Understanding S-Parameter vs Equivalent Circuit-Based Models for RLC Component Simulations With NI/AWR Microwave Office

The S-parameters provided commonly in the industry for passive RLC components are typically obtained using a split-block fixture and loose devices. However, practical circuits do not use components in this environment; instead they are soldered onto custom pads on a specific substrate. Pad geometry, substrate characteristics and other standard assembly features have significant effects on the performance of a surface mount component, especially at increasing frequencies. A more representative test fixture for obtaining S-parameters of a surface-mount component is to solder mount it on a substrate along with microstrip interconnects. Figure 1 shows the variation of S-parameters for a SMT inductor as measured on different substrates along with S-parameters downloaded from the vendor’s website. Note that all data are in agreement below 500 MHz, but at higher frequencies significant differences emerge. The frequency at which such substrate-dependent variations emerge will vary with component type, size, value and mounting configuration. Also shown in Figure 1 is how one might approach construction of a simple equivalent circuit model for such an inductor (Fig. 1b). Two different fixturing approaches are also suggested with the solder mounted approach (Fig. 1d) being preferred over the commonly used split block approach (Fig. 1c) for development of scalable models in-line with PCB design applications. S-parameter models are most commonly based on a standard file format known as “.s2p” or “Touchstone” file format. The data in these files represents the 2-port (or N-port) S-parameter data for a discrete set of frequencies from some practical start frequency and stop frequency. Common S-parameter start frequencies are 10 MHz or 50 MHz (sometimes lower, sometimes higher) to some upper frequency typically set by instrument or fixture limitations. Common upper frequencies for vendor S-parameters for surface mount devices are 3, 6 or sometimes 18 GHz, occasionally higher. S-parameter files and Sparameter-based models are sometimes provided by Modelithics. Our standard approach is to measure the device in a manufacturer’s suggested surface mount configuration using RF probing and custom calibration standards. Detailed information about the fixture and reference plane locations are provided,
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