RF Board Designers Confront Many Challenges

From data formats to signal interference, designers must support multiple-RF designs to stay competitive.

By John Blyler

No one doubts that the global market for all things wireless will continue to grow. Wireless connectivity in both voice and data-based systems is spanning the globe. Data-intensive wireless systems range from personal-area-network (PAN) technologies based on Bluetooth and Ultra Wideband (UWB) protocols to longer-range designs like the IEEE 802.11 wireless-local-area-network (WLAN) family and WiMAX systems. Because of these trends, analog, radio-frequency (RF), and wireless design challenges have moved to the forefront of both chip and board-level design concerns.

An example of one company that is poised to address both the chip and board-level issues facing analog-RF-wireless engineers is Cadence Design Systems (www.cadence.com). Recently, I had the opportunity to interview experts in both Cadence's North America and China headquarters. Their insights reflect the universal problems faced by RF designers.

The problem in designing board-level RF systems is twofold. The first challenge involves implementing the RF circuit with the existing—often "clock" noisy—digital design onto an already crowded printed-circuit board (PCB). The second obstacle deals with the signal-interference issues that are common to multi-RF front-end systems. Let's start with the board-level challenge.

RF Board-Level Challenges

Whether an RF circuit is being realized on a silicon chip or PCB, the first step is to model, simulate, and design the circuit. A host of specialized software tools are available to help engineers create the necessary design. But once these circuits are designed, how are they implemented on a board? To answer this question, one must first understand that two distinct design data formats are involved: one for the board and a different one for the RF circuit. As stated by Josh Moore, Senior Product Manager for PCB Design Tools at Cadence Design Systems, "The problem is one of data interoperability—i.e., getting information from the design and analysis tools into the tools that can actually create the physical circuit board."

Most board designers are familiar with PCB data formats like Gerber or GenCAD. In what format, however, are specific RF circuits captured? Most data comes out of the RF simulation and design tools in a format known as the Intermediate File Format or IFF (see the Figure). This format permits the bi-directional flow of data from schematic and layout design tools. It contains all of the RF front-end logic information for a particular circuit as well as the back-end RF physical elements. Board-level CAD tools use this information to properly place the circuit onto an RF-only or mixed-domain (RF and digital) board.

Yet this isn't as straightforward as it seems, notes Moore. "In the past, these tools lacked a highly integrative solution. For example, imagine that a mounting hole was mistakenly added right in the middle of your transmitter circuit. To fix this mistake on the board, you had to move the antenna, which could easily change the RF electrical characteristics of the circuit. Such a change would mean the circuit must be re-simulated." But there was no easy way to get these changes back into the simulators aside from verbally explaining to the RF engineer what specific changes were needed from the board designer's perspective. The RF engineer would then have to recreate the changes and redo the simulation—hopefully with favorable results.

In the past, board-level computer-aided-design (CAD) tools have looked upon RF designs as black boxes. Although the IFF data explained what the RF designer intended, CAD tools typically didn't understand the RF elements. For the CAD designer, Moore explains, this RF "black box" was little more than physical elements like copper construction pieces that could be a duct or exploiter. "It could be whatever RF element the circuit is trying to make happen." The CAD designer would just accept whatever data came from the tool—not really knowing which RF functions were to be performed by the various collections of copper elements.

RF Circuit Issues

We've highlighted some of the problems associated with importing RF design data into the established PCB CAD tools. But how do engineers know that the board-level implementation will work as well as it does on the simulators? Mr. Xiao, Product Engineer for Cadence in Shanghai, China, notes that the biggest challenges are electromagnetic interference (EMI) and electromagnetic compatibility (EMC). These challenges arise because most RF designs will go into consumer products (mobile phones, wireless networks, etc.) that must meet FCC and European standards. Designers must therefore be able to identify, locate, assess, and resolve EMI and EMC issues before the product can be manufactured.

By their very nature, EMI and EMC problems are often very difficult to trace to a single cause. The inability to define the complete context of the EMI-EMC problem makes it very hard to simulate. Xiao agrees, noting that EMI-EMC challenges are really system-level issues that involve such factors as mechanical structure, functionality, layout, power disturbance, heatsinking, shielding, and spectral planning.

Another design challenge involves the growing need to support multiple RF subsystem protocols like Bluetooth, Wi-Fi, and others. Incorporating multiple RF front-end designs into consumer products is a popular way to remove the interconnect wire tangle. This approach also works to add more features, shrink existing form factors, reduce overall weight, consume less power, and lower system costs. Unfortunately, multiple RF front ends present a number of tradeoffs. Coupling the board's signal paths can degrade system performance while separating them can result in higher cost.

Xiao observes that still another issue faces RF designers: the lack of a proper design environment and tools. Organizational structure and tool provisioning haven't evolved in order to facilitate the design trend to combine RF and digital features on the same board. Often, the RF design team and digital design team continue to be two separate groups—each having its own set of equipment.

On the process side of the development issue, analog-RF-wireless designs have never scaled into smaller silicon sizes as easily as digital designs. Technological advances, such as direct downconversion or zero intermediate frequency (IF), have helped to reduce the size of RF front ends. Today, traditional superheterodynes and zero-IF architectures co-exist on the same board—albeit for different applications. Superheterodyne architectures are more widely used in classic designs. In contrast, zero-IF approaches offer the advantage of reduced external parts count, which saves board space and enables smaller-form-factor chips. Zero-IF technology is now widely accepted. Xiao believes it will be the future direction for all RF front-end applications.

What can be done to address all of these challenges—starting with the data format interoperability? More and more software vendors are providing better bi-directional mechanisms to exchange data between simulation, design, and testing tools. Some of these tools even provide reuse functionality.

While RF and digital designers continue to operate in separate camps, both know that they must co-exist in order to survive in the marketplace. Mixed-domain technology advancements, which require the integration of both analog and digital signals onto a single board, help to encourage a healthy respect for each other's point of view.

As they become more knowledgeable of the inherent challenges that the other faces (i.e., RF designers with electromagnetic field issues and digital designers with electrical circuit issues), the designers should become more tolerant and accepting of the other's position.

Another factor that encourages this sharing of perspectives has resulted from college courses, which integrate analog and digital disciplines in the same curriculums. Companies have followed this example by cross-training their engineers.

Yet the best motivator for the closer integration of RF and digital designs—and design teams—is probably the growing global market for wireless products. But meeting shrinking board sizes and time-to-market demands means that RF circuit and board designs must have an accurate and efficient bi-directional data flow. Fortunately, this seems to be a high priority for both chip- and board-level EDA companies.

Sample list of tools:
(1) Board design-layout tools: Cadence's Allegro RF PCB platform including Board Station, ADS, and HFSS for RF design and simulation (www.cadence.com)
(2) Simulation and design tools: Microwave Designer from Applied Wave Research (www.appwave.com)
(3) Testing tools: BTG's Bird Diagnostic System (BDS); Agilent ADS (www.agilent.com)