RF Issues

[jkurina86]

• We are routing the Iridium directly from the module to the antenna, so it doesn't need any connectors on the board.
• The ublox EVA-M8 series hardware integration manual (pgs 20-21) states the following regarding the antenna routing:

• Add a ground plane underneath the GNSS module to reduce interference. This is especially important for the RF input line.
• For improved shielding, add as many vias as possible around the micro strip/coplanar waveguide, around the serial communication lines, underneath the GNSS module, etc.
• The micro strip / coplanar waveguide must be 50 Ω and be routed in a section of the PCB where minimal interference from noise sources can be expected. Make sure that there is only GND around and under the RF line.

These instructions were not followed and have resulted in degraded GPS performance. To resolve this we need to do the following:

• Use a copper-pour on the top layer and ensure that the ground copper layer is present beneath the RF trace to create a coplanar RF waveguide. Use via stitching on both sides of the waveguide.
• https://www.microwaves101.com/calculators/864-coplanar-waveguide-calculator use this to calculate the RF trace width and length to impedance match ZO=50 Ohms. Use ER=4.7 for FR-4 board. Substrate Thickness (H) is in mils and our boards are set to approximately 18.9 mils thick.

[noahlawrence-slavas]

Is the ground plane still needed, I was confused vs the last hallway conversation we had about this?

[jkurina86]

Yes. The Ublox datasheet mentions the ground plane beneath the trace being used in the RF calculations. Below is a cross-section illustration of a standard coplanar waveguide: