Impedances on the test bench
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 then, a lot of applications for fields as different as geology and food technology have been addressed.
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 utilize 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 by a PC, which is fully integrated in these in case of high-end TDRs and just attached via e.g. an USB- or Ethernet-connection in case of more economic ones.
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 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.
Figure 2. Reflectograms (TDR signals) of a disturbed (red) and undisturbed (green) transmission line
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 impedance-matched 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 cost-efficient to extremely expensive 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 product-specific measurement technology, 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 which is suitable for impedance measurement of differential and single-ended transmission lines up to approx. 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. Reflectograms of RG 405 coaxial cables with properly (1, green) and erroneous (2, red) installed SMA jacks.
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 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.
Figure 4. Reflectogram of a differential line routed on two different layers on a FR4 substrate.
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 approx. 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 time-consuming, tedious, and thus unpopular work is dramatically simplified by the included automatic reporting tool, reducing the effort for creation of extensive graphical and statistical evaluations to a few clicks. Additionally, an online impedance calculator for the most common line types is available.
Figure 5. Reflectograms of an USB 3.0 adapter with an open circuit (3) and two different USB 3.0 cable assemblies (4 and 5)
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 (figure 6). The DTDR-65 also features excellent electromagnetic shielding and can ‑ due to its compact form factor and low weight – comfortably be used in mobile applications. In addition to this, its battery-operability and high IP-class allow for measuring of RF cables and connectors in outdoor applications (e.g. in control cabinets or outside facilities) with a consistently high quality.
Figure 6. Different probes and accessories for the time domain reflectometer DTDR-65.
About the authors:
Dr. Ing. Ove Schimmer studied and acquired his doctorate at the Christian-Albrechts-Universität in Kiel/Germany where he researched the applicability of high-frequency dielectric spectroscopy for the determination of food quality until 2007. In 2007, he co-founded the Sequid GmbH in Bremen/Germany and has since been working there as managing partner.
Dr. Ing. Thorsten Sokoll studied electrical engineering at the TU Braunschweig/Germany and Optical & Electrical Engineering at the University of Rhode Island/USA. He acquired his doctoral degree at the TU Hamburg-Harburg/Germany in the field of microwave measurement systems for in-situ monitoring of buildings. He has been with Sequid GmbH since 2009 and became its managing partner in 2013.