Page 22

EETE APR 2014

TEST & MEASUREMENT Impedances on the test bench By Dr. Thorsten Sokoll and Dr. Ove Schimmer The implementatio n 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 with inhomogeneous dielectric constants (DC) across the entire base material are employed. Moreover, the pressing of cores and prepregs to multilayer PCBs 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 PCBmargins and thus only partially Fig. 1: Block diagram of a TDR-based impedance measurement system. (all pictures: Sequid) 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. As a consequence, clock edges degrade and inter-symbol interferences occur, causing inacceptable bit error ratios and finally, performance degradation or even system malfunctions. Line impedances can be determined with a high degree of precision by means of a time domain reflectometry (TDR). TDR technology has been in use since the 1970s to detect faults in underground or submarine cables. Figure 1 shows the block diagram for a TDR-based impedance measurement setup. The TDR itself only consists of a voltage step generator and broadband sampler accompanied by a data acquisition unit. The basic measurement principle is as follows: the generator emits a step signal travelling via adapters, cables and a probe to the device under test (DUT). While interacting over the entire length of the DUT, the signal experiences partial reflections, which travel back to the detector and thus allow the spatial determination of the DUT’s wave impedance. Many people know this basic principle from radar applications, which is also the reason, why TDRs are frequently called Cable Radars. The rise time tr of the step signal determines the spatial resolution and should thus be as short as possible (for Sequid DTDR-65, this is tr ≈ 65ps, allowing a spatial resolution of approx. 5mm). The synchronisation between the generator and the sampler (which should feature an analogue input bandwidth of at least 10GHz) is crucial for low-noise operation, i.e. for jitter values of only some picoseconds. Ideally, a “real-thru” sampler is used; hence no external signal dividers or couplers are necessary. This is highly beneficial, since broadband signal dividers are usually built resistively and thus would add insertion loss and noise. Finally, a TDR features a data recording unit, usually implemented by a microprocessor or FPGA. High-frequency TDR-devices normally do not use real time, but sequential or random sampling techniques. Similar to stroboscopes, these permit the recording of rapidly changing periodic signals with reasonable technical effort. Data processing and visualisation are normally executed on a PC, which can be fully integrated in high-end instruments, or which can be connected via an USB or an Ethernet-connection. The adaptation of a measurement object to the TDR is a demanding task. So for example, precisely phase-matched cables and probes have to be used for Fig. 2: Reflectograms (TDR signals) of a disturbed (red) and undisturbed (green) transmission line. differential impedance measurements. If this requirement is not met, even- and odd-mode conversions will impair the measuring accuracy. In addition, the probe tips should be designed to match the DUT’s impedance as well as possible for most accurate measurements. Dr. Ing. Ove Schimmer co-founded Sequid GmbH in 2007 to become managing partner - www.sequid.de Dr. Ing. Thorsten Sokoll joined Sequid GmbH in 2009 and became managing partner in 2013. 22 Electronic Engineering Times Europe April 2014 www.electronics-eetimes.com


EETE APR 2014
To see the actual publication please follow the link above