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4. RF Circuit Routing Constraints
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 1. 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 figures
below show generic examples of good routing techniques.
Figure 2. RF Routing Example with Lead Type RF Connectors
4112719
Rev 2.1
January 07, 2013
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