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EETE APR 2014

TEST & MEASUREMENT Different systems on the market In the increasingly fast digital world, the measurement of line impedances has turned out to be the most important TDR application at this time. Figure 2 shows examples of such space-resolved measurements for undisturbed (green curve) and disturbed (red curve) transmission lines. Only transmission paths on which all components (not only including etched lines but also cables, connectors, and even terminating resistances within integrated circuits) are impedancematched allow reflection-free signal transfer between transmitter and receiver and thus highest bit rates. Impedance control therefore is an important aspect in evaluating signal integrity on both differential and single-ended lines. Developers and manufacturers can choose from a large variety of differential TDR systems (DTDR) for impedance control: from costefficient to extremely expensive Fig. 3: Reflectograms of RG 405 coaxial cables with properly (1, green) and erroneous (2, red) installed SMA jacks. ones. Some refined high-end TDR systems are offered by renowned measurement technology manufacturers. They are found in the area of high-speed oscilloscopes and available in combination with necessary accessories such as (D)TDR probes. These devices are very well suited for measuring transmission systems up to the 20 Gbit/s range and beyond. However, the impedance control seems to be only a niche market for the high-end device manufacturers. In consequence, no dedicated industrial solutions are offered and potential users are in danger of quickly getting lost in the jungle of the general RF-measurement technology before they reach the final goal of “impedance measurement”. Moreover, all of these systems belong to the high-price segment due to their high performance and general usability, making an investment unattractive especially if the TDR is not used continually. Less versatile TDRs are found in the area of industrial and productspecific measurement technology, Fig. 4: Reflectogram of a differential line routed on two different layers on a FR4 substrate. Fig. 5: Reflectograms of an USB 3.0 adapter with an open circuit (3) and two different USB 3.0 cable assemblies (4 and 5) where certain standard procedures have been established during the last two decades. These devices and the accompanying software are optimised for measuring impedances of test coupons and deployed by many PCB manufacturers. However, these TDRs are less suited in connection with design and testing of a random transmission line within a PCB. The reasons for this are the lack of appropriate probes and – even more severe – the too-low signal bandwidth caused by a too-slow signal rise time tr, which, in turn, only allows characterisation of lines with a minimum length of approximately 10cm. As a third version, there are “self-made” solutions. For these, there are some few cost-efficient (D)TDR devices on the market. Purchasing further components (TDR-probes and phase-adjusted cables) would thus generally meet the technical prerequisites. However, in this case, suitable software for data recording, error reduction, impedance calculation, and result documentation must be developed, so that it is questionable whether a solution from one source would finally not be more cost-efficient and safer. Sequid GmbH originally developed high-resolution and precise TDR-systems for determining the quality of fish and meat. In cooperation with the German PCB manufacturer Elekonta Marek GmbH, the existing basic technology was developed further to a very high-performance system (Sequid DTDR-65) fulfilling all demands of impedance control measurements. It is a highly stable differential time domain reflectometer suitable for impedance measurement of differential and single-ended transmission lines up to approximately 10 Gbit/s. It features a 65 ps step signal generator and thus allows for high-resolution measurements not only of test coupons but also in the real circuit. Furthermore, the DTDR- 65 exhibits an extremely good jitter performance (Jrms < 500 fs) usually reserved for high-end devices. At the same time, a software solution was developed enabling even non-RF-experts to perform impedance measurements. It contains not only basic functions (such as device control) but also intuitively operable functions for displaying line impedances. Tolerance masks make it easy to make PASS/FAIL statements. Below, some simple application examples are presented. Figure 3 illustrates reflectograms of RG 405 coaxial cables equipped with the SMA jacks being installed with (1) and without (2) adherence of assembly specifications. The line 24 Electronic Engineering Times Europe April 2014 www.electronics-eetimes.com


EETE APR 2014
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