Question

How should I route the traces on more-layer RF designs for optimal performance?

Answer

In order to achieve the possible best RF radiated performance the followings are suggested for more-layer RF board designs:

- Top Layer: Components and short traces. Top layer should use as large and continuous GND plane metallization as possible (with many stitching GND vias) on the entire PCB.

- 1st inner layer: GND plane and traces if necessary. The most important rule is to keep the GND pour metallization unbroken beneath the RF areas (between the antenna, matching network and RF chip). Traces can be routed under the non-RF areas and use GND pour where possible.

- 2nd, 3rd... inner layers: Traces. VDD and all other traces are suggested to be routed on these layers. Use GND pour where possible.   

- Bottom Layer: GND plane. Use as large and continuous GND plane as possible. Do not route traces on this layer, just if it is necessary, e.g. short connection traces to connectors.

- Generic for each layer: Try to avoid routing traces along or close to the board edges. It is recommended to place ground stitching vias with GND pour along the PCB edges. 

Question

What are the general layout suggestions for RF board design?

Answer

Here are some layout hints that are suggested to follow for nay kind of RF board designs.

- use as large and continuous GND metallization as possible on all layers

- use plenty of GND stitching vias on the enrite PCB, especially at the board and GND metal pouring edges.

- do not route traces at the board edges

- place the high frequency crystal close to the RF chip

- connect the crystal case to GND

- use short VDD traces

- if possible, shield the VDD traces by ground metallization (with vias) from both sides

- put the matching network as close to the RF chip as possible to minimize the parasitics in the RF-FE match and avoid de-tuning

- make sure that there are many GND vias on the RF chip's exposed pad

- further recommendations are in the layout-related application notes.

More layout suggestions can be found in application notes: AN629, AN928, for instance.