Virtual prototyping: this extra step can virtually pay for itself in time and money, from simulation through DFM.

EDA tool vendors have continued to expand their R&D innovation, even during parts of the recent recession. The main reason for this? Design complexity and time-to-market continue to ride on the heels of technology. Factors include innovations in silicon, improved techniques for design and PCB manufacturability, and the expectation of near-future capital equipment spending from end-users. How are we to keep pace with these trends?

Consider computer automation tools for virtual prototyping your design. In 2002, Gartner Dataquest reported that 38% of electronic designers used breadboarding as their means of design, with a high percentage of these likely to be analog engineers. But today a lot can be gained by simulating analog and mixed-signal circuitry, beginning with schematic creation through fault simulation. Don't worry; lab prototyping is still a viable design route. But you'll eliminate much of the headache associated with the lab by first using EDA tools to virtually prototype your work.

By the end of this article, you should have a better under standing of the benefit of virtual prototyping, and be able to weigh this against the disadvantages of not doing so.

What is Virtual Prototyping?

Virtual prototyping (VP) is a term that typically refers to wringing an electronic design through its paces to ensure compliance during the PCB design and fabrication processes. But I'll extend its definition several steps back. VP is a way to mimic the operation of a real-world design on a computer, debug the design using any number of techniques, analyze it for production and field worthiness, and ensure integrity between PCB design and fabrication.

Importantly, VP encompasses making your design reliable for the field. After all, you can ace all the design, signal integrity and manufacturability requirements for final PCB fabrication, but if your electrical design can't tolerate real-world woes like temperature and component tolerance variation, fault tolerance, worst-case timing and acceptable production yield, then even the best board layout won't see the inside of a chassis for long. Even if you make a lab prototype board in lieu of VP, or as a means to verify your simulated design, that board is set up for only one set of component values, temperature and layout conditions.

Virtual design creation and simulation begin by building a schematic, typically dropping pre-existing component models onto a drawing from a well-categorized selection of several thousand parts. Software models contain all information for the simulator to reproduce the design's real-world operation. Wires, busses and test points are quickly connected across the drawings, while several varieties of input stimulus (i.e., piece wise linear, sine wave, arbitrary wave, single-frequency frequency modulated, and digital pulse) are prescribed by the engineer. Once the simulation engine ascertains the design's operation, the engineer can point and click anywhere on the schematic to view electrical signals, which are brought to view in a waveform-viewing application. The waveform-viewing tool accommodates several analyses. Examples include time domain (analog and digital simulators) and frequency domain, DC operating point, and DC sweep with analog simulators.

Also, different design versions can be quickly constructed from a base schematic and recalled for different tasks such as initial debug, regression testing, PCB layout, and high reliability. Importantly, the schematic entry tool enables special operations such as copy, cut, paste and rotate on components, and displays things like DC operating point, special artwork and waveform thumbnail plots directly on the drawing. These tools also enable efficient design control and component management, which can otherwise be an involved task prone to error.

Next, several debugging tools enable fast change of component values or parameters, and re-simulation of the design. New waveforms are brought to view in a new plot, or can replace old data. A sweep is performed by quickly filling in a dialog box that will alter a component's value or parametric data in a stepped fashion (e.g., 100--1,000 [OMEGA] in 100 [OMEGA] increments). This capability can be extended to a "nested" sweep, which performs a complete sweep, but during each assigned step of another component's sweep. Families of waveforms are generated, and often component values are displayed per corresponding waveform.

Next, a special design optimization feature in analog simulators automatically calculates passive component values (e.g., resistor, capacitor) based on design criteria the engineer prescribes. Examples include 45[degrees] phase margin, amplifier gain of 100, filter bandwidth of 12 kHz and minimum overshoot of 250 mV.

Other steps are eliminated using VP, such as incorporating lead times for initial parts, management of messy schematic respins and versions, and the need to maneuver several pieces of equipment (signal generator, scope, signal analyzer, power supply and spectrum analyzer), as well as lab instruments' capturing of signals at critical junctures. Using VP, all signal analysis data is available within the waveform viewing application (in one window), and at any time or frequency prescribed. Some analog EDA providers also supply virtual benchtop instruments for all the aforementioned equipment. These mimic the look and operation of actual lab gear, but on your computer.

Looking further, a full waveform-processing tool can perform complex calculator functions on waveforms, such as copious mathematical operations, signal filtering, Fourier transformation, Bode plot, gain and phase margins, and other signal conversions. This conserves design time by providing "what if" analysis on signals without having to first create corresponding circuitry. A good waveform viewer also provides numerical data like delta between multiple signal edges, average voltage, peak current and rise time. Some analog/mixed-signal viewers can even plot two independent variables against each other, such as Beta vs. temperature.

