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7. Routing Constraints and
Recommendations
Layout and routing of the AirPrime AR7558 device in the application is critical to maintaining the
performance of the radio. The following sections provide guidance to the developer when designing
their application to include an AirPrime AR7558 device and achieve optimal system performance.
7.1.
RF Routing Recommendations
To route the RF antenna signals, the following recommendations must be observed for PCB layout:
The RF signals must be routed using traces with a 50 characteristic impedance.
Basically, the characteristic impedance depends on the dielectric constant (εr) of the material used,
trace width (W), trace thickness (T), and height (H) between the trace and the reference ground plane.
In order to respect this constraint, Sierra Wireless recommends that a MicroStrip structure be used
and trace width be computed with a simulation tool (such as AppCAD, shown in the figure below and
available free of charge at http://www.avagotech.com).
Figure 30.
AppCAD Screenshot for Microstrip Design Power Mode Diagram
The trace width should be wide enough to maintain reasonable insertion loss and manufacturing
reliability. Cutting out inner layers of ground under the trace will increase the effective substrate height;
therefore, increasing the width of the RF trace.
Caution:
It is critical that no other signals (digital, analog, or supply) cross under the RF path. The figure
below shows a generic example of good and poor routing techniques.
4116922
Rev 1.0
January 21, 2015
52
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