RF Circuits On-board Adds Design Complexity

There’s a virtual explosion of products that communicate with each other. Fueled by the Internet of Things, these products depend on integrating wireless technology with a myriad of components, and cramming increasingly diverse capabilities, technologies, and requirements into an amazingly limited space.

RF circuits have moved from large mil-aero applications into a broad and rapidly growing range of commercial products in medical, communications, and industrial settings, morphing from desktop environments into small, low-power, handheld devices.

More cramped quarters mean PCB layouts with RF/microwave circuits on-board are substantially more complex. Layout designers thus face many pitfalls, especially when implementing high-frequency 500 MHz to 2GHz RF and 2 GHz microwave frequency. While typical analog signals have a small range between DC and a few hundred MHz, high-frequency RF signals can literally be at any voltage and current within the minimum and maximum stated range.

RF/microwave signals are highly sensitive to the impact of noise, crosstalk, and power. Impedance matching is critical for RF and the tolerance exhibited by digital signals often just isn’t there. RF components require controlled impedance-transmission lines to transport RF power to and from IC pins. These lines can be implemented on an exterior layer of the PCB or internally.

As a result, designers must protect against accidental or unintended coupling between signal lines. For example, RF-transmission lines should be kept far apart, and at the very least, not be positioned close to one another for any extended distances. Lines crossing on separate layers should have a ground plane to add distance. Signal lines carrying high power levels should also be kept away from all other lines. High-speed digital signal lines should be on a different layer than RF and routed separately to prevent digital noise from coupling onto RF lines.

VCC/power lines should also be separate from any RF lines that will transmit large amounts of RF power. Ground vias between layers placed throughout the board’s RF segment can prevent parasitic ground inductance due to ground-current return paths, preventing cross coupling from RF and other signal lines across the PCB. Adjusting for many differences in mixed-signal design also involves shape issues. RF circuits, for example, have symmetric structures; however, many irregular and large shapes are used for RF layout, making layout even more challenging.

There are many considerations when integrating RF onto PCBs, including tight-component densities, multiple-surface-finish possibilities, board-thickness considerations, wide ranges of reliability requirements, and even substrate-material choices. In addition, RF/microwave designs can require special manufacturing equipment. On top of the increased challenges, however, ever-lower cost, greater throughput, fewer processes, and higher yields are demanded.

Combining RF technologies on one device involves adding the capability while keeping the performance high and the price tag low. Unfortunately, combining such RF technologies as cellular, Bluetooth, GPS, FM, and WLAN into one device drives up both time spent and cost, while demanding greater design and layout expertise to ensure that the end product not only works, but excels at the expected performance, reliability, and cost.

 
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