Long a bottleneck to rapid product development, new RF simulation tools are faster, use less memory, and are easier to use than the tools they replace
By John H. Mayer, McHale n Keller
Despite shrinking product lifecycles and escalating time-to-market pressures, RF design remains for the most part an iterative process. Designers typically build board prototypes and continually tweak designs as they deal with extremely sensitive layouts, component non-linearities, and interactions between components on a board. With each iteration, critical circuit and systems parameters such as noise figure (NF), gain, and phase noise require frequent, time-consuming and error-prone recalculation.
For these same reasons, engineers continue to design RF circuits on most wireless communications projects separately and on different time schedules from the rest of the system. That same practice continues to limit the designer`s ability to perform systemwide analysis and tradeoffs between RF and digital or mixed-signal segments. This dramatically increases the chances that the final RF and baseband components will not interact as expected.
"Over the last several years the development time of the digital and lower-frequency parts of the system has shrunk dramatically while the development cycle for RF and higher-frequency analog components has not," observes Ted Miracco, vice president of business development at tool vendor Applied Wave Research in Redondo Beach, Calif. "It`s still an 18-month cycle on many RF subsystems and it`s become a problem that`s going to require a lot more attention over the next few years."
The key to faster design turnaround lies in better RF simulation tools than have been available recently. Historically, building models in older SPICE-based tools to represent a component on a printed circuit board accurately has often proved a challenging task. New RF simulators, however, offer a variety of high-level functions that, for instance, enable the designer to see how a particular change in a board layout will affect overall system performance characteristics.
These new tools not only can predict parameters such as signal-to-noise (SNR) and third order intercept point (IP3) for a receiver, but they also can also automatically determine how much gain each block must have or how much gain control range an intermediate frequency (IF) amplifier will need.
The payoff in terms of design time can be dramatic. Tuning options can help identify sensitive sections of a design early in the design cycle, while back-annotation during the layout stage can help the designer track down difficult-to-find board parasitics. Newer simulators like SpectreRF from Cadence Design Systems in San Jose, Calif., handle full-chip circuit simulation of RF designs with more than 5,000 devices, and provide critical performance characteristics such as noise, conversion gain, IP3, and intermodulation distortion.
The big sell
Perhaps the greatest challenge tool vendors face, however, is convincing designers that currently available tools can meet their needs. As wireless system designs - particularly in commercial markets - have grown increasingly complex and moved to higher operating frequencies, designers have expressed reservations about using time-domain-based simulators. At a forum at the 1997 Custom Integrated Circuit Conference in San Diego, for example, wireless system designers continually argued that RF simulators could not deliver the speed or accuracy they needed.
"Usually designers have reservations because they believe time-domain simulators will be much too slow for the types of multi-rate simulations that they have to do and the simulators just can`t handle folding noise and noise that`s been aliased in," says Tony Stone, analog and mixed signal product marketing manager at Cadence Design Systems. "With SpectreRF we`ve addressed the multi-rate problem that most designers will encounter and we also perform all the necessary measurements that RF designers are interested in, including IP3, folding noise, and noise figure, without requiring any special modeling."
The next key advancement to minimizing the many development delays traditionally associated with RF systems will be the creation of a common platform for the tools that designers now use for digital signal processing (DSP), analog, and RF design. Only then can designers effectively cope with the interactions between the different segments of a system design.
Engineers at Hewlett Packard Co. of Westlake Village, Calif., took the first step down that road with their introduction last year of their Advanced Design System. Designed to address the challenges inherent in building wireless, electronic warfare, radar, and other communications circuits, the system offers Spice, harmonic balance, circuit envelope, and data-flow simulation techniques in a single user interface. Intended to give designers the opportunity to explore different ideas and then model the electrical and physical circuitry, the Advanced Design System simulates what HP officials call the entire "communications signal path."
