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Using EDA tools to design and develop RF/microwave circuits helps reduce design iteration cycles and design time, which can translate into substantial R&D savings in today's fast-paced and competitive marketplace.

Today's designer requires a high degree of circuit simulation accuracy. This accuracy can be particularly hard to achieve as frequencies climb ever higher. A better way to achieve good results at higher frequencies is by performing electromagnetic (EM) simulation. When the circuit is a mixture of discrete and distributed components, however, EM simulation is not as easy as it is for distributed circuits. Designers find it difficult, and most of the time they avoid performing EM simulations on these types of circuits.

The main challenge is that EM simulators solve problems using Maxwell's equations, which solve the circuit in the physical domain. The user must separate the physical, distributed layout from the discrete components and then perform the EM analysis on each distributed section, finally combining the EM results with lumped elements in a schematic environment to see the composite result. This process can become complex and requires a lot of effort on the designer's part. It also introduces many potential sources of error. EM analysis can be particularly difficult in the case of multistage amplifiers or complex circuit designs where dividing the circuit is not as easy as with simpler circuits.

A method for performing EM simulations without dividing circuits into many parts is EM/circuit co-simulation, which carries out the EM and circuit simulation under a single simulation setup, and with little user intervention. In this article, we use Agilent's Advanced Design System circuit and simulation software for co-simulation of a single-stage amplifier.

When using this co-simulation method, it is important to place the layout component in the schematic with an exact idea and description of the pins on which various lumped elements need to be connected. In this case, layout look-alike components are much easier to use than traditional black-box representations, which are difficult to understand and set up and require the user to remember the various pin details in order to connect the correct discrete components. Layout look-alike components help with proper visualization of the layout and allow the user to easily identify the location for the various discrete elements and perform the simulation while keeping potential errors to a minimum.

Designers must take care when connecting discrete components to the physical layout during the simulation, so that parasitics resulting from unwanted coupling between the ports are taken into account. After assigning all the port properties, the user can start the simulation. The simulation automatically uses the EM solver to simulate the distributed components, the circuit simulator to simulate the discrete parts, and then displays the composite results.

Figure 1 shows how a layout look-alike component can help the designer to set up composite simulations. The black shapes in the figure show the distributed part of the amplifier design and show how the discrete components are connected to the layout. The setup is simple, and the user can easily identify the sections of the layout for the appropriate discrete component connection.

Figure 2 compares the measured and simulated (circuit as well as EM/circuit co-simulation) gain performance of the example C-band, single-stage amplifier. The plot shows that co-simulation results match measured results quite well. It also shows that co-simulation predicts the amplifier's gain behavior much better than does the circuit simulation.

In conclusion, EM/circuit co-simulation can help designers to achieve accurate results without too much complexity in terms of circuit setup, and it reduces sources of error and saves time that otherwise would be needed for post-production tuning to achieve the desired results.

The EM/circuit co-simulation technique could be further extended to help designers who find it hard to simulate cascaded layouts where the substrate material is different for the cascaded circuits. This utility helps designers get the required cascaded results under one simulation setup without performing a number of EM simulations separately and combining them into a schematic by importing their S-parameter files to see the composite results.

Anurag Bhargava is an applications engineer with Agilent Technologies' EEsof EDA division.

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