[Design Application]
Accurate Measurements On High-Speed Rambus Traces Present Challenges
Using time-domain-reflectometry normalization and fast oscilloscopes, measurements can be made on very short traces.
Making highly sensitive measurements on high-speed computer motherboards and memory boards is tough, especially when the traces are short. Such short traces (e.g., 1 to 2 in. in length) are common in the Rambus memory architecture, which uses 250-ps rise-time signals. They also can be found in the smaller-form-factor pc boards used in laptop computers and network equipment applications.
The issues brought about by short traces can be conquered using the test procedure called time-domain-reflectometry (TDR) normalization. Through this process, high-speed oscilloscopes can accurately measure the characteristic impedance of short traces by removing sources of error.
Three basic measuring tools are needed to measure the characteristic impedance of those traces: a high-speed digitizing oscilloscope with a modular TDR plug-in (Fig. 1), a 10-GHz TDR probe, and a TDR calibration substrate (see R&D Equipment List). The only caveat is that the TDR oscilloscope must have normalization capability.* Ideally, the versatile TDR equipment is suited for an R&D environment. If a high-volume manufacturing capability is required, additional test equipment is recommended (see Manufacturing Equipment List).
A TDR Calibration Substrate is used to remove the errors that are usually introduced by test fixtures. Probe-tip ground-lead inductance, reflections from connectors, and cable loss are typical examples of these errors. The normalization process demands that the user perform a two-point calibration at the probe tip. An appropriate digital filter can then be automatically configured in the oscilloscope firmware. It's recommended that this calibration process be performed while probing Rambus In-line Memory Modules (RIMMs) and Continuity RIMMs (CRIMMs). When probing short traces on motherboards and Small-Outline RIMMs (SO-RIMMs), however, the process is mandatory.
The calibration substrate has precision, thick-film resistors that are laser trimmed to yield the utmost in accuracy (Fig. 2). To ensure a long life with various probe-tip configurations, durable gold electrodes are fired onto the alumina substrate. That substrate is a non-reactive standard. Due to the lower-frequency content of time-domain reflectometers, it's favored for Agilent TDR measurements. A 28-Ω airline isn't recommended for the company's TDR calibration, but it can be used effectively with the vector network analyzer.
Calibration And Verification Using calibration and verification together will enhance the confidence of Rambus board designs. To minimize measurement error, calibrate a 50-Ω output-impedance instrument in a 50-Ω environment. Next, verify the calibration by measuring a well-known impedance value that's close to the characteristic impedance of the device under test (DUT). This two-step process guarantees that the measurement equipment is highly accurate in the region of interest.
The two-step process is as follows: Using one TDR probe with suitable bandwidth and an oscilloscope with normalization capability, probe the short standard on the calibration substrate. This is the electrode area indicated on the left side of Figure 2. Next, probe the 50-Ω precision resistor. Be sure to place the ground pin of the TDR probe on the larger electrode pad, because the excess capacitance of the larger electrode effectively compensates for the excess inductance associated with most probe ground tips. After confirmation from the TDR scope, the calibration is complete.
The next step is verification. Start out by probing one of the 28-Ω, ±0.25% thick-film resistors located on the calibration substrate. For the 28-Ω standard, the ±0.25% resistor tolerance translates into ±70 mΩ. This resolution is important for Rambus applications, in which the ±2.8-Ω tolerance is essential for proper operation of the signal channel. Some Asian pc-board manufacturers have recently tightened the specification to ±1.4 Ω.
Activate the normalized waveform with the following button-press sequence: plug-in setup (hard key), normalize response (soft key), TDR normalize on (soft key). When performing verification, use the normalized TDR waveform. The standard TDR waveform will include secondary reflections that will introduce significant inaccuracies at the front end of the DUT (up to 3 Ω of error for Rambus SO-RIMMs).
On pc boards, TDR provides useful insights for the digital design engineer trying to understand signal-integrity issues. When no connectors are conveniently located near a circuit trace on a board, it's common to launch the TDR step into the trace with a high-bandwidth TDR probe. But finding the optimal location on the board to probe isn't always easy, especially since the results obtained will vary depending on the launch point chosen.
As seen in the memory architecture of the Rambus circuit topology, shown in Figure 3, the first microstrip segment under test is located between the Rambus memory-controller chip (RMC) and the first RIMM connector. Figure 4 reveals the impedance profile of this motherboard microstrip trace when probing from the RIMM #1 connector side.
Sometimes, it's useful for the designer to troubleshoot the whole Rambus channel with all of its components, although most TDR measurements are performed on bare pc boards. The motherboard with the test waveform in Figure 4 has already been populated with the various components associated with the Rambus physical layer. Check out the higher impedance of the RMC package's ball-grid-array (BGA) structure. This particular BGA adds excess inductance to the 28-Ω characteristic impedance of the motherboard microstrip. It thereby pulls the impedance higher.
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