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Contactless Flanges and Rail Positioning Systems Streamline Waveguide Component Tests

Parent Category: 2021 HFE

By Eravant

In microwave and mm-wave manufacturing operations, it can be a constant struggle to achieve good measurement accuracy at reasonable cost when large quantities of waveguide components are tested. Incoming inspection and production-line personnel require advanced skills and high levels of discipline to minimize their contributions to measurement variability. Without adequate levels of skill and discipline, different operators produce different test results. Excessive test variability can impede efforts to control quality and may lead to unproductive design iterations as engineers strive to improve manufacturing yields.

Additional measurement variability can also result when test system components are degraded by heavy usage. When a traditional waveguide test port is connected to a Device Under Test (DUT), screws must be carefully engaged and tightened to assure full mechanical contact between the waveguide flanges. Unfortunately, the process of tightening the screws usually causes small lateral and rotational movements between the flanges. Such movements slowly diminish the protective metal plating on test port flanges. Eventually the flanges are deformed to the point where they are unable to provide adequate electrical performance.

In high-volume production scenarios, operator variability and test system degradation can conspire to dominate observed process variations. In such cases the controllable factors that contribute to observed process variations are difficult to isolate, thus limiting the extent to which manufacturing processes can be controlled. To address these concerns, Eravant Inc. (Torrance, CA) developed their Proxi-Flange™ contactless flanges and Wave-Glide™ rail positioning systems (Figs. 1-2).

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Fig. 1 • Proxi-Flange™ waveguide is packaged individually in rugged equipment boxes with additional hardware and tools.

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Fig. 2 • Wave-Glide™ rail positioning systems simplify the task of maintaining good alignment between test ports and DUTs.

 

Working together, Proxi-Flange™ and Wave-Glide™ products can greatly reduce observed process variations caused by Variation from Appraiser (operator variability) and Variation from Gauge (test system accuracy, stability, and repeatability). When these tools are used in manufacturing operations, test results are far less dependent on operator skills. Additionally, mechanical stresses on test system components are greatly reduced. Design and manufacturing engineers are thus able to focus more on the controllable factors that affect manufacturing yields.

Contactless Waveguide Flanges

The Proxi-Flange™ family of waveguide flanges is designed to interface with conventional (flat) waveguide flanges. Together, they provide contactless signal transmission across junctions between the test system and the DUT. By eliminating the need to carefully adjust and tighten waveguide screws, Proxi-Flange™ products remove much of the variability and fatigue encountered when testing waveguide components.

Covering waveguide bands from 18 to 220 GHz, Proxi-Flange™ components are constructed with special waveguide flanges that are populated with arrays of small pin-like structures. The pin arrays provide a highly effective RF choke that surrounds the waveguide junction (Fig. 3). Small gaps between the flanges can be tolerated because the RF choke presents very low electrical impedance across the gap. This low gap impedance connects the two flanges electrically without requiring intimate physical contact.

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Figure 3 • Proxi-Flange™ products include an RF choke that surrounds the waveguide opening.

 

Very low gap impedance is achieved because the mating surface of the pin array approximates a Perfect Magnetic Conductor (PMC). Conversely, the flat surface of the adjoining flange approximates a Perfect Electrical Conductor (PEC). When PMC and PEC surfaces are in close proximity the gap between them does not support electromagnetic wave propagation. This results in the total reflection of any signal power that would otherwise flow into the gap.

Eravant has applied for patents covering this new technology. Meanwhile the company is rapidly developing new components and new applications to address the needs of high-volume component manufactures and other potential users of contactless waveguide flanges.

Proxi-Flange™ components solve the problem of poor return loss and high insertion loss caused by imperfect contact when conventional waveguide flanges are mated. The captive screws normally used in waveguide test sets can cause cocking issues if tightened unevenly, but Proxi-Flange™ eliminates the need for any waveguide screws, resulting in improved durability, measurement consistency, and repeatability.

All Proxi-Flange™ components are constructed from gold-plated beryllium copper for long life and high performance. Holes for alignment pins are included in the flanges to avoid excessive cocking when components are mated. The use of waveguide screws is optional but unnecessary in most cases. When screws are used, their adjustment is much less critical compared to conventional waveguide flanges.

For example, model STQ-WG-22025-FB-CF is a straight WR-22 waveguide section that includes UG-383/U anti-cocking flanges (Fig. 4). Measuring 2.5 in (6.3 cm) long, the section operates from 33 to 50 GHz. Other Proxi-Flange™ models include STQ-WG-42025-FB-CF operating from 18 to 26.5 GHz (WR-42 waveguide with UG-595/U square flanges), and STQ-WG-05025-FB-CF spanning 140 to 220 GHz (WR-05 waveguide and UG-387/U-M anti-cocking flanges). Both units are 2.5 inches in length.

