Issue link: https://iconnect007.uberflip.com/i/1534120
24 DESIGN007 MAGAZINE I APRIL 2025 Unreinforced materials may be needed at very high frequencies, but a lack of experience handling these materials increases the risk of misregistration. Some manufacturers handle unreinforced PTFE well, but others trying to make a sale will push for glass-reinforced PTFE, which may not be acceptable in some RF designs. In some of your video presentations on RF design, you stress the importance of floor planning. Walk us through that. Floor planning refers to pre-planning place- ment and routing. I've always approached RF designs by first tracking where I will place the RF components and where I'll route the RF signals or printed circuits, and then I figure out the rest. is is a good approach because you can quickly find a spot for the most impor- tant RF elements while accounting for other design constraints, e.g., connector placement. Because most of the designs I've worked on include some type of digital interface, I can put the digital portion in its own area and deter- mine whether additional layers will be needed for the digital section. Once you have the placement for the RF ele- ments and the routing path roughly determined, you can allocate some real estate to power and digital routing. e approach is essentially the same as in other mixed-signal designs at lower frequencies: Try not to route the digital stuff next to the analog stuff and try to control loop currents with your ground planes. What material considerations should design- ers and engineers keep in mind with RF designs? When are alternative materials a good option? ere is a decent amount of misconception about RF materials, specifically PTFE, and when they are needed. If you look at most peo- ple pretending to know about RF design, they always say to use a low-Dk PTFE laminate. If you're operating below a few GHz, in most designs you don't have to worry about using an advanced low-loss material. You are unlikely to notice a difference from standard FR-4. In the seminars I give on RF design, I make sure attendees understand that high-Dk mate- rials are a viable option. I always like to show an example of a high-Dk laminate and explain why we would use it. For example, certain PTFE laminates have Dk = 10 with low loss tangent. If you use this material, you'll still have 75% lower dielectric loss than in standard FR-4 and can reduce circuit size by about 40%. What recommendations would you give regarding RF stackup design? As I mentioned, not all RF designs need to be built on low-Dk PTFE materials. Standard FR-4 stackups will work for lower frequen- cies as long as the layout and routing are cor- rect. If you're planning to do a standard type of stackup with RF-specific materials like PTFE, it never hurts to send it to your fabricator and make sure they can produce it. When you start looking at some RF sys- tems you will see some interesting stack- ups. For example, RF designs are sometimes hybrid or have asymmetric stackups. You are not required to follow the typical approach as with multilayer FR-4 PCBs. It is okay to use Zachariah Peterson