The presence of RF circuitry on printed circuit boards (PCBs) was once limited to military and aerospace industry requirements. Now, the proliferation of the wireless handheld communications and remote-control devices is driving the need for mixed analog, digital and RF designs at a significantly increasing rate. Handhelds, base stations, remote controls, Bluetooth devices, computer wireless, many consumer devices, and mil/aero systems now all contain RF.

For years, RF design has been a special art, requiring specialized design and analysis tools, used by specialized designers. Typically, the RF portion of a PCB was designed by that specialist in a separate environment and then merged into the rest of the mixed-technology PCB. This process was highly inefficient, often required iterations to marry the mixed technologies together and resulted in multiple, unrelated databases representing the final product.

In the past, design functionality was performed (and repeated) in two design environments through non-intelligent ASCII interfaces (Figure 1). Both the PCB system design and the RF specialized design systems had their own libraries, RF design databases, and design archiving. It required that design data (schematic and layout) and libraries be managed (and synchronized) in both environments through the cumbersome ASCII interfaces.

With this old methodology, the RF designer was developing the RF circuitry isolated from the rest of the PCB system design. The RF portion was then translated into the PCB design using ASCII files to create schematic and physical implementation on the host PCB. If problems exist with the RF circuitry, the design must be corrected in the stand-alone RF solution and re-translated into the host PCB. The result was a total replacement versus an incremental change.

RF simulators only simulate the ideal RF circuit. The actual implementation in the mixed system with fractioned ground planes, ground vias and neighboring RF circuitry has been extremely difficult to analyze and it's well known that these additional shapes will have a profound impact on the RF circuit operation.

This old methodology has been used successfully for years to design mixed-technology boards, but as the RF content in products increases, the problems with having two separate design systems is starting to impact designer productivity, time-to-market, and quality of the products.

With these issues in mind, Mentor Graphics has developed a dynamic link that integrates the PCB schematic and layout tool with RF design and simulation tools, resulting in a solution that overcomes the shortcomings of the classic RF design. Working with RF design experts, a set of requirements was identified and a new solution designed.

No integration — no matter how good — can help maintain design intent between PCB design and RF design unless there is a common understanding of the technology-specific environment between the tool sets. In other words, the typical layer-oriented structures in PCB layout has to be understood by the RF design tools and the parametric planar microwave elements used in the RF design environment must be understood by the PCB system.

Another key issue is that PCB systems regard RF shapes as short circuits and this prevents proper design rule checks (DRC) of the design. With today's complex RF system designs, functional RF aware DRC is a must to enable a correct by design methodology.

All these contribute to the design intent. Preserved design intent is critical as this is the foundation to support multiple iterative roundtrips of design data between tools without losing information.

RF design is an iterative process. A design is tweaked or optimized in many steps. It was difficult in the past to do this in the context of the real PCB design. When the optimized RF module was implemented on the PCB, there was no guarantee that it would still work in an optimal manner. As a validation, the PCB implementation was sent to electromagnetic field analysis (EM).

This design flow has several problems. First, the circuit is pushed to simulation as simple metal polygons, so there is no way to modify the metal in the RF tool and send the optimized result back to PCB design and still have an intelligent RF circuit. Second, EM solutions are time consuming so it may be best to wait until it's needed.

In the new flow, as the PCB and RF tools share an understanding of the design intent, the circuit can be looped back and forth between the tool sets multiple times without loss of design intent. This means that circuit simulation (which is very fast) and EM analysis (when needed) can be repeated and results can be compared for every change made to the circuit. This is done within the context of the real PCB with fractioned ground planes, RF shapes, traces, vias, and other components.

Libraries have always been a hurdle in RF system design. The standard components in the RF library (capacitors, resistors, transistors, etc.) frequently lack some of the parameters required for the PCB design and manufacturing processes. Likewise, the PCB design libraries usually don't contain the planar microwave elements used in the RF domain to build up RF circuitry.

In the past, a snapshot has been taken of the microwave element library, but as with any snapshot, it could be outdated in no time, forcing designers to manually ascertain that the PCB and microwave library is kept in absolute synchronization. And not just synchronized, but perfectly synchronized to ensure performance on the PCB is 100% identical to what you simulate. Obviously, as this process involved people, it failed now and then. The new integration solves this dilemma using an inter-tool dynamic link to synchronize the library.

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