Precision Cables Preserve Microwave Measurement Accuracy

May 18, 2010
Precision cables are usually the last addition to a microwave test system. Often, they are an afterthought. The wrong cables, however, can add unstable electrical characteristics to the system and unreliable test results. For that reason, it ...

Precision cables are usually the last addition to a microwave test system. Often, they are an afterthought. The wrong cables, however, can add unstable electrical characteristics to the system and unreliable test results. For that reason, it is important to select coaxial interconnects for testing with great care. It is best to choose cables that will not only deliver phase and amplitude stability and low loss, but are rugged and reliable.

For use with RF/microwave vector network analyzers (VNAs), for example, W.L. Gore& Associates has developed cables capable of operating to 110 GHz. According to the maker, the PHASEFLEX test cable assemblies are designed for ruggedness and minimal phase and amplitude changes and achieve a minimum bend radius of 1 in. The test cables were "lifetime" tested using a procedure in which they were first bent 90 deg. and then reverse bent 180 deg. The stranded center conductor cables in these assemblies performed beyond 100,000 cycles (or more than 200,000 total cable bends) while meeting electrical specifications.

When tested as per the procedure outlined in the firm's literature on microwave/RF test assemblies, a 24-in. PHASEFLEX assembly exhibits phase stability to 4.7 deg. (2.0 deg. typical) and amplitude stability to 0.08 dB (0.03 dB typical) at 18 GHz. At 67 GHz, a 24-in. assembly provides a maximum phase stability of 14.6 deg. (8.0 deg. typical) and amplitude stability to 0.10 dB (0.05 dB typical). The flexible assemblies' crush resistance is rated at 250 lbs./linear inch. At 110 GHz, a 16-cm cable assembly typically exhibits 2.14 dB insertion loss while a 10-cm assembly exhibits 1.34 dB insertion loss. The VSWR performance in both cases is 1.50:1.

By providing superior phase and amplitude stability, Robert John, Gore's Manager for Microwave Cable Products, notes that these flexible coaxial-cable assemblies ensure high measurement accuracy and repeatability. In doing so, they allow the time interval between calibrations to be extended. The PHASEFLEX test-cable assemblies come with a variety of connector options, which determine the ultimate high-frequency limit. For example, a PHASEFLEX cable assembly with 1-mm coaxial connectors operates to 110 GHz while assemblies with 3.5-mm connectors reach 26.5 GHz. Those with 2.4-mm connectors hit 50 GHz while assemblies with 1.85-mm connectors reach 67 GHz.

By enhancing flexibility and adding an abrasive-resistant cover, Micro-Coax () also has improved the ruggedness of its test cables. Designed for operation to 40 GHz, the new line of UTiFLEX rugged microwave test cables, which is labeled MKR, promises to provide superior mechanical strength (see figure). The supplier claims that the MKR test cables have passed strenuous qualification testing to ensure long-term reliability. The 26.5-GHz MKR300C series is currently available while the 40-GHz version MKR246A is slated for release this summer. The minimum bend radius for the MKR300C is 1.25 in. It exhibits insertion loss of 0.09 dB/ft. at 1 GHz and 0.49 dB/ft. at 26.5 GHz. From DC to 26.5 GHz, insertionloss change is no more than 0.061 dB/ ft. when wrapped one time (360 deg.) around a 4.5-in. mandrel versus when the cable is straight. According to the maker, the cable is capable of withstanding 150,000 unrestrained flexes with minimal degradation.

For applications in which phase stability under bending, torsion, or temperature variations is paramount, Switzerland's Huber+Suhner has developed the SUCOFLEX measurement cables. They promise to maintain stable electrical characteristics when subjected to severe conditions. SUCOFLEX 104PE with N, TNC, and PC7 connectors range to 18 GHz while the SUCOFLEX 400 is usable to 26.5 GHz. Based on extruded, very-low-density polytetrafluoroethylene (PTFE) dielectric, the new 50-Ω SUCOFLEX 404 exhibits 0.3 dB/m attenuation while handling 689 W of continuous-wave (CW) power at 2 GHz. At 26.5 GHz, attenuation drops to 1.15 dB/m while handling 189 W. The cable has 3.74 ns/m signal delay while exhibiting insertion-loss stability versus temperature of better than 0.0023 dB/C. With a straight SMA connector, return loss is kept to 20 dB to 18 GHz.