"That`s our concept of being able to use a design tool that`s applicable to the whole span of the design, from the baseband data through whatever digital processing you might want to do, and then to the RF link and back down to the baseband data," explains Andy Potter, product manager with Hewlett Packard`s EEsof Division in Westlake Village, Calif. "We want to encompass that whole path."
The system offers the co-simulation of RF and DSP functions in a single environment by combining HP`s RF and analog simulation tools with DSP data-flow simulation software based on the University of California at Berkeley`s Ptolemy project.
Co-simulation enables users to review the interaction between DSP and RF portions of the design and make architectural tradeoffs. The system lets the user choose either a DSP or analog/RF track. All of the tools in the Advanced Design System are transparently interoperable to minimize the need for data or design transfer. Circuit designers can build physical design portions of the circuit in the system and then integrate them into popular electronic design automation (EDA) frameworks from Mentor Graphics of Wilsonville, Ore., or from Cadence.
For areas in an RF simulation where designers need a high degree of precision, such as components that exhibit a large variety of unique physical parasitics, they have typically turned to electro-magnetic simulators - also known as EM simulators.
"Any software simulation is only going to be so good," notes Mike Hillbun, a consulting engineer who specializes in satellite communications and serves as president of Diamond Engineering in Diamond Springs, Calif.
"In this industry you begin to get into situations where you need greater precision for predicting outcomes and trying to figure out why bandwidth is constricted," Hillbun says. "As an simple example many of the KU band satellite down converters that I`m involved with are all discrete components. We`re using 06035 physical resistors and those resistors just flat out don`t work unless they`re used in a structure and will make that resistor work over that particular bandwidth. But to do that you have to get in and measure a wide range of parameters."
One of the leaders in the electromagnetic arena over the last decade or more has been Sonnet Software of Liverpool, N.Y. Company officials offer a line of electromagnetic simulation software targeted at the planar metalization of layered dielectrics in printed circuit boards, packaging, and microwave/millimeter wave integrated circuits (MMICs). Historically Sonnet Software officials have served a wide range of military and avionics manufacturers such as TRW in Redondo Beach, Calif., Northrop Grumman Corp. in Los Angeles, Raytheon Co. of Lexington, Mass., and Hughes Aircraft in Los Angeles. But recently the market for electromagnetic simulation has begun to broaden.
"Military systems manufacturers have always been good customers of ours because you cannot create good, higher- frequency MMIC circuits without having something like this," says Senior Engineer James Willhite. "But now we`re seeing more and more people in communications technologies using them, even though they`re operating at somewhat lower frequencies, because they`re pushing so hard to get excellent performance out of very compact circuits.
Sonnet offers a suite of 3D Planar High Frequency Electromagnetic software tools in five modules as well as translation tools to convert designs from a preferred layout or design package. Company leaders recently added a new electromagnetic analysis capability for 3D structures that uses the Transmission-Line Matrix technique in the time domain. Like many tool vendors, Sonnet officials also extended their support to Microsoft`s Windows NT and Windows 95.
Hewlett Packard`s electromagnetic capability comes from two products: High Frequency Structure Simulator (HFSS) supports designers who build machined components, antennas, and other high- frequency 3D parts such as connectors. The latest HP revision, version 5.0, offers 10 times the performance of its predecessor.
HP`s second product, called Momentum, is a planar electromagnetic simulator that takes into account real-world design geometries to simulate the effects of coupling and parasitics, and can analyze multilayer arbitrary shapes. The simulator helps designers compute S-, Y-, and Z-parameters of general planar circuits and to analyze microstrip, stripline, slotline, coplanar waveguide, and other circuit topologies. "It`s coming into use more and more as designers cram more functionality onto a circuit," says HP`s Potter.
The newest option in electromagnetic simulation comes from Applied Wave Research. Company engineers have developed a new 3D electromagnetic simulation package for the analysis of planar high frequency circuits and antenna structures that attempts to dramatically shorten the learning curve for engineers using these types of tools. Called EMSight, it applies an intuitive Windows graphic user interface to a tool that takes a full-wave spectral domain simulation approach.
