Integrating System and Circuit Simulation Capabilities
Applied Wave Research Inc. (AWR)
El Segundo, CA
Version 5.0 of Microwave Office Design Suite will begin shipping in the fourth quarter of this year. The new release, called Microwave Office 2002, includes integrated system simulation capabilities that close the gap between digital signal processing (DSP) and circuit-level simulation tools for high frequency wired and wireless applications. A revolutionary new multi-rate discrete-time simulation engine for the analysis of end-to-end communications systems is the cornerstone of an impressive array of additions and enhancements to the popular design suite. Also included is an extensive collection of RF component models that allow RF subsystem analysis using harmonic-balance and Volterra simulations. Microwave Office 2002 also incorporates a comprehensive Filter Synthesis Wizard, a Load Pull Wizard and an optional third-party interface to link test equipment to the simulation capabilities in Microwave Office. Enhancements to the existing circuit simulator include the addition of oscillator phase noise analysis and an automated load-pull capability that can incorporate both measured and simulated load-pull contours. The 3D planar electromagnetic simulation engine has been enhanced to simulate large problems more efficiently by incorporating state-of-the-art solvers.
DISCRETE-TIME SIMULATION ENGINE
Microwave Office 2002 is the first release that offers an optional multi-rate discrete-time simulation engine for analyzing end-to-end communications systems. Derived from the ACOLADE core technology, this new system simulation solution offers unprecedented capabilities for evaluating the performance of such systems. The efficiency of the simulation engine is immediately apparent as large bit streams can be processed through extensive arrays of components in real time. Users can control any of the model parameters, such as signal-to-noise ratio or third-order intercept point, with a series of "tuners," and instantaneously watch modulated waveforms change on an eye diagram, I/Q constellation or bit-error-rate (BER) plot. The simulator includes an extensive library of models such as modulators, encoders, filters and propagation channels as well as digital signal processing elements. Analyses include adjacent channel power ratio (ACPR), error vector magnitude (EVM), noise power ratio (NPR), third-order intermodulation (3IM), BER and many others. Figure 1 shows the output spectrum from a satellite receiver system.
CIRCUIT AND SYSTEM-LEVEL PHASE NOISE SIMULATIONS
The design of microwave oscillators and synthesizers provides many engineering challenges that have been difficult to address with traditional simulators. The most important of these is analyzing nonlinear phase noise, in which low frequency (1/f) noise mixes with the oscillator's clean steady-state waveforms, resulting in noise sidebands and fluctuation of the oscillation frequency.
Microwave Office includes new capabilities that overcome these challenges at both the circuit and system levels. Oscillator noise analysis is predicted at the circuit level by rigorous computation of the oscillatory steady-state, allowing precise determination of the circuit's large-signal waveforms and operating frequency. The method used in Microwave Office is highly sophisticated, easily accommodating even high Q oscillators. Accurate predictions of noise spectra result from a perturbation analysis of the oscillatory steady-state, making use of rigorous models of low frequency device noise.
Accurate prediction of oscillator phase noise has been an important challenge, as phase noise directly impacts the performance of digital communication systems. Typical effects, such as phase jitter and reciprocal mixing, where a neighboring channel mixes with the noise side bands of the local oscillator, are a direct result of oscillator phase noise and can produce interfering signals. Reciprocal mixing is particularly severe in multicarrier systems (such as orthogonal frequency division multiplex (OFDM)), where the carriers are closely spaced in frequency.
Figure 2 illustrates the capability to design an oscillator at the circuit level, then use the characteristics of that oscillator at the system level to analyze BER performance. A system-level simulation can automatically extract spectral information such as noise and spurious characteristics from a circuit model. This information is used in an end-to-end simulation to calculate desired system-level measures of performance, such as BER or EVM.
Microwave Office also allows the user to model DSP-based synthesizers such as digital phase-locked loops (PLL). Digital PLL techniques increase oscillator frequency resolution, reduce spurious effects and improve the noise characteristics of oscillators. A simple PLL-based frequency synthesizer is shown in Figure 3. This design shows a single-loop configuration containing a fractional-N divider. This loop is a time-domain model and can be used directly in the discrete-time-domain simulation at the system level. Alternatively, the frequency-domain characteristics of the PLL synthesizer output may be accurately measured and used to drive a frequency-domain behavioral model of the synthesizer, which can in turn be used in a time-domain simulation. The time-domain output waveforms from the behavioral models are illustrated in Figure 4.
