the placement of components has been chosen properly,
in order to keep the line length as short as possible, thus leading to lowest power losses possible.
A Grounded Coplanar Waveguide (G-CPW) line has been chosen, since this kind of
transmission line ensures good impedance control and can be implemented in an outer PCB layer
as needed in this case. A SMA female connector has been used to feed the line.
The interface board is realized on a FR4, 4-layers PCB. Substrate material is characterized by
relative permittivity ε
r
= 4.6 ± 0.4 @ 1 GHz, TanD= 0.019 ÷ 0.026 @ 1 GHz.
A characteristic impedance of nearly 50 Ω is achieved using trace width = 1.1 mm, clearance
from coplanar ground plane = 0.3 mm each side. The line uses reference ground plane on layer
3, while copper is removed from layer 2 underneath the line. Height of trace above ground plane
is 1.335 mm. Calculated characteristic impedance is 51.6 Ω, estimated line loss is less than 0.1
dB. The line geometry is shown below: