Impedance test scheme on the test platform

The realization of the broadband impedance controlled system poses a daunting challenge to the designers, manufacturers and quality assurance managers of the central electronic building component-the printed circuit board (PCB). This challenge is not due to lack of electromagnetic design knowledge, but also from the huge price pressure in the PCB industry: that is, the ideal radio frequency (RF) substrate that is perfectly suitable for the clock rate in the GHz range in the eyes of developers has hardly been used.

In contrast, low-cost FR4 materials with non-uniform dielectric constant (DC) throughout the substrate are often used. In addition, pressing materials and prepregs into a multilayer PCB often results in geometrical unevenness, which further increases uncertainty. However, in order to meet the specified tolerances, many PCB manufacturers provide inspection services for line impedance, and then require additional impedance test boards. These test boards are usually located on the edge of the PCB, so they can only partially represent the characteristics of the actual transmission lines of interest distributed throughout the production panel. In a bad case, the test board under test may be within the specified range, but the actual transmission line of interest does not meet the requirements.

Impedance fluctuations are often intolerable

In addition to special changes in materials and production processes, design parameter changes (such as layer changes, too short distances to GND planes, PCB boundaries, or other transmission lines) also occur from time to time, eventually leading to intolerable transmission line impedance fluctuations. The consequence of impedance fluctuation is clock edge degradation, inter-symbol interference, and unacceptable bit error rate, which eventually leads to performance degradation and even system failure.

Through time domain reflectometry (TDR), the line impedance can be determined with high accuracy. TDR technology has been used since the 1970s and is mainly used to detect faults in underground or submarine cables. Figure 1 shows a block diagram of an impedance measurement device based on TDR technology. The TDR itself only contains a voltage step generator and a broadband sampler with a data acquisition unit.



Figure 1: A block diagram of an impedance measurement system based on TDR technology. (All photos are from Sequid)

The basic measurement principle is this: a voltage generator generates a step signal, which is transmitted to the device under test (DUT) through an adapter, cable and probe. When the interaction occurs over the entire length of the device under test, the signal will undergo partial reflection and be transmitted back to the detector, thus realizing the spatial measurement of the waveform impedance of the device under test. Many people understand this basic principle from radar applications, so TDR is often referred to as cable radar.

The rise time tr of the step signal determines the spatial resolution, so it should be as short as possible (for Sequid DTDR-65, tr≈65ps, so the spatial resolution is about 5mm). The synchronization between the generator and the sampler (with an analog input bandwidth of at least 10 GHz) is essential for low-noise operation (that is, jitter values of only a few ps). The ideal is to use a "true straight-through" sampler, without the need for external signal separators or couplers. This benefit is obvious, because broadband signal splitters are usually resistive, which increases insertion loss and noise. , TDR instrument also has a data recording unit, this unit is usually realized by a microprocessor or FPGA.

High-frequency TDR equipment does not use real-time sampling technology under normal circumstances, but uses sequential or random sampling technology. Similar to stroboscopes, these devices can record fast-changing periodic signals with reasonable technology. Data processing and visualization tasks are generally performed on a PC, which can be fully integrated in high-end instruments, or connected via USB or Ethernet.

Measuring the adaptation of objects to TDR is a very demanding task. For example, differential impedance measurement must use high-precision phase-matched cables and probes. If this requirement cannot be met, the even mode and odd mode conversion will reduce the measurement accuracy. In addition, the probe head should be designed to match the impedance of the device under test in order to achieve possible accuracy measurements.

Different systems on the market

In the increasingly fast digital world, the measurement of line impedance has been shown to be an important TDR application at present. Figure 2 shows an example of this spatially resolved measurement for transmission lines without interference (green curve) and interference (red curve).



Figure 2: The reflection of the RG 405 coaxial cable when the SMA connector is installed correctly (1, green) and the SMA connector is installed incorrectly (2, red).

Only when all components on the transmission path (not only etched lines, but also cables, connectors, and even terminal resistors in integrated circuits) are impedance-matched, can the transmitter and receiver achieve non-reflective signal transmission, thus obtaining The bit rate. Therefore, impedance control is an important factor when evaluating the signal integrity of differential and single-ended lines.

Developers and manufacturers can choose from a large number of different types of differential TDR systems (DTDR) for impedance control: from extremely cost-effective systems to extremely expensive systems. Some well-known measurement technology manufacturers provide high-precision high-end TDR systems. These systems can be found in the field of high-speed oscilloscopes, and are generally combined with necessary accessories, such as (D) TDR probes. These devices are very suitable for measuring transmission systems up to 20Gbit/s and above. However, for high-end equipment manufacturers, impedance control seems to be just a niche market. Therefore, they do not provide dedicated industrialized solutions, and potential users will soon get lost in countless common RF measurement techniques before reaching the final "impedance measurement" goal. In addition, due to their high performance and versatility, all these systems belong to the high price area, which makes investment unattractive, especially if the TDR is not in continuous use.

