Design Articles

Designing RF Mixed Technology Boards

A single, integrated design flow that accounts for the use of printed circuit components for RF elements is capable of saving a great deal of time, re-spins, and headaches.

By John Isaac and Per Viklund, Mentor Graphics

chip graphic

Although most people don’t think about it, we are surrounded by, and use daily, all kinds of different wireless devices.  The average user does not seem to equate “wireless” to “radio”, or RF, but in truth RF circuits are showing up on a surprisingly large number of products. Indeed, RF in one form or another — WiFi, cellular, Blue Tooth, garage door opener — is nearly as common as a keypad, and in most cases, the radio signals used are very low power.

For most of the century-plus that RF circuits have been designed and built, RF has been treated as a highly specialized task, and as such has been more-or-less been isolated, both in design and on the printed circuit board (PCB). With the advent of the handheld cell phone, there was no longer the luxury of having an isolated PCB just for the radio components. Other small wireless devices contributed to the drive, and now boards containing both analog and digital signals, plus RF circuitry, have become the norm.

Unfortunately, this isolation also meant that the EDA tools used by RF engineers were not the same as the PCB designer…or the logic designer for that matter. What happens is that the designs are merged together on a single, mixed-technology PCB way down the development timeline. This process is highly inefficient, often requires iterations to marry the mixed technologies together and resulted in multiple databases representing the final product.

EDA tool designers have been working to make the incorporation of RF data more smooth and seamless for designers of all disciplines. We worked closely with Agilent to develop a new approach and set of integrated design tools that address these issues and results in significant improvements in designer productivity, design cycle time and product documentation.  The integrated design solution enables the complete logical and physical design to be accomplished in one environment (tools and RF shape libraries) that is tightly integrated. 

Need for improvement

RF designs have typically used two separate design environments coupled via an ASCII interface. Each system has its own libraries, RF design databases and design archiving. This means that the schematic and layout libraries have to be constantly managed and kept synchronized in both environments.

Thus, the RF design is done in isolation from the rest of the design and the PCB design. Once the RF design is ported, any changes must be done in the RF tool, and then re-translated and sent back to the PCB designer — in other words, the entire RF data set was sent, not just incremental changes. This defines a very inefficient system.

Further, when RF designers simulate their circuits, they do not include the nearby geometry of the PCB, since it isn’t know to them at this point. Of course, PCB traces, components, vias, etc. can have significant influence on the operation of RF circuitry.

This methodology has been used successfully for years to design mixed technology boards but as the RF content in products increases, and the size of the products decrease, the problems with having two separate design systems (Figure 2) is now significantly impacting designer productivity, time-to-market and quality of the products.

Integrate the two systems

Now, the introduction of an integrated solution (Figure 3) allows RF circuitry to be included in the PCB design right up front. The system integrates together:

  • PCB design flows, which act as the master system containing the complete board design (schematic, and layout).
  • The RF sections can be designed in either system.
  • RF design system is used for simulating the RF sections.
  • A dynamic, bi-directional link (rather than ASCII interface) transfers the data and libraries back and forth between the systems in real time.  This provides for a single design database, library and archiving mechanism.

Not only does this integrated approach make it easier to transfer data between the systems, but in addition, the RF designer can become an integrated member of the design team.  This integrated system eliminates the need for duplicate databases and libraries thus removing the time consuming and error prone steps with translations and synchronization.

Changes made to the RF circuit, within the RF development tool, are immediately reflected into the PCB design tool. Since the PCB tool knows that RF components can be printed as traces, it does not disrupt RF designs or clearances. All this results in fewer design iterations.

The result of this new methodology is significant increase in productivity, reduction in design cycle times and time-to-market, and improvement in product functionality and quality.

A Design Example

An example of a real world design will illustrate the ease of use and the greatly-reduced potential for error when using an integrated tool approach.

Developing the Schematic

Using the common RF library, a designer selects RF component symbols and places them on the schematic from either the PCB tool or the RF Design tool, as shown in Figure 1.  The designer adds parameters to these generic schematic symbols setting up for the layout synthesis function that will generate the physical shapes. The key is that the PCB tool knows that these symbols result in a particular set of traces that cannot be modified.

figure 1
Figure 1: RF shapes are defined and placed in the schematic

Synthesizing the RF Shapes

RF shapes are synthesized on-the-fly based on user entered values for parametric shapes. The RF shapes can be designed on a schematic and forward annotated to layout, and can then be tweaked in layout to adhere to placement and clearance constraints on the board.

