One of the challenges in developing high-accuracy compact models is enabling sufficient versatility for designers to select layout configurations that are different from the test fixtures used in the characterization and model extraction process. For modeling passive surface-mount components, series two-port fixtures are typically used to obtain the S-parameter data that are subsequently utilized in model fitting. These test fixtures have well-defined reference planes to establish the boundaries of the model. An example of such a fixture is shown in Figure 1; here linear tapers are used to transition from the 50 Ω feed-lines to the dimensions of the mounting pad stacks. In practice, designers are likely to use mounting layouts that vary in one or more ways from this configuration.
This application note focuses on co-simulation – which combines circuit and numerical electromagnetic (EM) simulation – of surface-mount capacitors in shunt configurations. This is an important topic especially for microwave power amplifier designers, who commonly use shunt-mounted capacitors in impedance matching networks that require very precise impedance transformations. The accuracy of the simulations is important at the fundamental design frequency as well as at several harmonics. One situation addressed herein is that when a portion of the mounting pad stack is embedded in the interconnect transmission line that runs orthogonally to the major axis of the component (Figure 2). A second configuration, sometimes used by designers for load sharing purposes, consists of two closely-spaced capacitors mounted side-by-side (Figure 2, with the second pair of pad stacks occupied). The distribution of current on the pad stacks, and how it enters the capacitor(s), varies between the series and shunt two-port fixtures and must be properly emulated in the simulator in order to accurately predict the circuit performance.