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Figure 4 • Proxi-Flange™ waveguide section, model STQ-WG-22025-FB-CF, operates from 33 to 50 GHz.

 

Repeatability and Reliability

Contactless waveguide flanges are highly effective when used as test ports in network analyzers and other test systems. Calibration procedures may be performed without tightening waveguide screws, provided that the gaps between the test ports and the calibration standards are kept sufficiently small. This requirement demands fixtures that can sufficiently limit misalignment and gaps between the waveguide components involved.

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Table 1 • Standard Proxi-Flange™ waveguide sections

When good alignment is maintained, contactless calibration procedures can achieve impressive results. Measured return loss for a through-line standard is a good indicator of calibration quality. For a VNA test system spanning 60 to 90 GHz, through-line return loss is typically better than -55 dB when Proxi-Flange™ test ports are used (Fig. 5). Repeated calibrations show similarly low levels of through-line return loss, indicating that reliable calibrations are possible without engaging waveguide screws.

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Fig. 5 • Return loss and insertion loss for a calibration standard. The two-port calibration was repeated six times.

 

Test results are similarly consistent using Proxi-Flange™ waveguide components. When a bandpass filter was measured six times with complete removal and reinsertion of the DUT, the test results were unchanged (Fig. 6). This measurement consistency makes it possible to test large numbers of waveguide components, with each measurement taking only a few minutes to complete.

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Fig. 6 • Return loss and insertion loss for a bandpass filter. The measurement was repeated six times with complete removal of the DUT between tests.

 

In addition to greatly improving test system consistency and productivity, Proxi-Flange™ components can also reduce variability in system integration and servicing. Waveguide connections are far less sensitive to how waveguide screws are tightened. As a result, technicians with ordinary skills can combine or replace components without risking unacceptable system performance.

Another application for contactless waveguide flanges is in cryogenic testing. If the DUT is thermally isolated from the test ports using small air gaps, it may be tested at cryogenic temperatures without having to cool the test ports to the same temperature. This approach to cryogenic testing could save time and cost by greatly lowering the amount of thermal energy that must be extracted from the test setup.

For high-power components that operate at elevated temperatures, thermal expansion can result in high mechanical stresses being applied to surrounding components. If adequate alignment is maintained, contactless waveguide flanges can operate with small gaps that would allow thermal expansion of high-temperature components and avoid the unwanted transmission of mechanical forces to other components.

Rail Positioning Systems

The alignment of two VNA extenders is critical for getting the best results from any two-port test setup. Unfortunately, the mechanical tolerances of the extender mounting plates and flange locations are not always well-controlled, as they are typically designed and manufactured for manual use in laboratory environments.

Eravant’s Wave-Glide™ test fixtures were developed to streamline component testing (Fig. 7). They provide a fast and reliable system for connecting DUTs with waveguide frequency extenders that are mounted on sliding rail-mounted platforms. The fixtures are custom-built from a standard set of mechanical rails and sliding mounts to address specific customer needs.

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Fig. 7 • Wave-Glide™ test fixtures were developed to streamline component testing.

 

To ease the alignment process and accommodate variations in the extender geometries, a combination of pitch, yaw and roll movement adjustment is necessary. To aid this requirement a simple mechanism has been developed which consists of two plates separated with screw guided springs. The screws are easily accessible and can be tightened or loosened causing the springs to contract or retract, thus providing the adjustment required along the x, y or z axis. An additional fixture is provided to aid the alignment between the flanges, as well as to have the flanges parallel and centered with respect to the rails.

Currently the plate for standard Wave-Glide™ models has mounting holes compatible with several extender modules (Table 2). For all other extenders, custom plates are available.

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Table 2 • Several families of waveguide frequency extenders are compatible with standard Wave-Glide™ models. For all other extenders, custom plates are available.

In future design iterations, Wave-Glide™ positioning systems will include right-angle test port configurations, greater numbers of test ports, and motorized positioners that further streamline the task of loading and unloading DUTs. The goal of these efforts is to make high-volume production of waveguide components easier and less costly, thus enabling faster and more diversified growth in microwave and mm-wave applications.

Eravant’s rapid development of Proxi-Flange™ and Wave-Glide™ products is a continuation of their long-term commitment to support, and in many cases lead, new advances in high-frequency electronics technology.

November 2021

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