If the RF circuits are designed in the PCB systems, the dynamic link allows real-time validation of RF circuits both in schematic and in layout using RF simulation tools for doing quick circuit simulation checks and also for very in depth and elaborate electromagnetic simulation.

Creating a Circuit from the Schematic and Shapes

With RF circuitry, it is not good enough to just connect the components with random net lines.   The component shapes must be directly connected (all shapes are functional RF circuits, including net lines) and configured to perform the correct RF function.  Mentor has developed an “auto arranger” that automatically performs this task.  It combines the logic of the schematic, the shapes and a set of default rules that govern the final configuration, as seen in Figure 2.

figure 2
Figure 2: The auto-arranger takes pre-defined shapes and forms RF circuits based
on the schematic and rules set

Simulating the RF Design

Using the real-time interface, the RF designer can simulate and optimize the circuit using simulation to verify it against the latest wireless, industry radiation safety, or high-speed digital standards. Simulation of the actual PCB layout provides the best assurance of RF design verification accuracy possible.

The link between systems allows real-time cross probing between the systems.   Shapes can be probed either environment, and be reflected in the other; see Figure 3.

figure 3
Figure 3: Cross probing can be done between the two systems, and observed in both

If the simulation of the shapes does not meet the desired functionality, the shapes can be changed either by adjusting the parameters (re-synthesis) or by manually editing the RF shapes or other “components/shapes” (custom shapes, packaged parts, traces, plane shapes and thermal ties) in either environment. Groups, described in the next section, allow much easier handling of RF shapes when editing.

Manipulating Groups

RF elements exhibit influence on other elements based on proximity and geometry. Placing the RF circuitry on the PCB then being able to adjust the RF its position relative to the rest of the analog or digital circuitry requires that the RF shapes be tightly locked in place relative to each other.  Otherwise, a slight change in their relative position can drastically change their functionality.  For this purpose, capabilities exist to hierarchically group the RF circuitry and then manipulate it as a group rather than individual shapes.

Also in conjunction with groups, a designer can specify clearances between a group and other shapes that might cause improper RF circuitry operation.  This applies to both X-Y and relative Z axis clearances.  The relative Z axis clearance to various objects allows for maximum control of RF design as they are re-arranged by the PCB designer on the same or different layers of the PCB.

Automating Stitch Vias

Properly interconnecting ground planes or to shield RF shapes often requires the addition of several (up to hundreds of) vias.   If done manually, this could be a very long process.  Instead, Mentor’s layout tool provides the ability to specify a via pattern and then instantiate those vias automatically. This can occur in very specific patterns or to merely flood and area with vias, illustrated in Figure 4.

figure 4
Figure 4: Automatic Stitch Via generation can produce a specific pattern, or flood an
area with stitch vias

Interface to Manufacturing

The complete design database is now complete and stored in the layout design tool.  The RF sections have been completely simulated and any problems that surfaced have been resolved.   The database and libraries are synchronized to ensure that what was simulated exactly matches what is resident in PCB system databases.  Now the manufacturing data can be generated from the master database for the complete mixed technology board including the RF circuitry.

The Bottom Line

Integrating the PCB design, layout and simulation tools with RF design and simulation tools is incredibly powerful in this age of ‘low-power, wireless-everything’. A single, integrated design flow that accounts for the use of printed circuit components for RF elements is capable of saving a great deal of time, re-spins, and headaches!

The Authors

John Isaac, Director of Market Development

John Isaac Mr. Isaac has worked in the Electronic Design Automation (EDA) industry with PCB and IC technology for over forty years.  His career started with IBM where he managed the development of EDA systems for IBM's internal design of their high-end ICs and PCBs.   Mr. Isaac then joined Mentor Graphics where he has held marketing positions in both PCB and IC product areas.  He is currently responsible for worldwide market development for the Systems Design Division.

Per Viklund, Director of IC Packaging & RF

Per Viklund Per Viklund is responsible for IC packaging RF Design and Embedded components technologies at Mentor Graphics. He is a long time IEEE & IMAPS member with close to 30 years experience with electronic design and has spent the last 20 years with development of advanced EDA tools. He is a recognized industry expert in Advanced packaging and RF design and has published numerous papers on IC Packaging, Embedded passives, RF and high speed design. Prior to joining Mentor, Viklund was chief technical manager of DDE-EDA for 10+ years.

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