In the field of industrial and special product measurement technology, some TDRs with lower versatility can be found. In the past two decades, specific standard procedures have been established in these fields. These devices and related software are optimized for measuring the impedance of the test board and are deployed by many PCB manufacturers. However, these TDRs are not suitable for the design and testing of random transmission lines inside PCBs, because of the lack of suitable probes—and worse—too slow signal rise time tr leads to too small signal bandwidth, and then only allows Characterize a line with a length of about 10 cm.

As the third version, there is also a "self-made" solution. In this regard, there are few extremely cost-effective (D)TDR equipment on the market. Such further purchase of components (TDR probe and phase adjustment cable) generally meets the technical prerequisites. However, in this case, it is necessary to develop appropriate software in terms of data recording, error reduction, impedance calculation and result archiving, so that the solution derived from a certain solution is not ultimately more cost-effective and safe. .

Sequid GmbH has initially developed a high-resolution and high-precision TDR system for judging the quality of fish. In the process of cooperation with German PCB manufacturer Elekonta Marek GmbH, the existing basic technology has been further developed into a very high-performance system (Sequid DTDR-65), which can meet all the needs of impedance control measurement. This is a highly stable differential time domain reflectometer, suitable for impedance measurement of differential and single-ended transmission lines up to 10Gbit/s. This instrument has a 65ps step signal generator, so it supports high-resolution measurements on test boards and actual circuits. In addition, DTDR-65 has particularly good jitter performance (Jrms<500fs), which is usually only available in high-end equipment.

The software solution developed at the same time allows non-RF to complete impedance measurement smoothly. This solution includes not only basic functions (such as device control), but also intuitively operable functions for displaying line impedance. Tolerance templates make it easy to make PASS/FAIL statements. Some simple application examples are introduced below.

Figure 3 shows the reflection of the RG 405 coaxial cable. The coaxial cable is equipped with SMA connectors according to the assembly specification (1) and not according to the assembly specification (2). The line impedance of the two RG 405 cables is Z0≈51.5Ω, and the transition in the connector area is very obvious. In the case of incorrectly installed connectors, a drop in capacitance (deformation toward low impedance) is visible. This effect occurs frequently when the outer and inner conductors are installed too close (that is, a capacitor is built).



Figure 3: The reflection of the RG 405 coaxial cable when the SMA connector is installed correctly (1, green) and the SMA connector is installed incorrectly (2, red).

Figure 4 shows the impedance curve of the differential transmission line on the 4-layer printed test circuit. The transmission path starts with the microstrip line in the layer (top layer), then passes through a via to the second layer, where it is still the microstrip line, and then returns to the surface of the layer through the second via. After several iterations of this route, it finally ends at the level. Obviously, this test circuit cannot reach the target impedance of 100Ω: the characteristic impedance of the microstrip line and the strip line are Z0≈120Ω and Z0≈110Ω, respectively. It can be clearly seen from this picture that the capacitance factor of the via will seriously affect the signal integrity in the actual system, especially at high data rates.



Figure 4: The differential line reflection diagram of wiring on two different layers of the FR4 substrate.

As an example, Figure 5 shows the reflection of the USB 3.0 connector and cable. The rated impedance of USB 3.0 components is Z0=90Ω±7Ω. The TDR device still works on a reference impedance of 100Ω (time range t<12.2ns). The secondary reflection caused by the conversion from the test adapter to the USB 3.0 connector occurred at approximately 12.3 ns, which was consistent with all measurements as expected. Curve 3 (green) represents the result of the open adapter, where the rapid impedance rise indicates the (high impedance) end of the adapter. Curves 4 and 5 (red and blue) represent two different USB 3.0 cable assemblies, each of which consists of an adapter and a subsequent cable. Although the cables are within the specifications, the adapter does not meet the specifications. In particular, the red curve shows that the impedance is about 122Ω, which produces serious reflections, which may cause the USB 3.0 controller to reduce the data rate.

In short, all the examples clearly show that developers can use DTDR-65 to intuitively and deeply observe the transmission path. The tasks of developers and quality inspectors often include easy-to-understand documentation of the results obtained. This task is very important, but unfortunately it is very time-consuming and tedious. However, this unpopular work can now be greatly simplified with the built-in automated generation tools, which can generate graphical and statistical scalability evaluation results with just a few clicks. In addition, online impedance calculators can also be used for most common line types.



Figure 5: Reflected image of a USB 3.0 adapter with open circuit (3) and two different USB 3.0 cable assemblies (4 and 5).

supports a wide range of applications

The necessary accessories include high-quality phase-adjusted coaxial cables and TDR probes, which can be used in different types of applications: industrial probes are used for serial measurement in the production process, and high-precision probes are used for research and development-see Figure 6. .DTDR-65 also has excellent electromagnetic shielding performance and can be used in battery-powered mobile applications.



Figure 6: Different probes and accessories for time domain reflectometer DTDR-65.