"Software development has changed dramatically in the last 15 years and there have been advances in software technology that are at least as significant, if not more so, than what`s gone on in the hardware world," says Miracco of Applied Wave. "Yet most of the design software used today was written 10 years ago using obsolete programming techniques and without a lot thought or concern about the user interface."
Designers can use the package to analyze the electrical behavior of arbitrary conducting traces found in RF ICs, MMICs, patch antennas, and high-speed printed circuit boards. Early users include Northrop Grumman, Hughes Aircraft, and TRW.
"I had looked at some other products and became very disenchanted with the user interfaces," Hillbun says. "Granted all the theory behind them is heavy stuff, but it doesn`t do you any good if you can`t use it. I downloaded EMSight, tried it, and I could immediately see they`d gone a lot further in perfecting the user interface than anything I`d seen."
EMSight is the first in a series of simulation and design tools that Applied Wave specialists are building based on their object-oriented design framework. The framework provides interfaces for importing and exporting EMSight data to other RF/microwave design tools. Designers can save simulation results in industry-standard data formats such as Touchstone and then read into popular linear and non-linear circuit simulation packages from vendors such as Hewlett-Packard, Ansoft Corporation of Pittsburgh, and others.
To boost accuracy, EMSight uses the Galerkin Method of Moments. The software also features a Fast Frequency Sweep algorithm that reduces simulation time by as much as an order of magnitude, company officials claim. Advanced animation features let users view true time-domain and frequency-domain current animations that indicate the magnitude and direction of currents. Post-processing capabilities include Smith charts, Cartesian graphs, polar graphs, and tabular outputs.
"One thing I like about it is it`s very fast," says John Gipprich, a senior engineer with Northrop Grumman`s Electronic Sensors and Systems Division in Baltimore. He bought the software because he wanted something to run on his PC. "Most of our field solvers here are on workstations and the price was right," he says, noting his organization uses Hewlett-Packard`s HFSS and products from Sonnet and Ansoft. To date he says he has used EMSight primarily on multilayer striplines and problems with interconnects and vias.
Department of Defense efforts to advance RF simulation tool capabilities are being driven by the DARPA-financed Microwave and Analog Front-End Technology (MAFET) program. The program actually ranges well beyond design tool issues. Its three primary goals are:
- to develop a "first-pass success" design environment, called the MAFET Design Environment, for low-cost MMICs and RF systems;
- to promote the development of new semiconductor materials, MMICs, and passive components for next-generation MMIC subsystems; and
- to explore new technical approaches to improve the ability of MMICs to handle the large power levels required by most military radar and communications systems, and to lower their cost.
The program brings together tool vendors, microwave integrated circuit manufacturers, and academic institutions. Tool vendor participants include Cadence Design Systems, HP EEsof, and Compact Software of Patterson, N.J. - now owned by Ansoft Corp.; IC makers Texas Instruments in Houston, TRW, Hughes Aircraft, and Raytheon Electronics; and colleges include the University of California at San Diego, the Massachusetts Institute of Technology in Cambridge, Mass., the University of Colorado at Boulder and the University of Massachusetts at Amherst.
With half the initial $59 million funding for the program coming from DARPA, the subcontractors include Northrop Grumman, ITT Avionics in Clifton, N.J., and M/A Com Inc. in Lowell, Mass.
Prospective tool users complain, how-ever, that despite intentions to commercialize new development, projects such as MAFET unfairly benefit only a segment of the RF industry. Critics claim these efforts target primarily systems operating at 30 to 120 GHz frequencies, and offer little benefit to commercial designers building RF and wireless systems running between 1 and 2 GHz.
Moreover, they argue that tools designed to serve military and aerospace applications have historically proven far too expensive for commercial users, and more often than not are intended for a design cycle with extremely long lead times. Designers can amortize the costs of military systems components against decades-long deployments.
