VNAs Characterize Supercomputer Cable Assemblies

Contemporary supercomputers comprise clusters of nodes. The collective performance of those nodes depends on the data bandwidth across the interconnect fabric. Because cable assemblies are part of the critical path to supercomputer system performance, they must be accurately characterized. A microwave vector network analyzer (VNA) supported by a software tool for managing calibration and measurement data provides a sound methodology for interconnect characterization. Proper error-correction techniques are required, however, to ensure that these measurements are accurate.

Most modern high-speed cabled data links employ a differential configuration. Key performance parameters for these cables include differential insertion loss, differential return loss, near- and farend crosstalk, eye opening, and jitter. Other parameters include in-pair skew and transient impedance as well as common- mode conversion, which affect electromagnetic compatibility (EMC) and crosstalk. When measuring these parameters, many different factors can affect accuracy and cause less than ideal measurement results. One such influence is the test fixture itself. Ideally, its effects must be removed from the measured data so that critical electrical parameters of solely the device under test (DUT) can be obtained.

Various approaches, which are classified as either pre-measurement error correction (a.k.a. calibration) or post-measurement error correction (a.k.a. de-embedding, can be used to accomplish this goal. The most common calibration technique is the short-open-load-thru (SOLT) method, which is based on the use of one standard for each of the four circuit types. To be usable, the reference structures must be carefully designed and fabricated to metrology-grade standards. The electronic-calibration (ECal) accessories recently developed by several test-equipment companies also can be used to greatly reduce the time required for calibration.

The three most common methodologies for accomplishing the removal of fixture effects are as follows:

1. Direct Touchstone de-embedding: Assuming that the S-parameter file for the fixture is readily available, deembedding is a mere click of a button on a standard signal-integrity-specialist software tool (Fig. 1).

2. Thru-reflect-line (TRL) calibration: This highly flexible calibration approach allows the user to set the reference plane anywhere within the channel. Although TRL printed-circuit-board (PCB) fixtures must be fabricated, new tools have simplified this normally complex errorcorrection methodology (Fig. 2).

3. Automatic fixture removal: If the test fixtures are symmetric from top to bottom and left to right, the normally complex algorithm is simplified to a three-step process. First, measure the complete channel including the fixtures. Next, measure a pre-fabricated 2x thru structure. Finally, click "de-embed" on a signal- integrity-specialist software tool (Fig. 3).

Clearly, a VNA can be used to accurately measure and characterize supercomputer cable assemblies. Yet the number of tests that must be performed and the test time required to characterize multi-pair cable assemblies can be problematic. In recent years, the availability of multiport instruments has provided the ideal solution to this challenge.

Offering an expanded number of measurement ports, multiport test systems are especially useful for measuring all combinations of near- and far-end crosstalk in one measurement setup. For example, a complete measurement of insertion loss, return loss, and crosstalk for three multi-pair cables (one pair of interest and its two adjacent-pair neighbors) can be done in one measurement using a 12-port VNA. This approach eliminates the complicated data manipulation that would be required to post-process the results from 15 separate measurements on a four-port analyzer. Furthermore, the full 12-port calibration can be completed using an electronic calibration module that has all of the calibration standards built into one small box under Universal Serial Bus (USB) control (Fig. 4). The control and data management of multiport calibration and measurements are typically handled by a signal-integrity-specialist software tool, such as Agilent Technologies' N1930B Physical Layer Test System (PLTS).

Today's telecommunication systems are pushed to the limit by demanding video, voice, and data requirements. Engineers must utilize advanced design tools to create sophisticated network equipment that can transmit serial-channel data at 10 Gb/s and above. Understanding how to accurately measure and characterize key cable electrical parameters, such as insertion loss and crosstalk, within differential channels is crucial to creating high-performance devices and systems. A multiport VNA and data-management software provide an optimal solution for accomplishing this task. Yet calibration techniques like SOLT also are crucial for the correction of test equipment. The ability to achieve accurate measurements will enable engineers designing future supercomputer interconnects to realize even higher data rates, resulting in more teraflops per second than ever before. Agilent Technologies, 1400 Fountaingrove Pkwy., Santa Rosa, CA 95403; (707) 577-5000, e-mail: [email protected], Internet:

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