Printed Circuit Design & Fab - December 2007 - (Page 26)

RF DESIGN INTEGRATING RF into PCB Design In RF designs, it’s important to identify and correct circuit interactions as the system design evolves to reduce cycle time and prevent unnecessary re-spins. by PER VIKLUND Today, RF (radio frequency) circuitry is sneaking into more and more products. As this happens, more designers are faced with allowing RF circuitry to share board space with other technologies such as high-speed digital and analog circuits. The combination of RF and non-RF circuitry in close proximity can cause a lot of pain to design teams. Multiple design iterations and very long design times are common. Traditional design techniques have not been very helpful as they typically focus on RF alone and don’t consider the interactions of the complete system. In this article, we will take a practical look at modern RF design in the context of a complete system and see how careful planning and design can significantly improve the success rate, as well as shorten design times. RF design can be painful, especially so when seen in the context of system design. Why is that? In the past, RF was typically isolated modules – boards with solely RF content. Today, we combine all kinds of technology on the same dense board, and as dimensions are constantly shrinking, the risk of interaction between technologies, such as high-speed digital and RF, increases. Currently, this interaction cannot fully be avoided. We are typically not able to design out all interactions but are instead faced with designing a system with interaction but not more than the circuit can accept and still perform as intended. With this in mind, we can identify three areas of interest (each topic could fill a book or more): Design methodology. How to best design a mixed-technology product. Circuit interaction. How to manage the interaction between modules of different technologies. Verification. How to verify and validate a mixed-technology design. 26 Design Methodology Today’s methodologies go back over 10 years when RF design was a different ball game. In the classic flow, the RF designer is a contributor of technology that eventually will end up on a circuit board. The RF modules are designed in a dedicated RF design solution and, when ready, transferred to the system PCB layout using design translation. The benefit of this method is that the designer can focus on the RF by itself, not bothered by non-RF issues, and there are cases where this is a real advantage. The downside is that the RF engineer can’t see the impact he may cause or what will be affected by other circuits, and he stays as a contributor while the desired situation, in this case, is a team player role. For multi-technology designs, much can be gained in a methodology where all technologies can be co-designed in a manner where everyone is a team player rather than module contributors. By providing a collaborative environment, concurrent design teams can design their technology in parallel while being aware of what is being designed by other teams, thus eliminating some re-spins. Circuit Interaction Multi-technology design is about circuits living in harmony. You avoid being a noisy neighbor, and you don’t walk in your neighbor’s garden. The issue is that the circuits are becoming noisier. Plus, they are placed closer together than in the past … a trend that will continue no matter what. Hence, circuits do impact each other, and the challenge is to manage how and how much so the circuits still perform as specified. There are several areas impacting the interaction; the most obvious one is parts placement. Unfortunately, parts placement is not free since everything has to fit, and each technology has specific placement requirements. DECEMBER 2007 PRINTED CIRCUIT DESIGN & FAB

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Printed Circuit Design & Fab - December 2007 Contents Our Line Market Watch Around the World Happenings ROI 2007 PCB Designer Salary Survey Interconnect Strategies Positive Plating RF Design Ad Index Noise Reduction Supply Chain DfM Off the Shelf Marketplace The Signal Doctor

Printed Circuit Design & Fab - December 2007

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