AWR Design Environment Version 2009 Boasts Enhanced Harmonic Balance, New Features
By Sherry Hess, Vice President of Marketing, AWR Corporation

The AWR Design Environ- mentTM has been growing in both depth and breadth with every new version of the high-frequency design software since its initial release back in the 1990s. The latest release, Version 2009, offers a broad array of new features in AWR’s flagship Microwave Office® software, APLAC® harmonic balance/transient analysis simulator, AXIEMTM 3D planar EM simulator, and Visual System Simulator™ (VSS) communication system analysis software.

Perhaps the most significant enhancement within Version 2009 is the addition to AWR’s APLAC harmonic balance technology of a new patent-pending algorithm coined multi-rate harmonic balance (MRHBTM). MRHB dramatically increases the speed and reduces the computer memory required to perform steady-state analysis of complex nonlinear systems that employ multiple signal sources. Overall, it makes it possible to analyze frequency-rich complex circuits that are beyond the reach of traditional harmonic balance techniques alone.

The Multi-tone Challenge
Harmonic balance has long been the primary tool for nonlinear frequency-domain simulation and its ability to analyze more complex circuits has grown continually over the past two decades. Today the most formidable high-frequency harmonic balance simulators such as APLAC can tackle designs with thousands of analysis frequencies and scale almost linearly as elements, nodes, and frequencies increase. However, consumers’ hunger for all things wireless continues to push the envelope on communication systems and consequently complexity – more complicated systems processing signals of greater diversity.

Existing (or traditional) harmonic balance simulation (HB) assumes that the frequency content is the same in every part of a circuit. In the simplest case, for a system without frequency conversion or a multi-tone source (defined as a fundamental frequency and several harmonics), the input of the circuit experiences the same spectral composition as the output, as does every component in between. While filters and quasi-unilateral circuit behavior may limit the amplitude of certain spectral components, they are inherently present. When this is the case, analysis is comparatively straightforward to perform, since designers can use single-tone harmonic balance analysis with five harmonic components—more if the system is operated nonlinearly.

Systems may require multiple tones to describe either the single signal source or because mixers are strict harmonic balance formulations. A multi-dimensional space would then be constructed with a dimension for each tone and then the harmonic balance simulation would be performed for each sum and difference of the tones at their fundamental and harmonic frequencies. An early improvement on the basic harmonic balance algorithm was able to eliminate some of these sum and difference frequencies based on the assumption that they will have relatively little energy content and therefore minimal effect on overall circuit performance. This simplification, a technique called “diamond truncation” (Figure 1) has the overall effect of reducing the number of frequencies that must be solved.

Nevertheless, “adding another tone” is a far greater computational problem than it might appear to be. Using a single tone as an example, analysis must be performed at the fundamental frequency and (perhaps) four harmonics. It must also be conducted at the number of nodes in the circuit multiplied by the frequency points. If another tone is added, the equation grows from 6 frequency points (including DC) to 61 for traditional harmonic balance and 31 with diamond truncation. Complexity continues to grow rapidly with further increases in the number of harmonics but geometrically with the number of tones. If four tones are required, simulation must be performed for well over a thousand frequencies, and at several million frequencies for an 8-bit digital communication bus. The result is that the computational difficulty in solving the design very rapidly becomes unruly, taking so long to solve that it is rendered unsolvable in an acceptable time span and with any available memory.

MRHB was created to address this problem, simulation of an entire transceiver in a mobile phone, for example. This task is simply not possible using traditional harmonic balance tools, since the phase-locked loop alone, which includes a divider, requires 5,000 to 10,000 frequencies to solve. MRHB is instead based on the premise that for a given component, it is possible to determine which tones and their harmonics really matter and therefore must be solved. If the design employs a variety of filters, the tones and harmonics that must be considered after the filters can be limited because many of the tone/harmonic combinations or even some of the fundamental tones themselves have no significant effect and can safely be ignored. MRHB also makes it possible to reduce the number of frequencies that must be solved by constructing dynamic tones “on the fly,” using the sums and differences of the tones defined as “sources”.

For example, to analyze downconversion of an RF signal at 8 GHz with a local oscillator at 7.5 GHz, their difference (500 MHz) can be used as a tone for all analysis after the mixer and subsequent filtering. This can reduce the size of the problem by a factor of two, and in actual five-source simulations has shown that reductions of an order of magnitude can be achieved. This makes it possible to actually consider solving an entire transceiver in a practical amount of time with a realistic amount of memory, as well as any complex receiver with multiple stages of downconversion, multi-band power amplifiers, and complex high-frequency digital circuits.

A good example of this is shown in Figure 2, which is an upconversion circuit with two tones and three distinct spectral regions for its four components. The amplifier at the input is solved for one tone and five harmonics, the mixer, bandpass filter, and local oscillator section with two tones and five harmonics, and the amplifier at the output with one tone and five harmonics. The results of a simulation of this circuit using traditional harmonic balance and MRHB is shown in Figure 3A and B. Figure 3A shows that the output waveforms are virtually identical using both techniques, illustrating the accuracy of the latter method. Figure 3B shows the significant reduction in frequency content of MRHB when compared to traditional harmonic balance. The benefit from the user’s perspective is that MRHB solved the circuit in a fraction of the time (thanks to its “intelligent” frequency-selective ability) while delivering the same level of accuracy.
AWR has applied for a patent covering MRHB, which has been in development for several years. Its addition to the Microwave Office suite provides immense benefits to customers analyzing specific types of complex circuits that were either too difficult or simply impossible to solve previously.

