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8 CHANNEL PC OSCILLOSCOPE For just ¤1688 • High resolution • USB powered • Deep memory INCLUDES AUTOMATIC MEASUREMENTS, SPECTRUM ANALYZER, SDK, ADVANCED TRIGGERS, COLOR PERSISTENCE, SERIAL DECODING (CAN, LIN, RS232, I²C, I²S, FLEXRAY, SPI), MASKS, MATH CHANNELS, ALL AS STANDARD, WITH FREE UPDATES 12 bit • 20 MHz • 80 MS/s 256 MS buffer • 14 bit AWG www.picotech.com/PS282 Fig. 6: Different probes and accessories for the time domain reflectometer DTDR-65. impedance of both RG 405 cables is Z0 ≈ 51.5 Ω, whereas the transitions in the area of the connectors vary strongly. In case of the improperly installed connector, a capacity drop (deformation towards low impedance) is visible. Such effects frequently occur when the outer and inner conductors are mounted too closely together (i.e. a capacitor was built). Figure 4 shows the impedance curve of a differential transmission line on a printed 4-layer test circuit. The transmission path starts as a microstrip line in layer 1 (top layer), changes through a via into layer 2, where it continues as a stripline, and comes back to the surface in layer 1 through a second via. This is repeated and the line finally ends in layer 1. Obviously, the test circuit does not reach the target impedance of 100 Ω: the microstrip and stripline feature impedances of Z0 ≈ 120 Ω and Z0 ≈ 110 Ω, respectively. The capacitive influence of the vias that may – especially at high data rates – severely impair the signal integrity in real systems is clearly visible here. As a last example, figure 5 shows the reflectograms of USB 3.0 connectors and cables. The specified impedance of USB 3.0 components is Z0 = 90 Ω ± 7 Ω. The TDR device still works on a reference impedance of 100 Ω (time range t < 12.2ns). The first reflection, caused by the transition from the test adapter to the USB 3.0 jack, occurs at approximately 12.3 ns and is, as expected, identical for all measurements. Curve 3 (green) illustrates the result of the open-ended adapter, with the fast impedance rise indicating the (high impedance) end of the adapter. Curves 4 and 5 (red and blue) represent two different USB 3.0 cable assemblies, each consisting of an adapter and a subsequent cable. While the cables are both within the specifications, the adapters are not. Especially, the one represented by the red curve shows a maximum impedance of approx. 122 Ω, causing severe reflections, which, in turn, may result in data rate reductions by USB 3.0 controller. In summary, all examples clearly show that developers can acquire a deep and intuitive overview of transmissions paths with a DTDR-65. The tasks of developers and quality inspectors usually include an easy-to-understand documentation of the achieved results. This very important but unfortunately timeconsuming, tedious, and thus unpopular work is dramatically simplified by the included automatic reporting tool, enabling to create extensive graphical and statistical evaluations with only a few clicks. Additionally, an online impedance calculator for the most common line types is available. Broad application spectrum The necessary accessories comprise high quality phase-adjusted coaxial cables as well as TDR probes for different kinds of applications: industrial probes for serial measurements in production processes and highly precise ones for R&D – see figure 6. The DTDR-65 also features excellent electromagnetic shielding and can comfortably be used in mobile applications, battery-operated. www.electronics-eetimes.com Electronic Engineering Times Europe April 2014 25


EETE APR 2014
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