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Design Consideration For A High Performance Production Test Cable

Tue, 07/31/2001 - 6:17am
By Robert Thiele, Semflex

Today's market demands shorter product development cycles while simultaneously applying constant pressure to reduce costs and increase performance. The goal is to accomplish more with less, while doing it better and faster. In test and measurement, the specific challenge is to accurately and efficiently characterize components and systems, measure performance, verify design integrity and ensure consistent quality — at the lowest possible cost. The selection of the proper test equipment and accessories is obviously essential to meeting this challenge. Toward this end, Semflex's RHT test cables were designed for production environments where a cost-effective, stable, predictable and repeatable connection between the test equipment and the devices being tested is essential.

Flexible test leads should meet specific criteria in order to function effectively without risk of degradation of the measurement. The challenge in designing cost effective flexible test cables is selecting the optimum balance between price and performance. The user should have test leads that function well without superfluous and costly options. The following are key points to consider when selecting a flexible test cable:
•Operating frequency range,
•Importance of phase stability in the application,
•Importance of low VSWR performance in the application,
•Length of time between calibrations,
•Protection for rough handling,
•Flexibility for routing, and
•Type of connector interfaces needed.

The answers to these questions will drive the selection, and ultimately the cost, of a flexible test cable.

When Semflex (Mesa, AZ) began development of a new cost-effective production test cable, each of these variables was carefully weighed from the onset. To cover the frequency spectrum up to 50 GHz, two RHT cable sizes were developed to optimize the size of cable to a particular connector interface. Larger connectors like the 7 mm and N type naturally fit better on larger cable sizes, while small connectors like the SMA or 2.4 mm fit better on small cable sizes.


When making VNA measurements, the phase and amplitude stability of the test leads is critical for accurate and repeatable measurements. The RHT series is well suited for these measurements because of its superior phase stability verses flexure.

Since most high frequency measurements are performed with vector network analyzers, phase stability and low VSWR are important factors in making accurate calibrations. Semflex chose a helically wrapped microporous cable design optimized for phase stability verses flexure. The microporous dielectric is also phase stable over temperature. This is beneficial in applications that experience calibration drift due to temperature variations over time. Overall cost savings are also realized from phase matched interchangeable heads because they reduce the amount of recalibration required by the user.

Production test cables need to remain accurate and repeatable over the course of thousands of measurements. With that in mind, ruggedization was added to the RHT series to enhance both the mechanical and electrical performance of these precision test leads. The rugged construction incorporates an interlinked armor with a soft jacket that provides 250 lbs. of gradual crush resistance and resists pull forces up to 150 lbs. This armor also limits tight bends and kinking of the cable that typically cause premature failure.


Figure 1 shows the typical performance of RHT test cables.

Another cause of premature service life in high volume production test cables is failure of one or both connectors. To address this problem, Semflex added interchangeable connectors with precision interfaces offering tight tolerances for repeatability and low VSWR. All center contacts on the interchangeable connector heads are recessed for protection from mismating and damage from handling. All outer conductor surfaces are made with stainless steel in order to increase durability and reliability in high volume applications. The coupling nuts feature both knurled surfaces and wrench flats, which allows the technician the option of either hand tightening or utilizing a wrench to torque the coupling nuts to specific limits. Connector heads are phase matched which eliminates degradation of VNA calibrations when connector heads are interchanged, providing a simple way to test inter-series and intra-series components using a single set of test cables. Currently, Semflex offers two different series optimized for popular connector sizes and frequency performance. Connectors are sold in kits or separately, and can be removed and replaced without having to return the cable to the factory for refurbishment. One RHT test cable and a connector kit provides up to 55 combinations of connector interfaces.

RHT series cables are stocked and available for 24-hr delivery from Avnet in standard 24, 36, and 48-inch lengths; other lengths are available upon request. The assemblies can be ordered without connector heads or as complete kits that include the cable and all connector heads and accessory tools needed. Individual connector heads are available also. The RHT series comes with a 1-year service warranty and is guaranteed to be free from defects in materials or workmanship. RHT assemblies are priced from about $400 to $800 depending on the configuration.

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