While SUCOFLEX 404 with a male SMA connector is available now, other members are also in the works. For instance, SUCOFLEX 406 with N and TNC straight male connectors are in qualification and expected to be released this year. Designed to handle frequencies to 18 GHz, 406 is said to offer the lowest loss in the SUCOFLEX 400 family. Furthermore, the company said that the connector range will be expanded with thinner versions and lower insertion losses for frequencies to 40 GHz.

For field use, the German maker has developed heavy-duty flexible test cables for severe outdoor conditions. With an operating temperature range of -55 C to +85 C and waterproof resistance in compliance with IP68, the SUCOTEST line of cable assemblies operates to 18 GHz with low insertion loss and excellent phase and loss stability with flexure.

Radiall has launched a range of ruggedized cable assemblies that guarantee a high degree of stability over nearly 5000 mating cycles. While the phase-stable TestPro 4.2 is aimed at production test, the TestPro 5 and 8 lines are tailored for remote test stations and anechoic chambers. The TestPro 4.2 offers phase stability with flexure of 2 deg. at 18 GHz, states Business Development Manager Michel Maximilien. In addition, the low-loss TestPro 5 and 8 allow long length use in test applications. For category 5 and 8, the assemblies exhibit insertion loss of 0.06 dB/ft. at 2 GHz and 0.21 dB/ft. at 18 GHz. "The retention of the center conductor in dynamic use is a key feature of these cables," notes Maximilien.

For ultra-low loss, Radiall uses specially designed, ultra-low-density PTFE dielectric material. Currently, the frequency range for TestPro 4.2 cables is DC to 20 GHz while the TestPro 5 and 8 families operate to 18 GHz. Efforts are underway to extend the frequency capability to 40 GHz without compromising performance. The 40-GHz version is expected to be released this summer.

Triple-shielded Silverline test cables from Times Microwave Systems also target production, field, and lab test environments. They promise to deliver superior phase and loss stability with repeated flexing and mating cycles to 18 GHz. In addition, Pasternack Enterprises ( has readied test cables for a variety of test and measurement applications in both precision-test and harsh environments. The firm's 18-GHz precision test cables offer wideband coverage with greater flexibility for easy connection and bend radius.

This triple-shielded cable comes with molded strain relief and passivated stainless-steel connectors to provide ruggedness, durability, and longer life. In addition to passivated stainless-steel connectors, the 26.5-GHz test cables (PE315 and PE317) use external armoring to guard against a bend radius that might damage the cables. These cables operate from (-55 to +165C), making them suitable for environmental test systems. With the introduction of the PE319 and PE321, the manufacturer has pushed its cables to 40 GHz.

Recognizing the growing need for high-performance test-cable assemblies, passive-component suppliers like Mini-Circuits and Synergy Microwave also have entered this fray. Mini-Circuits, for instance, offers rugged, triple-shielded flexible cables for laboratory and field measurement applications. Covering a frequency range of DC to 18 GHz, its CBL series cables deliver low insertion loss while withstanding 20,000 flexures without failure. According to the supplier, the CBL series test cables are available from stock in a variety of lengths and male connector types including SMA to SMA, SMA to Type N, and Type N to Type N connectors.

Using PTFE dielectric material and gold-plated coaxial connectors with brass center pins to minimize reflections and insertion loss, Synergy Microwave is offering precision test cables for use to 18 GHz. While the SF series is tailored for use to 18 GHz, the thinner SFR series ranges to 8 GHz. The cables are specified for temperatures from -55 to +105C for use in environmental test chambers. A 2-ft. cable like the model SF-SMNM typically exhibits 1.21 dB insertion loss at the upper-frequency range (18 GHz) and room temperature.

At Florida RF Labs, the Mini-Flex 105MC is the latest highreliability, flexible coaxial-cable assembly for measurement applications. Tailored for operation to 50 GHz, the assembly is covered with a stainless-steel monocoil (MC), which uses a silicone jacket for added protection without sacrificing flexibility. An overlapping helical shield enables 90 dB shielding. According to the manufacturer, this cable assembly targets applications that require flexibility but can tolerate higher losses. The cable exhibits 1.06 dB/ft. insertion loss at 18 GHz, which drops to 2.01 dB/ft. Currently, the supplier is evaluating its performance relative to phase and amplitude stability.

Meanwhile, RF Precision Products, a division of RF Industries, has launched a line of precision test cables for applications that require multiple connector attachments and continuous flexing. Offering triple shielding for optimum EMI protection, the test cables are configured for low loss and low VSWR. Other key features include high velocity of propagation, a wide operating temperature range, and MIL-DTL-17 type IX compliance.

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