When designing an RF system that operates at a high frequency such as the 5 GHz band, it is necessary to take parasitic effects into consideration. It is important to understand from where the parasitic effects are coming. When designing the PCB layout, you must consider landing pad size in order to reduce junction effects, as well as parasitic capacitances which could exist at your operating frequency. Here are five PCB design tips to help you work around parasitic capacitances.
First you can remove the inner ground referenced copper layer. Removing this copper increases the distance to the reference ground which reduces the capacitance value.
The second option is to reduce stub junctions as much as possible. Stubs can cause problems with signal integrity when operating signal frequencies rise above approximately 100 MHz.
Third, reduce any abrupt changes in your trace width. A 90 degree bend has an adverse effect on an RF signal. Smooth transitions and curves are best. Think of the trace as a water pipe. If the pipe bends at a 90 degree angle, then the water does not flow through smoothly.
Fourth, reduce the junction on the matching circuit.
Lastly Make sure the return ground path is clean. For RF signals, the ground return path is under/ follows the RF signal trace. Sometimes when designing layouts, engineers do not consider signal path and forget that the signal returns through the ground. Ground continuity and ensuring the ground path is clean is critical for a successful design.
The ground plane must not be broken or interrupted during transmission line routing. Ground vias are necessary to ensure that the RF trace has proper ground reference. Ground vias prevent accrual of parasitic ground inductance caused by ground-current return paths. They also help to prevent cross-coupling between RF and other signal lines across the PCB. It is also important to consider bias and ground layers. The layers assigned to the system bias (DC supply) and ground must be considered in terms of the return current for the components. The general rule of thumb is to not have signal routed on layers between the bias layer and the ground layer. Ground is the key to signal integrity.
One last helpful tip. Remember the three S's, which are key to reducing parasitic effects in PCB designs. Keep your designs as short, straight, and smooth as possible.
This blog post was originally published in Laird's "RF Design Considerations" white paper. For best practices on how to select and implement an antenna, check out LSR's webinar "Designing for Success: The Engineer's Approach to Antenna Selection and Implementation".