An integrated flow compresses the traditional simulation-to-design cycle.
You can’t build a house without blueprints. So why do so many RF design flows try to build a board without a schematic? Often, this crude process came about due to lack of availability of RF design tools. But the wireless industry is the new cool kid on the block. As a result, EDA software vendors are scrambling to disencumber the rigid PCB world of tailored processes and streamlined user flows. RF engineers demand freedom from constraints, so EDA vendors are peeling back the layers of traditional PCB design and opening some excellent solutions. But with these solutions, RF engineers are challenged to rethink their request for primitive simplicity and consider a higher level process.
To successfully examine the RF design challenge, it is important to first examine a “traditional” RF design flow. The term “traditional” is meant to convey the flow is a common solution for many cutting-edge companies that had no software tools to help them. It is fair to say many of these design flows were established years ago, before software tools gained strength in the RF specialty.
This traditional RF design process is most easily identified in its use of DXF file transfer to bridge the RF and PCB design gap (Figure 1). But there are real problems with using primitive shapes in an advanced PCB design software package. Designers often have to “trick” the software system by copying the primitive objects into component geometries, or by trying to manipulate the shapes as area fill objects.
In addition, the primitive shapes cannot be easily modified. And if they are modified, design rule checks (DRCs) can’t be fully exercised, introducing instability into the design. In short, designers are left trying to use software methods never intended for RF design.
Even for the many RF design companies that have navigated their way through these issues, one glaring problem remains: There is no schematic. This means there is next to no control over the connectivity, and the presence of a schematic for the PCB portion of a mixed RF-digital-analog design becomes almost useless because the schematic and layout cannot be synchronized without dangerous workarounds. Reverse-engineering solutions can help these issues, but even then, the layout-driven design process can be messy and time-consuming. In the end, designs are difficult to reuse and only partially archived.
An Integrated RF Design Flow
As RF designs have gained a larger share, RF engineers and EDA software vendors have teamed to work out a better design process. EDA vendors have taken on more of a consulting role, where the needs of the RF community are heard, and then steps are taken to try to ease their pain points in the design process. Most vendors now are gravitating toward a fully integrated solution with bidirectional data transfer between the RF simulation applications and schematic and layout tools (Figure 2).
While this new process is not as simple as the traditional DXF data transfer method, it offers many more advantages. The simulation-to-design cycle is now shortened into direct bidirectional data exchange with the schematic. This means the RF circuit can be designed and simulated at any time during the PCB design and layout process, and changes transferred much more quickly.
In addition to shortening the design cycle, the schematic has finally entered the picture. Just as building a house requires blueprints, the RF and mixed signal circuits are now complete “blueprints” that can be reused in new designs, used for future design revisions, and properly documented and archived.
This is just the first step forward: With the intelligence of RF models and connectivity represented by the schematic, the same intelligent RF models can be placed into the layout. This is superior for any RF design that utilizes parametric models, because it facilitates changes. The layout software can provide the model’s parameterized options for editing, and even help readjust surrounding microstrip or stripline elements to properly reflect shape changes. These changes can be back-annotated to the schematic and then returned to the RF simulation tool for analysis to ensure the circuit still performs properly.
Now that intelligent circuits are placed into schematic and layout, they can be organized into blocks for use in other designs or for fast placement of arrays of repeated circuits that occur in layout. The blocks can be stored in a library and managed just like standard component geometries, so every designer can have instant access to the latest design revisions.
It is fair to say the benefits gained by using an integrated, schematic-based system for RF design and layout far outweigh the arguments for sticking with tried-and-true DXF file transfer methods. It is understandably difficult for design environments to embrace process changes such as those proposed here, but the same effort and coordination that it takes to keep a design environment running could easily be channeled into embedding a new way of doing things. In an RF design environment that must stay on the cutting edge, some consideration should be given to the processes that EDA software tools have defined and are now supporting in established RF design environments.
Abby Monaco, CID, is a product manager at Intercept Technology Inc. (intercept.com); This email address is being protected from spambots. You need JavaScript enabled to view it..