In this case, the average noise power of the raw reference was adjusted to provide a very noisy waveform. The effect of smoothing of the PLL is obvious in this plot. Frequency-domain spectral characteristics are shown in Figure 5. The reference characteristic is a scaled version of the noise spectrum from the transistor oscillator circuit and the output spectrum is the phase-noise spectrum of a second-order phase-locked loop. Note that the effect of the loop filter has been to drastically reduce the high frequency phase noise.
INTEGRATED LOAD PULL WIZARD FOR POWER AMPLIFIER DESIGN
Microwave Office 2002 includes a new Load Pull Wizard that provides new simulation capabilities to the power amplifier (PA) designer. With the Load Pull Wizard the main independent parameter of the measurement is not frequency, power or bias, but the source or load impedance at the fundamental or a harmonic frequency, presented to the device under test (DUT). These impedances are generated by a virtual tuner element that sweeps the impedance, at a given frequency, over a range of values. The integrated load-pull system can generate harmonic load-pull contours using either measured data or linear/nonlinear device models. Figure 6 shows the Load Pull Wizard, where the user selects the tuner element and specifies the desired measurements and simulation points. The wizard then automatically runs a series of linear or nonlinear simulations and generates the load-pull contours. Microwave Office is unique in incorporating measured load-pull data from measurement systems, including those from Focus Microwave Inc. and Maury Microwave Corp. PA designers can compare simulation results with measured load-pull data to develop better nonlinear models for high power transistors. Alternatively, the measured data can be used in Microwave Office, instead of device models, for designing matching networks.
FILTER SYNTHESIS WIZARD
A Filter Synthesis Wizard, shown in Figure 7, is another enhancement built into the Microwave Office design framework. Users activate the wizard by selecting it from the project tree and then proceed through a series of choices to generate lumped element or distributed filters including ideal transmission lines or physical structures. The wizard can synthesize Chebychev, Butterworth, Bessel, quasi-elliptic, linear phase and other filter topologies. One of the major advantages of the wizard is that the networks are created directly within the simulation environment rather than forcing users to employ a separate and often incompatible filter-design application. The synthesis routine uses the models and equations within the circuit simulator so users are never presented with conflicting results. The synthesis routines can generate planar physical structures including microstrip, stripline and coplanar topologies. It will also generate nonplanar physical structures, including dielectric resonator, slabline and coaxial filters. Once the network is synthesized, users can begin working with the results by immediately tuning, optimizing or laying out the filter, or combining it with other networks within the project space. The extensive libraries within Microwave Office, including popular surface-mount inductors and capacitors, can be used to implement the design, generate parts lists, and complete artwork and assembly drawings.
EM SOLVERS REDUCE SIMULATION TIMES
Enhancements to the electromagnetic simulator significantly reduce simulation times for large problems while also increasing the variety of structures that can be analyzed interactively. Microwave Office 2002 incorporates new iterative solvers that can process large matrices much more efficiently, often reducing simulation times by an order of magnitude. These enhancements make it practical to solve larger structures including those found in microwave monolithic integrated circuits (MMIC), low temperature co-fired ceramic (LTCC) packages and high speed printed circuit board (PCB) applications. Results are from the EM simulation.
RF SUBSYSTEM ANALYSIS CAPABILITIES USING BEHAVIORAL MODELS
While the discrete-time simulator is ideal for handling complex signal environments and time varying systems, its inherent feed-forward nature limits its ability to model RF interactions between components. However, for subsystem analysis, including SWR effects, users typically use frequency-domain simulation because of its ability to completely model interactions between components. Microwave Office 2002 includes an extensive new library of behavioral models for filters, amplifiers, mixers and other RF components. These elements are ideal for quickly establishing component specifications and making trade-offs at the system or subsystem level. In addition, with the direct integration of these models users can replace higher-level models with detailed transistor-level schematics as a design progresses from concept toward production. Figure 8 shows the two-tone IMD simulation of a Ku-band satellite receiver.
Microwave Office 2002 will be available to users in the fourth quarter of 2001. All of the enhancements to the electromagnetic simulator and circuit-simulation tools (including the add-on Filter Synthesis and Load Pull Wizards) will be a no-cost upgrade to existing users with valid maintenance agreements. The discrete-time simulation engine and DSP models can be purchased as either a stand-alone application or as an add-on license to an existing Microwave Office. Prices for the system/DSP simulation capability begin at US$12,000. For additional information contact AWR or a local sales representative.
Research Inc. (AWR),
El Segundo, CA (310) 726-3000.
Circle No. 301