Physical principles of interface design

In the past, the test interface did not need to be modeled well, operating at low speed. As bandwidth increases past 1 GHz, the interface needs to be modeled according to its distributed properties. By comparing S-parameters of a correct model with an ostensibly similar RC filter model, the author critiques the suitability of oversimplifying the test interface model. Bandwidth limitation in PCBs, as caused by the physical properties of materials and characteristic trace geometry, is examined as the central theme. The two most significant material effects, conductor and dielectric losses, are isolated, modeled, and derivations of the frequency and step responses are presented. As mismatches are reduced, it is shown that these losses become the dominant limitations to bandwidth until a change to superior dielectric material in load boards and probe cards is made.