Cables Connect Test Equipment

A large number of suppliers offer custom configurations and standard lengths of low-loss coaxial cables for use with test equipment, many with online calculators to predict performance.

Cables and connectors are often taken for granted in a communications system, radar, or test systemthat is, until they fail to perform. While their function is simple (transferring signals from one point to another), doing so with minimal loss or phase distortion is not always a trivial matter. Fortunately, the RF/microwave industry boasts a long list of high-quality suppliers of coaxial cables, connectors, and cable assemblies in a variety of diameters and materials for applications through 60 GHz and beyond.

Coaxial cables for RF/microwave applications generally fall into two categories: flexible and semi-rigid types. In terms of superior loss performance and phase stability, the choice was once clear in favor of semi-rigid cables. But flexible cables have improved in performance to levels much closer to those of their semi-rigid counterparts. And while semi-rigid cables and cable assemblies (with connectors attached) can provide the highest levels of electrical performance, they do so with the least amount of mechanical flexibility. Such cables are generally machined to specific lengths and bend radii so that they fit precisely within a system and its necessary interconnection points. They are meant to remain in place, except during maintenance cycles, and not to be removed and reinstalled repeatedly.

When a cable or cable assembly must be inserted and removed repeatedly, as is the case with a cable assembly used for connecting a device under test (dUT) or test fixture to a test system, a flexible cable assembly is much more suitable for the task. Of course, the phase stability of the cable then becomes a concernespecially for applications (such as a phased-array radar) in which phase is one of the key operating parameters. The same is true in test systems where phase is being measured, such as with a vector network analyzer (VNA). Any flexible cable or cable assembly used for those applications must provide acceptable phase stability with flexure and with temperature to minimize the introduction of phase distortion to the system. another key parameter for a flexible cable is its minimum bend radius, which determines how the cable can be shaped to fit a particular system requirement, presumably by achieving that small bend radius without damage to the cable.

Some suppliers, such as Astrolab, offer both types of coaxial cablesboth in raw cable form and as cable assembliesalong with advice on which cable is suitable for a particular application. The firm provides cables in diameters from 0.041 to 0.500 in. for applications through 50 GHz, and can terminate any required length of cable with a wide range of precision coaxial connectors, including 7-mm, 3.5-mm, and SMA connectors. Astrolab can also supply cable assemblies as amplitude-matched or phase-matched pairs or sets, for use in demanding applications such as radar warning receivers (RWRs) and phased-array radar systems. The company manufactures its own flexible cables for these assemblies, including minibend cables and many variants of them, such as low-loss and ruggedized versions. Ideally, the connectors at the end of a cable assembly should have the same characteristic impedance as the cable itself, and not introduce any impedance discontinuities to the assembly. Cables for use with high-speed signals are also often compared by means of their velocity of propagation; this is a measure of the speed of a radio wave through a cable relative to the speed of the radio wave in a vacuum, which would be at the speed of light (c). Depending on the materials used in the construction of a coaxial cable, including its dielectric material, the velocity of propagation can vary widely. Usually, though, it is 90% or less.

Times Microwave Systems offers several cable performance calculators, both downloadable and online, for predicting such parameters as loss and VSWR for a given length and type of coaxial cable. An operator simply selects the cable model number, such as the company's popular LMR-100a cable, a flexible indoor/outdoor low-loss RF cable which provides better than 90-db shielding effectiveness for connecting antennas to WiMAX and wireless-local-area-network (WLAN) systems. Once a frequency range and cable length are then entered, it displays the average power level that the cable can channel, attenuation in dB/ft. and dB/m, total run attenuation, and efficiency. It does not, however, include the electrical effects of connectors if considering a cable assembly.

Some companies, such as Flexco Microwave, offer cables assembled for different applications, including test and measurement use. The company's NTC195 and NTC182 VNA test cables, for example, are usable through 26.5 and 40.0 GHz, respectively, with velocity of propagation of 74% and 69%, respectively. They incorporate a convoluted outer conductor for flexibility, manufactured and sealed using a patented process to form the tubular outer conductor from high-quality copper. The inner conductor from silver plated copper, while dielectric materials range from PTFE, expanded PTFE, and polyethylene dielectric. Both cable types are designed to provide excellent amplitude and phase recovery from a flexure, allowing cables to be repositioned during a test without penalties in measurement accuracy.

