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Mixed-system RF Design: Problems and Solutions
December 31, 1969 |Estimated reading time: 4 minutes
By Per Viklund, Mentor Graphics and How-SiangYap, Agilent
Video 1. The video shows an integrated RF design flow using Mentor Graphics design tools in an integrated design flow with the Advanced Design System RF and microwave design solution from Agilent Technologies.
In the past, single technology-point tools all specializing in their niche and bridged using first-order ASCII-based translation interfaces was sufficient to reach design convergence in acceptable time. Often, the only available integration was based on ASCII IFF a format that cannot represent all the required data and therefore makes true integration impossible.
In recent years, we have seen a trend shift in system design towards advanced multi-technology systems having an extreme function-to-area ratio. RF, analog, and ultra-high-speed digital share board space as a single integrated system.
Initially, the same design tools and design methodologies were applied to these more advanced designs and corporations experienced major issues in meeting their goals of "correct by design" and acceptable lead times at known quality levels. These traditional methodologies and tools have failed in a way that costs excessive design iterations and long cycle times. Issues arose from poor tool integration, and time was wasted fighting tools that did not understand RF and overcoming issues in the differences in design models between RF and system design tools.
The Root of the ProblemUpon close examination, root problems become obvious: translation between RF and PCB design tools is a pain because the data models differ to a level where seamless integration can never happen. RF shape libraries must be in absolute sync with the system design library to ensure that what you simulate is identical to what you build. Integrated design and library transfer between RF and PCB design tools must be a real-time, two-way integration that can sustain multiple round trips without loss of data or design intention. The system-level board design tools have to understand RF and support the designer in implementing the RF circuitry at mixed-system level.
Finally, to really have a major impact on design cycle time, real-time concurrent design must be available to allow multiple designers to operate in real time on the same design, either to cut cycle time by working in parallel or to use individual circuit specialists in analog, microwave, and digital technology in a concurrent design flow, whether the designers are located locally or globally dispersed.
The solution to truly (not just marginally) improve on the number of design cycles and cycle time is to address the root causes directly, rather than keep improving existing solutions incrementally. With a board design system that directly understands the RF circuits from the RF design tool, translation is, in theory, eliminated. Passing the data between tools allows it to be understood. That way, designs can be passed back and forth between RF and board tools in a way that closely matches the iterative nature of RF design. By ensuring that the RF shapes library of the board tools area 100% matches that of the same RF tool library, we ensure that what we manufacture is identical to what we have simulated. A mismatch here leads to design defects that are discovered so late in the design cycle that the cost of correction becomes enormous.
Why Is This Important?RF and microwave technology is being incorporated continuously into more designs. From what was once primarily limited to military and aerospace designs, RF now is a vital consumer electronic building block making its way into automotive technology, handheld communication, Bluetooth and WiFi applications, and many other areas that engender tight cycle times and product cost requirements. Design space is shrinking, and is likely to keep doing so, leading to electromagnetic interaction between RF and non-RF circuits. This has become a common design concern.
In the design flow of a system with RF content, the RF module is simulated initially using fast circuit-level models. This is a good fit, as we will need many fast iterations at the beginning to narrow down the circuit solution. Real electromagnetic (EM) simulation at this stage simply would take too long. When the circuit is found to operate satisfactorily, it has to be implemented on the system PCB. In this process, it is common to turn, flip, bend, and fold the circuit to make it fit on the board. A tight integration lets you re-simulate at circuit level immediately to ensure that the board implementation still meets design requirements.
However, circuit-level models don't take ground vias into account and they assume a near-perfect power and ground system, which we know is not the case at the finished board level. Above all, they do not include electromagnetic coupling to surrounding circuitry. Therefore, as the design gets near to completion, we need to validate the actual implementation using EM simulation to model the imperfect power and ground as well as any vias or other surrounding metal.
New Design FlowThe design flow becomes a series of iterations along the following steps: fast circuit simulation based circuit creation; implementing RF circuit on the multi-technology system PCB; adjusting the RF circuit to fit available board space; re-simulate circuit level, iterate as required; validate the actual PCB implementation using EM modeling and simulation; correct issues found, repeat as required.
Given a seamless integration between RF and non-RF tools, and using design tools that support round-trip passes without losing either data or design intent, this becomes a very fast iterative flow.
While this flow has been the desired one for a long time, it used to be a distant dream due to poor integration capability. With the introduction of tools to the market that support a new methodology, users see the possibility to cut 50% or more off the design time. Results include fewer design cycles and the ability to reach design closure at known quality and circuit margins.
Per Viklund, director, advanced packaging, Systems Design Division, Mentor Graphics Corporation, may be contacted at per_viklund@mentor.com. How-SiangYap, Agilent EEsof EDA product marketing manager, may be contacted at how-siang_yap@agilent.com.