Impedances on the test bench

March 14, 2014 //By Dr. Thorsten Sokoll, Dr. Ove Schimmer
Impedances on the test bench
<p>Almost all designers and producers of digital high-speed systems are about to face the challenge of having to consider high-frequency design aspects. Specifically, this is about optimal data transfer being only possible on transmission paths with correct impedance ratios. Learn more about adequate measuring technology here.</p>

The implementation of broadband impedance-controlled systems challenges designers, manufacturers, and quality assurance managers of the central electronic building component: the printed circuit board (PCB). This does not stem from a lack of electromagnetic design knowledge, but from the enormous price pressure in the PCB industry: i.e. adequate radio-frequency (RF) base materials – which are quite justified at clock rates in the Gigahertz range from the developers' point of view ‑ are hardly ever used. Instead, low-cost FR4-materials, exhibiting inhomogeneous dielectric constants (DC) across the entire base material, are employed. Moreover, the pressing of cores and prepregs to multilayer PCBs – (these being mandatory, e.g., in most sophisticated embedded systems and backplanes) – causes geometrical inhomogeneities, adding another source of uncertainty. However, in order to meet specified tolerances, many PCB manufacturers offer inspection of line impedances, which, in turn, requires additional impedance test coupons. These are usually located at the PCB-margins and thus only partially represent the characteristics of the actual interesting transmission lines distributed all over the produced panel. In the worst case, the measured test coupons may be within the specified range, whereas the actual interesting lines are not.

Impedance fluctuations are often not tolerable

In addition to material and production specific variations, design specific ones (e.g. layer changes, too small distances to GND-planes, PCB borders, or other transmission lines) may occur as well, which eventually result in intolerably fluctuating transmission path impedances. In consequence, clock edges degrade and inter-symbol interferences occur which, in turn, cause inacceptable bit error ratios and, finally, performance degradation or even system malfunctions.

 

Figure 1. Block diagram of a TDR-based impedance measurement system. (all pictures: Sequid)

Line impedances can be determined with a high degree of precision by means of a time domain reflectometry (TDR). TDR technology has already been used for detecting faults in underground or submarine cables since the 1970s, where faults can simply be interpreted as large impedance variations. Since

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