Some suppliers of cable assemblies are better associated with their lines of RF/microwave components. Crystek Microwave, perhaps best known for its oscillators, also offers connectors, cables, and complete cable assemblies through 40 GHz.

Mini-Circuits, a broad-based supplier of RF/microwave components, recently introduced armored versions of their KBL series test cables, in standard lengths of 1.5, 2, and 4 ft. as well as 1 and 2 m. For example, model KBL-1MLOW+ is a 1-m-long, low-loss KBL cable assembly terminated with stainless-steel 2.92-mm male connectors for durability and long operating lifetime. Usable through 40 GHz, the cable assembly can handle 17 W power at 18 GHz and 11 W power at 40 GHz. Typical insertion loss is 1.06 dB from DC to 6 GHz, 1.87 dB from 6 to 18 GHz, 2.36 dB from 18 to 26.5 GHz, and 3.06 dB from 26.5 to 40.0 GHz.

The SF series of SYNFLEX test cables from Synergy Microwave Corp. is usable from DC to 18 GHz, while the SFR series is for applications from DC to 8 GHz. The SF series cable assemblies feature an inner conductor of solid silver-plated copper clad steel, with solid PTFE dielectric and blue fluorinated ethylene propylene (FEP) jacket. The SFR series cable assemblies are constructed with stranded silver-plated steel inner conductor with PTFE dielectric and gray polyvinylchloride (PVC) jacket. The connectors feature a gold-plated brass center contact with passivated stainless steel coupling nut. These cable assemblies are available in standard lengths of 2 and 3 ft. with SMA and Type N connectors.

Radiall supplies cables, connectors, and cable assemblies, and offers online "cable building" software to help specifiers select the best cable for their applications. Cables can be selected according to outer jacket, connector types, and even suggested price. Radiall's TestPro test cables are claimed to suffer minimal performance degradation even after 5000 mating cycles. TestPro 4.2 cable assemblies operate from DC to 20 GHz with only 0.58 dB/ft. attenuation at 18 GHz and only 2 deg. deviations in phase with flexure at 18 GHz. The amplitude stability is better than 0.05 dB at 18 GHz, with shielding effectiveness of better than110 dB at 1 GHz. For test applications through 40 GHz, Radiall's TestPro 3 cable assemblies suffer only 0.94 dB/ft. attenuation at 40 GHz with phase deviations with flexure of only 5 deg. at 40 GHz. As with the lower-frequency cable assemblies, they exhibit shielding effectiveness of 110 dB at 1 GHz.

Semflex, part of Emerson Network Power, produces a variety of cable assemblies for test and general-purpose applications. The company's RHT, HPT, and DKT series of cable assemblies are machined to provide excellent phase stability through 50 GHz. They differ in cable diameter and construction: The RHT and HPT cables capable of low-VSWR performance through 50 GHz, while the DKT cables are designed for use through 26.5 GHz. All three offer shielding effectiveness of better than 90 dB at 18 GHz, and are stocked in standard lengths with a variety of connector types for quick delivery.

Pasternack provides cable assemblies for use with VNAs, and even offers a handy PE310-KIT collection with a number of different lengths of flexible cable assemblies for measurement applications. The company's online "Cable Assembly Wizard" helps guide specifiers through the selection of coaxial and fiberoptic cables based on a variety of different terminating connectors.

MegaPhase's MegaPhase 67 line of test cables is aimed at the latest generation of millimeterwave VNAs operating into the 60-GHz region. These test cables, which are usable through 67 GHz, feature low attenuation of 1.05 dB/ft. at 34 GHz and only 2.05 dB/ ft. at 67 GHz, with minimum shielding effectiveness of 100 dB. They are available in standard lengths to 72 in. and are supplied with 1.85-mm connectors.

The list of suppliers for cables, connectors, and cable assemblies, including those specializing in products for test applications, is quite long. It includes ESM Cable Corp., Fairview Microwave, Huber+Suhner, Insulated Wire, Micro-Coax, Rosenberger, SRC Coax, Storm Products, Times Microwave, and WL Gore. It also includes numerous suppliers of test instruments, including Agilent Technologies, Amplifier Research, Anritsu Co., and National Instruments. Many offer standard products in set lengths, such as 1 ft. or 1 m, enabling most engineers to secure a highquality measurement cable for a test application almost overnight when needed.

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