Whether it’s a fancy new gaming PC, an HDTV or a 4G phone, all of them make use of printed circuit boards. A technological innovation dating back to the 1940s, the circuit board is part of what makes all modern electronics possible, and radio frequency products are no exception. Following best practices when designing a circuit board is one the first steps to building an RF system that performs well. This article will offer some basic tips for good RF board design and list some common mistakes that RF engineers should avoid.
The way that power planes are laid out across a circuit board is very important. One mistake that can cause a lot of problems is to design your RF boards with the power planes around the edges of the card. This can have the undesirable effect of causing a lot of parasitic radiation. It’s better to run them between two ground planes to decouple the ground and supply.
Layers, Lines, and Traces
One of the first decisions you’ll need to make when designing an RF board is the number of layers to you want to use. Single layer circuit boards simply won’t work for RF board design, since the traces require a good ground reference, which requires a coplanar stripline. A 2-layer RF board design is workable for some applications, but the space for DC power is limited. Generally, 4 layers is optimal for most RF board designs, as they facilitate distributed RF decoupling of a DC power plane, with 2 layers of mostly ground plane separating them. 4 layers allow for easier use of microstrip and stripline transmission lines. (If using a 2-layer RF board design, make sure that the total board thickness is no more than 30mils, otherwise the line trace will need to be a little too wide for this size.)
Circuit Loops and Decoupling
The location for decoupling components on an RF circuit board is very important. You want to minimize captive coupling as much as you can. Put decoupling caps to ground near the pin to reduce current loops as much as possible. You should make sure that every port or pin is decoupled to ground through its own dedicated via.
Parasitics and Thermal Relief
Circuit board parasitics must also be taken into account with RF board design. The term “parasitics” applies to any physical trait of the circuit board that can influence how well it performs. The higher the radio frequencies you’re working with, the more damaging parasitic effects can be.
There are several different design techniques you can use for dealing with parasitic effects. One of these is to avoid making hard, sharp changes (think right angles) in your trace width, as this can negatively impact the RF signal. Imagine a pipe transporting water or some other liquid. If the pipe took a hard 90-degree turn, this would slow down the flow of the liquid, since it has to abruptly change direction. The same thing happens with an RF signal when the board has traces with right angle turns. It’s much better to go with smoother, more curved and gradual turns in the traces.
For thermal relief, making use of thermal relief pads can be helpful. It’s best to connect the pad to component side ground connections. These should be connected the primary ground plane layer by multiple parallel thermal vias.
Related article: 18 CAD Tools Every RF and Microwave Design Engineer Should Consider
Circuit board design is complex and important work, and we’ve only just scratched the surface. For quick visual reference guide of Dos and Don’ts for designing RF boards, check out our free Common Guidelines for RF Board Design infographic.