Understanding S-Parameter Vs. Equivalent Circuit-Based Models For Surface Mount RLC Components

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. 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.