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Design007-Feb2020

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28 DESIGN007 MAGAZINE I FEBRUARY 2020 Power integrity components—such as bypass capacitors, inductors, ferrite beads, or other small discrete components—can be character- ized in fixtures. There is a wide range of fix- tures available, from the professional and very accurate [1] to the home-made and very crude [2] . In between these extremes, you will find vari- ous printed circuit board fixtures, such as the decoupling test board kit shown in Figure 1 [3] or the RF experimenter board set shown in Figure 2. The Picotest boards come with Touchstone files for de-embedding the measured data. I particularly like this kit because it has sepa- rate small boards with solder pads specifically for a wide range of surface-mount component sizes, including reverse geometry capacitors and some medium-size bulk capacitors. The range starts with the 0201 size and includes 0204 and 0612 reverse-geometry sizes. Large- size polymer capacitors can be tested on the D-size (7343) fixture; for surface-mount cy- lindrical capacitors, we get 5-mm and 8-mm sites. To test filter structures, two of the small- er sizes (0402 and 0603) also have a generic T scheme: pads for two components in the series path and for one component in the shunt path. There is a dedicated site for single-body 0402- X2Y filter elements. This test board kit also has a single-piece open-short-load impedance ref- erence board; you can find it in the oversized lower-middle compartment in Figure 1. You can also build fixtures straight out of small co- axial connectors [4] . With all of these fixtures, we need to keep in mind that the current path around the device we characterize does not necessarily match the current path that is created by the layout and stackup in our final application. In the solder- wick fixture, the current path is highly uncer- tain; the shape of the flexible connections will vary depending on how we achieve the pres- sure-mount connection. With the fixtures built en- tirely out of SMA connec- tors, we have a fixed ge- ometry for the connector pieces, but there are no dedicated pads to solder the parts down, so the ac- tual current path depends on how we solder the DUT between the center pins and outer frame. This means that the extracted mounted inductance val- ues will need to be used with some caution. PCB Fixtures for Power Integrity Quiet Power Feature Column by Istvan Novak, SAMTEC Figure 1: Picotest component characterization fixtures.

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