circuit design For example, in designing a laptop
computer, a team responsible for signal and power integrity might lay out a circuit board that minimises signal interference along a high-speed channel. But this same layout might lead to two high-power components operating in close proximity to one another, causing signifi cant overheating. Similarly, a thermal via might be placed under a high-power device to improve conductive heat transfer, but doing so might lead to signifi cant interference with the signal entering or leaving the IC. EMI/EMC engineers might want to reduce the size of an air vent to decrease electromagnetic emissions, but doing so creates the risk of overheating and product failure. Similarly, the group responsible for thermal design might add a heat sink on an IC, but this
and weight, but expose the computer to damage from shock and vibration during regular usage. Such system interdependencies make
it essential that electrical and mechanical engineers work together to optimise system performance rather than each optimising the product for their particular component or discipline. Simulation to address electronics
design can begin as soon as engineers have created the electronics circuits and long before a physical prototype exists. The engineering team can use a circuit simulator to model the behavior of the circuit down to the level of the amplitude and timing of individual pulses. Then, after the physical layout is complete, they can perform a 3D electromagnetic simulation to identify all the
SYSTEM INTERDEPENDENCIES MAKE IT ESSENTIAL THAT ELECTRICAL
AND MECHANICAL ENGINEERS WORK TOGETHER TO OPTIMISE SYSTEM PERFORMANCE RATHER THAN EACH OPTIMISING THE PRODUCT FOR THEIR PARTICULAR COMPONENT OR DISCIPLINE
puts increased weight on the chip, which can result in damage during shipment. In addition, the heat sink can act as an internal antenna that increases electromagnetic emissions. And the cycle continues… adding a fan to improve air circulation can increase noise and power consumption; increase the unit’s size and weight and reduce product reliability due to fan failure. Changing the laptop’s overall dimensions or even the materials it is made of may reduce costs
radiation released by the prospective design. Next, these stray signals can be added back to the circuit simulation to determine their precise effect on the product’s operation. Other simulation tools address a variety of issues involved in product design such as cooling and structural integrity. In the best practice scenario, these simulation tools are harnessed together. To add these extra interdisciplinary aspects means moving beyond the
capabilities in SPICE or similar simulators. What this often means is adding fi nite- element (FE) analysis to the mix. A number of approaches from various software houses have arisen: from traditional simulators adding FE capabilities to the opposite where FE software adds SPICE capabilities.
Some commercial approaches Agilent, for example, has assembled a set of packages that together allow you to create 3D components such as packages, shields, connectors and surface-mount components (using EMPro) that can be simulated (using either the fi rm’s FE or FDTD (fi nite difference time difference) simulator elements) together with 2D circuit layouts and schematics within the Advanced Design System (ADS) environment, which is targeted at RF/ microwave and high-speed designs. In EMPro you can draw arbitrary 3D structures or import CAD fi les. 3D structures can be analysed in that software using the same simulators available in ADS. The fi nite- element method is widely used for RF/ microwave applications, but for electrically large problems such as antennas and some signal-integrity analyses, the FDTD simulator presents a useful alternative. Next, parameterised 3D components created in EMPro can be placed in a layout design in ADS, where the FEM simulator models the combination of the 2D layout and 3D EM components. Example applications include the design of RF modules, which are typically constructed from multilayer ceramics or laminate dielectric material with embedded RF passive components between the layers. Another is RF components such as resonators, which are sensitive to interactions with the surrounding PC board traces and vias. Further, with data rates increasing, board traces must now be analysed as RF transmission lines, and for high-speed connector types supporting gigabit rates you can generate high- frequency S-parameter models for use in ADS.
Agilent offers multiple products that combine to form a circuit-analysis platform that considers far more than just traditional circuit models
www.scientific-computing.com
Importing SPICE into a multiphysics platform As noted earlier, the limitations of SPICE component models mean they simply can’t handle every aspect that arises in the real world. If these components are critical to system operation, it would be nice to be able to substitute a more accurate FE component model for that critical SPICE model. That’s exactly what Comsol allows users to do
DECEMBER 2011/JANUARY 2012 35
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