The implementation of broadband impedance controlled system to the center Electronic Construction Parts - Printed Circuit Board (PCB) designers, manufacturers and quality assurance managers have put forward arduous challenges. This challenge is not from the lack of electromagnetic design knowledge, and it originates from huge price stress in the PCB industry: that is, the ideal radio frequency (RF) substrate that is completely suitable for the GHz range clock rate is almost useless.
In contrast, low-cost FR4 materials in the entire substrate mediation (DC) are often used. In addition, the core material and semi-cured chip pressure synthesis multi-layer PCB often leads to uniformity of geometries, further increasing uncertainty. However, in order to meet the predetermined tolerance, many PCB manufacturers provide inspection services for line impedance, followed by additional impedance test boards. These test boards are typically located on the edge of the PCB, so it can only partially represent the characteristics of the actual interest transmission line distributed across the production panel. In the worst case, the test plates of the test may be within the specified range, but the actual transmission line does not meet the requirements.
Impedance fluctuations are often unable tolerable
In addition to the special changes in the materials and production processes, the design parameters change (such as the change in layers, the distance between the GND plane, the PCB boundary, or other transmission lines) also occurs, eventually leading to the impedable transmission line impedance fluctuation. The consequence of impedance fluctuations is the clock edge deterioration, interpretation interference, which in turn causes unacceptable error rate, which ultimately leads to performance degradation or even system failure.
The line impedance can be determined at a high precision by the time domain reflection method (TDR). TDR technology is started from the 1970s, mainly used to detect failures in underground or submarine cables. Figure 1 shows a block diagram of an impedance measuring device based on TDR technology. TDR itself contains only a voltage step generator and a broadband sampler with data acquisition unit.
Figure 1: Block diagram of an impedance measurement system based on TDR technology. (All photos from Sequid)
The basic measurement principle is such that the voltage generator generates a step signal, passed through the adapter, cable and probe to the toary device (DUT). When interacting occurs at the entire length of the device to be tested, the signal will undergo a partial reflection, and return the detector, thereby achieving spatial measurement of the waveform impedance to be tested. Many people understand this basic principle from radar applications, so TDR is often called 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 approximately 5mm). Synchronization between generators and samplers (at least 10 GHz thereof) is important for low noise (only a few PSs, that is, the jitter value). The most ideal is to use the "true straight" sampler, no external signal separator or coupler. This benefit is obvious because the broadband signal separator is typically resistant, which will increase insert loss and noise. Finally, the TDR instrument has a data recording unit that is usually implemented with a microprocessor or FPGA.
Real-time sampling techniques are not used under normal conditions of high frequency TDR equipment, but use sequential or random sampling techniques. Similar to the field, these devices can record rapidly changing periodic signals with reasonable techniques. Data processing and visual tasks are typically performed on the PC, which can be fully integrated in high-end instruments or through USB or Ethernet.
The adaptation of the object to TDR is a very strict task. For example, differential impedance measurements must use high-precision phase matching cables and probes. If this requirement, the even mode and odd mode conversion will reduce the measurement accuracy. In addition, the header of the probe should be designed to match the impedance of the devices to be tested to achieve possible maximum accuracy measurements.
Different systems in the market
In increasing digital world, the line impedance measurement has shown that it is currently the most important TDR application. Figure 2 shows such spatial resolution measurement examples of uninterrupted (green curves) and interference (red curves) transmission lines.
Figure 2: Reflection of the RG 405 coaxial cable when the SMA connector (1, green) and erroneous installation SMA connector (2, red) are installed.
Only all components on the transmission path (not only the etch lines are included, including the cable, the terminal resistance in the connector or even integrated circuits) are impedance matching, in order to achieve no reflection signal transmission between the transmitter and the receiver, thereby obtaining The highest bit rate. Therefore, impedance control is an important factor when evaluating signal integrity of differential and single-end lines.
Developers and manufacturers can select a large number of different types of differential TDR systems (DTDRs) for impedance control: from a very cost-effective system to particular expensive systems. Some famous measurement technology manufacturers provide high-precision high-end TDR systems. These systems can be found in the high-speed oscilloscope field, generally combined with the necessary accessories, such as (d) TDR probes. These devices are ideal for measuring the transmission system up to 20Gbit / s and above. However, for high-end equipment manufacturers, impedance control seems to be just a niche market. So they do not provide dedicated industrialization solutions, potential users will quickly lose in countless ordinary radio frequency measurement techniques before reaching the final "impedance measurement" target. In addition, all of these systems belong to high price fields due to their high performance and versatility, which makes investment lacks attractive, especially if TDR is not continuous.
