System design was once the demesne of a chosen few engineers who understand the behavior of different high-frequency functions working together. As more functions become integrated together onto common substrates and housings, however, it is more incumbent upon RF/microwave engineers to gain at least a fundamental understanding of system-level dynamics. Fortunately, with the emergence of powerful system-simulation software, designers can now model the most complex analog and digital system-level functions on a standard personal computer (PC).
System-level simulators are currently available from a number of dependable suppliers, including Agilent Technologies (Santa Rosa, CA), Ansoft (Pittsburgh, PA), Applied Wave Research (El Segundo, CA), Cadence Design Systems (San Jose, CA), Eagleware Corp. (Norcross, GA), Elanix (Westlake Village, CA), and Mentor Graphics (Beaverton, OR). For example, SPECTRASYS is a spectral-domain system simulator from Eagleware Corp. (www.eagleware.com) that complements the company's lineup of linear and nonlinear simulators, including SUPERSTAR and HARBEC, and electromagnetic (EM) simulation tools, such as EMPOWER. The system-level software is in keeping with the philosophy of the company's other design tools: provide a high level of accuracy in an easy-to-use package.
SPECTRASYS, which is a design module for the firm's GENESYS software suite, allows engineers to analyze full-node spectra at any node in a design. Operators can analyze parallel paths, view the phase of any spectral component, and identify the paths of any spectral component throughout the system. Rather than define each path in a system, the SPECTRASYS program allows an operator to create a single schematic diagram with any arbitrary topology, and then specify any path of interest for analysis, rather than creating a new schematic diagram for each spectral path.
SystemView from Elanix (www.elanix.com) has long been used by designers of both commercial and military systems. On the commercial side, the software has been used to model Bluetooth, wireless local-area network (WLAN), cellular/personal communications services (PCS), and various spread-spectrum communications systems. On the military side, the software has been used to model direction-finding (DF), electronic-intelligence (ELINT), radar, signal-intelligence, and sonar systems. The software, which allows operators to connect and manipulate function "tokens" to construct sophisticated analog, digital, and mixed-signal systems, allows analysis of a wide range of system signal characteristics, including magnitude, power spectral density, phase, and group delay. It allows the simulation of mixed time-continuous and time-discrete systems, multirate systems, multiple parallel systems, and distortion in RF and analog systems. The software is supported by a full complement of logic functions, switches, and nonlinear devices, and includes full libraries of sources, sinks, functions, operators, and MetaSystems (which are essentially multiple-function modules).
The company recently announced a pair of new application notes devoted to one of the more significant emerging wireless technologies: ultrawideband (UWB) communications. Available for free from the firm's website, these application notes (see p. 110 for a review) cover the simulation of UWB transmitters (Txs) and receivers (Rxs) based on both pulse-position modulation (PPM) and on/off-keying (OOK) modulation.
New application notes from Elanix, for example, highlight simulations of UWB Rxs and Txs based on pulse position modulation (PPM) and on-off-keying (OOK) modulation. UWB technology is based on the use of modulated pulses to send high-rate data at extremely low transmitter power levels (a few milliwatts). Although the pulses can occupy several gigahertz of bandwidth, they are transmitted at power levels designed not to interfere with existing applications within the operating bandwidth, including Global Positioning System (GPS), cellular systems, and WLAN systems.
Another relatively new modeling tool, the Visual System Simulator 2002 (VSS2002) from Applied Wave Research (www.mwoffice.com) features specialized "Design Studio" modules to speed and simplify the modeling of specific wireless systems. For example, the 802.11a Design Studio for VSS2002 models all the signal generation and measurements needed for evaluating wireless systems according to the IEEE Std 802.11a-1999 specifications (data rates as high as 54 Mb/s). Operators have access to a transmitter's complex offset-frequency-division-multiplex (OFDM) envelope as well as to the in-phase/quadrature (I/Q) constellation. Users can select the number of data bytes per frame, the number of samples to overlap the OFDM symbol, the oversampling rate, and the frame length. Operators can also turn off OFDM subcarriers individually or in groups.
The company's 3G Design Studio for VSS2002 supports the simulation and evaluation of third-generation (3G) wideband code-division-multiple-access (WCDMA) equipment for base stations and user equipment. The software is compliant with the latest Third Generation Partnership Program (3GPP) specifications (revision 3.9). All of the baseband processing found in a 3G system, including framing, encoding, interleaving, spreading, and rate matching, can be handled by the 3G simulation function blocks. The simulation tool handles uplinks and downlinks at data rates of 12.2, 64, 144, and 384 kb/s.
Ansoft Designer from Ansoft (www.ansoft.com) combines time, frequency, and system analysis capabilities within a common framework. It blends EM analysis with circuit- and system-level simulation and a new capability (called "Solver on Demand") to automatically select the best solver for a given simulation task. The software suite includes Full-Wave Spice for time-domain analysis, along with nonlinear simulation and digital communications systems simulation with support for systems through 3G configurations.
The Advanced Design System 2002C from Agilent Technologies (www.agilent.com) is the latest version of a powerful system-level design environment that may well be the tool of choice for designers of integrated-circuit (IC) systems on a chip (SoC). The software seamlessly combines device-, circuit-, and system-level simulation engines and models with connectivity to commercial test equipment. This latest version includes an improved tuning model, new 3G source models, enhancements to artwork importing functions, and new Ptolemy models.
The MMICAD suite (see p. 64) of software tools from Optotek (www.optotek.com), which is also commonly used by designers of ICs, is well suited for designers of SoC. The system suite supports filter synthesis, time-domain transient analysis, and small- and large-signal device modeling.
Version 2.0 of TESLA for Windows from Tesoft (www.tesoft.com) speeds the simulation of complex systems at the block-diagram level. The first true systems-level software simulator (introduced in 1988), TESLA works with OrCAD Capture for Windows to first create a block diagram model for simulation; operators can switch back and forth between Capture (for block-diagram creation) and TESLA (for simulation) with a single keystroke.
Last but not least, two system-level tools from software giants Cadence Design Systems (www.cadence.com) and Mentor Graphics (www.mentor.com) provide powerful analysis capabilities for both PC and workstation users. Cadence's OrCAD Unison Suite is a complete solution for front-to-back printed-circuit-board (PCB) design, with schematic capture, analog, digital, and mixed-signal simulation, layout tools, and autorouter functions. Mentor's SystemVision is a math-based prototyping tool that includes extensive model libraries, graphical-design tools, data-analysis tools, and model-creation tools. It can handle analog, digital, and mixed-signal electrical models as well as electromechanical, thermal, and hydraulic models.