This technical note is also available in PDF Format: S-Param Accuracy PDF (22k)
Johanson Technology offers MLCSoft® software which displays S-parameter data and SPICE models for our High Frequency Multi-layer Ceramic Capacitor lines. We have found that many factors can influence the actual capacitor S-parameters encountered in the final application. The S-parameters measured are actually very accurate for the fixturing used in the test at our factory. We presently have two fixtures, where one is for series-through configuration (the one that we use most), and one is shunt-through. Below we list three important points related to the accuracy of S-parameter data.
a.) The S-parameters encountered in a given application will differ from those reported because the individual application will have many factors different from the fixture used. These include line width, shunt vs series configuration, PCB epsilon (especially when used in series configuration), and soldering quality. To fixture for each possible permutation is obviously impossible, but we are trying to get a better handle on how to report the differences (see the last paragraph below). For example, the Series Resonant Frequency (SRF) could vary 10-20% (as indicated by the S11 magnitude dip) when the PCB material is changed while the capacitor is measured in a series configuration. At the higher microwave frequencies the capacitor, its mounting, the PCB material, etc. make up a "network" that defines the SRF, and the SRF of the capacitor by itself has no clear definition anymore.
b.) The MLCSoft® S-parameter files are generated with curve-fitting formulas. This is due to the fact that presenting the S-parameter data in raw form with look-up table would require the MLCSoft® file to be significantly larger and the accuracy gained would be mostly insignificant due to the inaccuracies mentioned above. In addition, the fixture effects used during measurement are mathematically removed in the .S2P files.
c.) Using S-Parameter data to derive ESR is not reliable since the accuracy of this data is limited by the accuracy of the network analyzer in a 50 ohm system (a typical amplitude calibration of ± .05 dB is not accurate enough in the low-loss region of a capacitor which could be .01 dB). For this reason, ESR and Q parameters presented in Johanson Technology’s MLCSoft® program are based on actual resonant line measurement methods.
In the near future, we intend to have a new version of our software with S-parameters available. It will have settings for the most popular mounting configurations by mathematically manipulating the underlying S-parameter formulas.
Last Updated: April 1, 1999
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