AXIEM Gets Even Faster
AXIEM also benefits significantly in Version 2009. Thanks to its proprietary solver and meshing algorithms, AXIEM made it possible for the first time to move EM analysis from its traditional place as a back-end, post-verification tool to use throughout the design process. The near-linear scaling achieved by its method-of-moments (MoM) engine has given AXIEM, even in its first year of commercial availability, better performance than competing 3D planar solutions that have been on the market for more than a decade.

Now, 64-bit, multi-core PC platform support further increases its speed, accuracy, and capacity in Version 2009 (Figure 4). AXIEM can now handle designs with more than 100,000 unknowns far faster than with 32-bit operating systems. In addition, loss model improvements, extensive new sources/ports, and de-embedding options combine to deliver more robust solutions at low frequencies and greater accuracy at all frequencies. The new de-embedding approaches include internal edge, finite difference/gap, as well as per-port, coupled line, and mutual group de-embedding. AXIEM can be used as a stand-alone EM point tool or can be accessed from within Microwave Office software. It is extremely well suited for EM analysis of planar components such as RF printed circuit boards (PCBs) and modules, low-temperature, co-fired ceramic (LTCC) packages, MMICs, and RFICs.

Advances in Microwave Office
Microwave Office Version 2009 includes a wide array of new features including expanded support for Open Access (OA) Process Design Kits (PDKs). These PDKs and layout views make it possible to create an interoperable design flow among EDA vendors and allow seamless exchange of design information between software vendors, foundry partners, and their customers by employing a standard, rather than proprietary, format.

Microwave Office now also supports constant output power simulation, which is extremely helpful in RF power measurements that provide more meaningful results when output power is fixed rather than manually derived by changing the input power with the software’s measurement search function. The result is that manual, post-processing steps can be eliminated along with the numerous simulation iterations required. The software now builds on the existing load-pull analysis with a much more sophisticated version driven by user requests. This enhanced capability accommodates data formats from device characterization specialists Maury Microwave and Focus Microwaves. The resulting benefit is reduced analysis time and the ability to add or remove load-pull data points directly on the Smith Chart.

Microwave Office software now also supports the active load-pull system and “Waveform Engineering,” pioneered at the Institute of High Frequency and Communications Engineering at Cardiff University, and implemented commercially by Mesuro Limited. The technology allows S-parameter concepts to be replicated within the nonlinear domain and to simultaneously measure the actual current and voltage of a device under test. Designers can then view and engineer their waveforms to match theoretical values. From this a resulting behavioral model, called a “Cardiff model,” can be extracted and invoked within Microwave Office Version 2009. This makes it possible for modeling and design engineers to fully characterize their devices or power amplifiers for any signal and impedance environment. Other enhancements include an improved project tree that enables nodes to be reconfigured to make it easier to organize large projects that have hundreds of schematics and graphs.

Expanded Tools in VSS
AWR’s comprehensive communication system analysis software now features a variety of new components, including RF switches (with P1dB and IP3 parameters), analog and digital phase shifters that use a digital control word, RF delay blocks that add frequency-dependent phase shift to signals, digital step attenuators and variable voltage attenuators, an RF combiner and splitter, a two-input, two-output 90-deg. hybrid directional coupler, and receive and transmit antenna models.

VSS also now benefits from the latest in AWR’s series of “AWR ConnectedTM” solutions, which blend AWR software with either the hardware or specialized software from other vendors to extend the capabilities of AWR software tools. AWR Connected for Rohde & Schwarz integrates the capabilities of R&S WinIQSIM2TM software within VSS to add many standards-based digitally-modulated waveforms to those already present within VSS. The addition of R&S WinIQSIM2 makes it possible to use real-world test signals throughout the design cycle to produce much greater accuracy than would generic waveforms. The waveforms in R&S WinIQSIM2 include 3GPP LTE, 3GPP FDD/HSPA/HSPA+, and WiMAX, arguably the three most current and demanding wireless signals to simulate and measure.

With R&S WinIQSIM2 within VSS (Figure 5) signals created by a signal generator are sent back into VSS so the device under test can be optimized within the software to meet the performance goals of specific wireless network standards. The solution is designed to be used with most Rohde & Schwarz vector signal generators such as the R&S® SMU200A, which provides signal processing capabilities such as real-time fading required for Multiple Input Multiple Output (MIMO) measurements.

Finally, AWR has recently created a comprehensive online training program within its Web site to help users become more proficient with the company’s high-frequency design tools. The training is module-based, each video presentation consisting of three 90-min. webinars (60 min. of content and 30 min. of Q&A). Customers can view live lectures via telephone and Web browser, or replay them at a more convenient time. Each lecture comes with a 90-min. take-home exercise designed to enhance understanding of the material.

Current topics include advanced EM concepts for planar simulators, setting up EM in Microwave Office, AXIEM concepts, an introduction to Microwave Office, controlling layout in Microwave Office, and using harmonic balance. The courses are developed and presented by Dr. John Dunn, a senior engineering consultant at AWR and recognized expert in electromagnetic modeling and simulation for high-speed circuit applications. The presentations are available by registering at www.awrcorp.com/online-training. More information about all of the enhancements in Version 2009 is available at our website.

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