From its inception, the holy grail for the design automation industry has been in the analog realm. Digital logic, with its relatively straightforward structures and topologies, has long been the chief beneficiary of the EDA industry’s efforts. Yet compared to the digital domain, automation of analog and mixed-signal design remains lacking. There are some obvious factors at work. Chief among them is the painstakingly hands-on, custom nature of analog design work.
Analog designers are paid to be finicky about their work and simply do not trust efforts to automate their flow, and it’s not just about the threat to their job security. They believe that they can do the job better than an algorithm can. In fact, they’re largely correct in that belief. But there are other, somewhat less obvious issues at work when it comes to the lack of automation in analog/mixedsignal design, particularly in the wireless realm.
CHANGE COMES SLOWLY IN ANALOG
“Every company that is integrating analog/ RF circuitry with digital is slightly different,” says Mar Hershenson, vice president of product development in the Custom Design Business Unit at Magma Design Automation. “The issue is that there are really three design flows: RF, analog, and digital.” For some vendors of RFICs, matters are even more complicated (see “The MEMS Wrinkle”).
“Analog design methodology really hasn’t changed in the last 20 years,” says Hershenson. Primarily, designers rely on editors and a Spice simulator. There’s no parallel in the analog world to the soup-tonuts design creation flow one would find in the digital realm. Thus, it’s entirely feasible for a digital designer to generate 100,000 transistors a week. It’s also entirely feasible for an analog designer to take three months to create 50 transistors.
That’s because the analog portion of the process is largely manual, leaving designers literally to their own devices. Designers must bring a great deal to the party in terms of circuit knowledge and layout skills. Often, they’ll begin with a previous design or some portion thereof, simulating and tweaking until they’re happy with results.
On the layout side, designers typically huddle with mask designers and tell them where to place output devices, how to approach interconnect lengths, and other performance-critical aspects. But little, if any, of this information is captured anywhere.
CO-DESIGN GAINING FAVOR
So for the analog/mixed-signal design team who must integrate digital logic with RF and analog circuitry, where are the bright spots? Most of the advances in analog EDA in the past few years have come on the simulation front. Modern simulators permit cosimulation using Verilog models and full netlists.
“Customers were still reluctant to use it two years ago, but now they do some sort of co-simulation for large chips. It’s the only way to catch simple connectivity mistakes,” says Hershenson. “They’re doing it at a high level but at least they do something.”
One of the latest packages to hit the scene with co-design capabilities is Agilent Technologies’ Advanced Design System 2009 (ADS 2009), the latest revision of Agilent’s flagship design platform. It directly targets the co-simulation sweet spot for wireless consumer electronics such as 4G Long-Term Evolution (LTE) smart phones.
Designing the physical layer of a wireless system means integrating a slew of devices and subsystems that must comply with various specifications, such as LTE, WiMAX, WiMedia, Wireless HD, USB, and so on. In the case of a 4G handset, this integration can be extremely difficult. “Some phone makers spin printed-circuit boards (PCBs) up to 30 times, with teams working in parallel,” says How-Siang Yap, Agilent’s ADS 2009 product manager.
The motivation for co-design in such scenarios is mitigation of the risk of designing various system elements in isolation. A codesign methodology can be the difference between a failed design spin and a successful one. In integrating an RFIC, package, and balun, ADS 2009 was used to find an unexpected resonance at 1.7 GHz (Fig. 1). The three elements were all known-good commodities. But when integrated and co-verified in ADS 2009, this unexpected resonance altered the RFIC response. Catching the problem enabled the integration team to avoid a failed design spin.
Co-design environments like ADS 2009 have to keep up with evolving telecom standards and trends. Both the LTE and WiMAX standards call for multiple-input, multiple-output (MIMO) antenna schemes. To accommodate MIMO technology, ADS 2009 incorporates a companion simulator that accounts for the characteristics of multiple antennas within a handset.
By doing so, the simulator aids in the design of adaptive antenna- matching networks to satisfy LTE system specs. The companion EMPro 3DEM simulator accounts for various phone-human juxtapositions as it verifies that the system meets the LTE specification even as it accounts for health compatibility.
HOW TO BRIDGE THE GAP
Some analog/mixed-signal/RF design teams have dabbled in system-level work, attempting to define their system architecture at high levels of abstraction. But they have found that the transition to a more concrete design representation is challenging.
For Chris Ouslis, vice president of IC technology at Fresco Microchip, the issue has as much to do with how to employ system- level methodologies as with the methodologies themselves. “You can find good people at system level and good people at circuit level, but it’s hard to find people who know how to create verification suites that bridge the gap,” Ouslis says.