Some versatile TDRs can be found in the industry and special product measurement technology. These areas have established specific standard procedures in these areas in the past two decades. These devices and related software are optimized for measurement test board impedance, and there are deployments in many PCB manufacturers. However, these TDR is not suitable for the design and testing of the PCB internal random transmission line, the reason is the lack of suitable probes - worse - too slow signal rise time TR leads to too small signal bandwidth, which only allows only Characterization of the wire with a minimum length of about 10 cm.
As a third version, there is also a "self-produced" solution. There are not many very cost-effective (D) TDR devices in this area. This further purchasing components (TDR probes and phase adjustment cables) generally meet the technical prerequisites. However, in this case, appropriate software must be developed in data records, error reduction, impedance calculation and results archiving, so that you can pursue a solution from some solutions and do not have higher cost efficiency and security. .
Sequid GmbH initially developed high-resolution and high-precision TDR systems to judge the quality of fish. During the cooperation with German PCB manufacturer Elekonta Marek GmbH, existing basic techniques further develop into a high performance system (Sequid DTDR-65), which can meet all the needs of impedance control measurements. This is a highly stable differential time domain reflectometer, suitable for the rate of differentials of 10 gbit / s and impedance measurements of single-ended transmission lines. Such instruments have 65 ps step signal generator, so high resolution measurements for test boards and actual circuits. In addition, DTDR-65 has a particularly good jitter performance (JRMS<500FS), and this performance is usually high-end equipment.
Software solutions developed at the same time can make non-radumper experts to complete the impedance measurements. Such solutions not only contain basic functions (such as device control), but also include intuitive operational functions for displaying line impedance. Tolerance template makes it very simple to make through / fail (Pass / Fail). Here is some simple application examples.
Figure 3 presents a reflection diagram of the RG 405 coaxial cable, wherein the coaxial cable is equipped with an SMA connector according to the assembly specification (1) and not in accordance with the assembly specification (2). The line impedance Z0 ≈ 51.5Ω in the two RG 405 cables is very obvious. In the case where the connector is installed, the capacitor is lowered (deforming toward the low impedance) is visible. This effect will occur frequently when the external and internal conductor mounts are too close (there is a capacitor).
Figure 3: Reflection of the RG 405 coaxial cable when the SMA connector (1, green) and erroneous installation SMA connector (2, red) are installed.
Figure 4 shows the impedance curve of the differential transmission line on the 4 layer printing test circuit. The transmission path is the first layer (top layer) in the first layer (top), then transferred to the second layer by a via, it is still a microstrip line, and then returns to the first layer surface by the second vias. This route is repeated several times and finally terminates at the first layer. Obviously this test circuit does not reach a target impedance of 100 Ω: the characteristic impedance of the microstrip line and the stripline is Z0 ≈120Ω and Z0 ≈ 110Ω. From this figure, it will be apparent that the via capacitor factors will seriously affect the integrity of signal integrity in the actual system, especially when high data rates.
Figure 4: Difference line reflection diagram of two different layers of the FR4 substrate.
As the last example, Figure 5 shows the reflection map of the USB 3.0 connector and cable. The rated impedance of USB 3.0 components is Z0 = 90Ω ± 7Ω. TDR devices are still working on 100Ω reference impedance (Time range T<12.2ns). The first reflection caused by the conversion caused by the test adapter to the USB 3.0 connector occurs at about 12.3 NS, as expected, consistent with all measurements. Curve 3 (green) represents the result of the open adapter, wherein the fast impedance rise indicates the end of the adapter (high impedance). Curves 4 and 5 (red and blue) represent two different USB 3.0 cable assemblies, each component consisting of an adapter and a subsequent cable. Although the cable is within specifications, the adapter does not meet the specifications. In particular, the red curve indicates that the maximum impedance is about 122Ω, which produces severe reflections, which may cause the USB 3.0 controller to reduce the data rate.
In summary, all examples clearly show that developers can use DTDR-65 to observe the transmission path inaccessively. The task of developers and quality inspectors typically contains archives that are easily understood from the results obtained. This task is very important, but unfortunately, it is very time consuming, and tested monotonous. However, this unwelcome work can now be used to generate tools with built-in automation. It is highly simplified, and only a few clicks can form graphical and statistical scalability assessment results. In addition, online impedance calculators can also be used for most common line types.
Figure 5: Reflection of the USB 3.0 adapter with open circuit (3) and two different USB 3.0 cable assemblies (4 and 5).
Support wide application
The necessary attachments contain high-quality phase-adjusted coaxial cables and TDR probes, which can be used for different types of applications: industrial probes for series measurements in the production process, high-precision probes for research and development - see Figure 6 .Dtdr-65 also has excellent electromagnetic shielding performance, which can be used in mobile applications that are powered by battery.
Figure 6: Different probes and accessories for time domain reflection meter DTDR-65.
Source: Electronic Technology Design
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