Necessary Components

So, what happens if an EDA tool provider doesn't furnish the component models you need? It's not always a pretty picture, but vendors do provide several model-building tools for this situation. For analog design, attachment of an existing or new graphic shape to a component's simulation model--found on the IC vendor's Web site--is possible. You can also enter a manufacturer's datasheet specs into a dialog box, which produces a corresponding model for discrete parts such as BJT, SCR, MOSFET, DIAC and others. Those parameters can also be modified to form a special rendition of a part. C-code and B-element are other modeling tools for the construction of analog components. Sometimes you can even get away with using an available model with behavior similar to the part you desire. In the digital domain, silicon (i.e., IC/ASIC, FPGA and sub-micron) vendors furnish design kits containing a wealth of component models that are compliant with foundries. VHDL and Verilog languages also provide modeling flexibility for large-scale digital and mixed analog/digital design. A hardware-modeling box allows placement of a VLSI part into a test bed for inclusion in a design simulation.

Designs in multiple engineering domains can be tricky to build in the lab for initial analysis. But most analog tool providers furnish a host of corresponding components and systems for simulation, and they can be combined with analog circuitry. Functions include motor, torque, mathematical operation, logic function, polynomial expression, spring, transfer function, load, torque, coefficient of friction, pressure, summation and multiplexing. This makes initial systems design quite doable.

Design Verification

Design automation tools help to ensure your virtual prototype is production-ready in several key ways. For analog design, this starts by furnishing inter-component sensitivity analysis, which indicates how sensitive a component is in relation to another component's change in signal behavior. This can help isolate which parts will be most vulnerable in critical areas of the design. Noise and distortion analysis can study these characteristics that are internal to certain types of analog components. Pole-zero analysis, two-port analysis and Smith charts all provide insight to circuit stability and transmission-line effects. In digital design, static timing analysis explores timing violations and synchronization problems among signals and components. Most of these analyses are impossible to implement by way of a prototype board.

Adding to the value of VP, analyses like Monte Carlo, worst-case and extreme value for analog, and mixed-signal design attempt to vary component values and/or parametric data (i.e., transistor beta and operating temperature) in ways that impose statistical and worst-case conditions found in production and the field. Curve families, histograms, cumulative density plots and numerical data all provide insight as to how a design stacks up to real-world variation. The engineer can go back and refine any portion of the circuit that needs better compliance, thus minimizing production respins and field failures.

Finally, fault synthesis and test-vector generation furthers VP design testing. In both the analog and digital world, this is accomplished by making final product fault-isolation capable. Highs, lows, "stuck-at" value and other conditions are assigned across the design. The conditions are simulated, and components and circuit nodes that have fallen out of tolerance are flagged, along with faults found during the test. For analog design, pass/fail limits are set when the engineer uses intuition or product specification, or perhaps by first running a Monte Carlo or worst-case analysis to gain a feel for signal variation limits at key nodes, as components are adversely varied through a number of runs.

When you're convinced your virtual prototype works, you can either construct a PCB or first build an actual prototype board for testing. With the latter option, you're usually 100 miles closer to your final design after running DFM simulation and verification analyses. You will also have saved a lot of time and eliminated lots of messy work on your prototype board.

PCB layout, route, DFM, DFA and signal integrity (SI) analysis for PCB traces encompass enough subject matter for dozens of articles. Briefly, all these fancy acronyms culminate to provide proper PCB manufacturing compliance, while reducing a multitude of electrical in effects that can occur in PCB traces. Factors affecting manufacturability include component height restriction, critical placement of heat/electrical sensitive parts, placing bypass components close to power supply pins, proper copper thickness to prevent overheating, minimum spacing between pads/traces, conformal coating for environmental adversity, trace distance from board edge, separating analog, digital ground routing and more.

For digital designers, SI tools simulate pulse streams from an equivalent output driver of a digital device, to a respective receiver on the other end of a trace. SI ultimately measures and displays parasitic effects in traces, including propagation delay, line capacitance, inter-trace mutual inductance and crosstalk. The engineer or layout designer can input design rules and electrical rules into any PCB-centric tool, as violations will be automatically flagged for manufacturability or electrical noncompliance.

Of course, analog designers would like to for the resistance, capacitance, inductance, impedance and crosstalk from SI analysis to be back-annotated to the schematic in the form of analog models and transmission lines for simulation with the original circuitry. These parasitics can be included with the actual circuit design. Well, that isn't happening yet with VP for two reasons. First, analog components have different driving, receiving, capacitive and impedance levels than do digital parts, so digital SI tools, per se, do not apply. Second, to my knowledge there is no automated back-annotation of electrical components into analog schematics. More EDA ideas for the future.

In summary, if you're still just prototyping your design, that may be OK for simpler designs, or those that demonstrate familiar operation or are production/field-compliant. But for more complex, cutting-edge designs, virtual prototyping can be a tremendous aid in maximizing final product reliability. PCD&M

TIM GHAZALEH is marketing director for Intusoft. He has been involved in EDA marketing for nine years, and engineering for robotics, instrumentation and test for eight years. He can be reached at [e-mail].