Vendors in the MAFET program responded last year with a white paper that detailed their progress since the program began. In particular, the document highlighted improvements in the user interface for EDA tools and major steps forward in simulation speed and accuracy. The initial phase of the product targets reductions in design cycle time and development cost for multichip assemblies by a factor of three. Still, participants in the program appear to be ahead of schedule.
One of the more interesting developments announced was a black box modeling capability developed at Hewlett Packard`s EEsof Division that enables designers to model and simulate microwave and millimeter wave systems at the behavioral level. Hewlett Packard officials also announced a waveform envelope simulator that enables RF systems designers to simulate a wide variety of modulation waveforms, including those used for phased radar.
To accelerate the simulation cycle, Sonnet Software experts have developed new circuit-partitioning techniques that break a microwave circuit into easy-to-simulate portions and then reassemble them. Along with other enhancements, the circuit-partitioning techniques have helped speed Sonnet`s software performance by more than 30 times over previous techniques, Willhite says.
And the improvements do not stop there. "In our next release, which is being benchmarked right now at Wright-Patterson Air Force Base [in Dayton, Ohio], we expect to accelerate performance by more than 100 times and reduce memory requirements by a factor of 40 or more," Willhite says.
In the meantime, Compact Software officials announced a stability analysis tool that predicts oscillations in high-frequency amplifier circuits. The tool applies Nyquist`s stability criteria in the frequency domain to locate spurious oscillations and examines the ac and dc signal path at various bias points to find the circuit branches most likely to generate the oscillations. Eventually the tool is expected to save IC, multichip module, and system manufacturers millions of dollars in unique device development and prototyping costs.
The latest version of the Hewlett Packard High Frequency Structure Simulator offers new finite-element simulation and mesh engines that perform 20 times faster than its predecessors, while using 50 percent less computer memory.
The Hewlett Packard Advanced Design System includes an RF board design module that enables designers to make system-level analyses to determine how best to partition designs, optimize each functional area such as the receiver front end, local oscillator, or transmitter, and ensure first-pass success.
European consortium focuses on fast RF design turnaround
Integrating RF designs seamlessly into the entire wireless design flow is a challenge for designers around the world. In Europe, leaders of a consortium of electronic companies are working within a three-year program called the RF Front End project to define design methods that will accelerate RF circuit design and help integrate it into the entire system design flow.
The project is attempting to address common wireless design challenges before the pan-European implementation of the Universal Mobile Telecommunications Service (UTMS) standard. Intended for next-generation mobile terminals, UTMS will operate at frequencies around 2 GHz and is due to deploy in 2001 or 2002.
The RF Front End project will focus on RF circuits for the UTMS and on applications in the unlicensed instrumentation, scientific, and medical band. Participating in the project are mobile-communications equipment maker Nokia Corp. of Espoo, Finland, Temic Semiconductor GmbH of Heilbronn, Germany, Thesys Microelectronics GmbH of Erfurt, Germany, Cadence Design Systems GmbH in Munich, Germany, and various German universities and technical institutes. Half of the $9.2 million funding for the project is coming from the German government.
A key goal of the consortium will be to develop or refine simulation technology that can bring the RF section into the main product development stream and the creation of a consistent flow and methodology that achieves that aim.
"There have been numerous advances in top-down wireless design flows for logic and DSP design, but until now, the RF section has been out on its own little island," explains Bill Portelli, vice president and general manager of Cadence`s Custom IC business unit. "The lack of a strong system-through-RF design flow is turning into a bottleneck, especially as designers try to put entire phones on one chip."
Consortium leaders expect to deliver the first commercial products based on their work during the second half of 1998. - J.M.
Who`s who in RF design and development tools
Applied Wave Research
Redondo Beach, Calif.
Cadence Design Systems
San Jose, Calif.
Hewlett Packard EEsof Division
Westlake Village, Calif.