Design007 Magazine


Issue link:

Contents of this Issue


Page 23 of 66

24 The PCB Design Magazine • May 2015 Surprising me on a regular basis is the way that demand for impedance control continues to extend into a broader range of product. Af- ter some 20 years of involvement in this field, I would have expected that everyone who need- ed to consider impedance control would have the capability nailed. Fortunately, that's not the case, and a steady stream of "how to" questions keep coming my way. The positives for new fabricators and design- ers lie in the fact that, even though impedance control may be new to them, there is a wealth of information available. Some of this informa- tion is common sense and some is a little coun- terintuitive. So, this month I'd like to go back to the fundamentals, and even if you are an ex- perienced hand at the subject, it can be worth revisiting the basics from time to time. Standards It's worth remembering that traceable ref- erence impedance standards are available and all impedance measurement systems should be able to be compared to a traceable standard. The reference air line is the de facto standard for impedance—a precision manufactured co- axial standard "air" referring to the fact that air is employed as the dielectric in this type of pre- cision reference line. Because a closed-form equation exists for the impedance of a coaxial structure, air lines may be calibrated by national standards bodies who can strip down the line and make precise measurements of the internal diameter of the outer body and the external diameter of the in- ner conductor. The bore measurement is usu- ally made with a technique called air gauging, by Martyn Gaudion POlar InsTruMenTs THE PuLSE Impedance Control, Revisited feature coulmn

Articles in this issue

Archives of this issue

view archives of Design007 Magazine - PCBD-May2015