Checking Cables For Test Systems

Jan. 19, 2011
A certain amount of flexibility is helpful in cable assemblies used for measurement applications, especially when making high-frequency or high-speed interconnections.

Coaxial cables are one of those components typically ignored in a system—until they fail. at their best, microwave cable assemblies add nothing: no loss and no phase distortion. at their worst, cable losses can be so severe that they must be offset by broadband amplifier stages. Coaxial cables and cable assemblies intended for test and measurement applications must truly be “invisible” to the system so that their electrical characteristics are not being measured as part of those for a device under test (DUT).

Although some engineers building an RF/microwave test system may prefer to “roll their own” cable assemblies by cutting flexible or semi-rigid cables to length and terminating those lengths in their coaxial connectors of choice (usually dictated by the frequency range of the test equipment), most prefer to go the route of ordering coaxial cable assemblies. The list of suppliers for such assemblies is long, even for the high-performance precision cables needed for test applications, and most offer assemblies in standard and custom lengths.

While semi-rigid cables have been often used in automatic-test-equipment (ATE) systems in which a DUT can be automatically placed within an appropriate test fixture, many measurement systems based on a microwave vector network analyzer (VNA) rely on connecting a DUT to the test system ports by means of flexible cable assemblies. Some companies, such as Flexco Microwave, offer precision cable assemblies specifically developed for use with a VNA. The firm’s NTC195 test cables, for example, can be used from DC to 26.5 GHz while the NTC182 test cables extend from DC to 40 GHz. The former uses a stranded center conductor while the latter has a solid center conductor. Both cables have an outer diameter of 0.5 in. and a minimum bend radius of 1.5 in. Both 50-Ω cables deliver minimum shielding effectiveness (SE) of 90 db. Depending on frequency requirements, the cable assemblies can be terminated in various different connectors, including K, 3.5-mm, SMA, TNC, and Type N connectors.

Semflex has developed test-grade cables with minimal phase change with temperature (-65 to +95°C) and with flexure (360°), with only 20 db loss per 100 feet of cable. Radiall uses a low-density PTFe dielectric in its TestPro 4.2 cables to achieve frequency coverage from Dc to 20 GHz. These cables feature an outside diameter of 4.81 mm and low insertion loss of 2.1 dB/m at 18 GHz. The company also manufactures TestPro 5 cable assemblies for use in measurement applications from DC to 26.5 GHz. With a slightly larger outside diameter of 5.85 mm, these cables feature insertion loss of only 1.02 dB/m through 18 GHz with amplitude stability of better than 0.05 db. Both types of cable assemblies have a minimum bend radius of 25 mm (about 1 in.).

Pasternack offers cable assemblies from DC to 26.5 GHz for use with commercial VNAs, such as those from Agilent Technologies and Anritsu Company. The firm’s PE310-KIT package includes various lengths of flexible cable assemblies. The company’s online “Cable Assembly Wizard” helps guide specifiers through the selection of coaxial and fiber-optic cables based on a variety of different terminating connectors.

The CBL series of test cables from Mini-Circuits cover a range of Dc to 18 GHz and 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. The SF series of test cables from Synergy Microwave use a PTFE dielectric with gold-plated coaxial connectors with brass center pins for low-loss coverage with a variety of connectors from DC to 18 GHz. The Mini-Flex 105MC cable assembly from Florida RF Labs incorporates a stainless-steel monocoil and silicone jacket for coverage from DC to 50 GHz depending upon choice of connector. The 160 series of flexible cable assemblies from Crystek Microwave include stainless-steel connectors for operation from DC to 40 GHz, with only 0.7 dB/ft. loss at 26 GHz and 0.9 dB/ft. loss at 40 GHz. Additional suppliers of high-performance cable assemblies include JMicroTechnology, MegaPhase, National Instruments, RF Precision Cables, Storm Products , and Times Microwave Systems.

MegaPhase, for example, recently introduced a low-cost alternative to original- equipment-manufacturer (OEM) test cables for applications through 67 GHz. Suitable for use with VNAs and other microwave test instruments, the MegaPhase 67 test cables exhibit 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. The test cables are available in standard lengths from 8 to 72 in. and supplied with 1.85-mm connectors.

Times Microwave Systems, for example, offers broadband coaxial cable assemblies for wireless and test applications, with the longer lengths (such as 100 ft.) required for communications systems applications, typically for use at 2.4 and 6.0 GHz. The company’s Silver- Line Test Cables consist of solid PTFE dielectric cables with stainless-steel connectors and an effective strain-relief system for stable amplitude and phase performance through 26.5 GHz. The flexible cables are available in armored and unarmored versions with phase stability within ±1.1º at 10 GHz. The unique strain-relief system can withstand a 175-lb. pull force without damage to the cable, making them ideal for “heavyhanded” test system operators.

Storm Products, a Teledyne Technologies company, offers semi-rigid cables, flexible cables, and test cable assemblies, including the Accu-Test line of flexible test cable assemblies. The Accu-Test 200 cables support operation through 26.5 GHz with outstanding phase stability: 2º phase variation with flexure at 18 GHz and maximum attenuation of 0.116 dB/ ft. at 18 GHz (and nominal attenuation of 0.108 dB/ft. at 18 GHz).

For higher-frequency applications, the Accu-Test 150 test cable assemblies are usable to 40 GHz. It matches the phase stability with flexure of the lower-frequency cable assemblies, with slightly higher maximum attenuation of 0.145 dB/ft. at 18 GHz and nominal attenuation of 0.135 dB/ft. The shielding effectiveness for both cables is better than 90 dB (and better than 100 dB for the 40- GHz cable assemblies).

Flexible cable assemblies designed for test applications are characterized in terms of voltage standing wave ratio (VSWR), amplitude stability versus frequency, and changes of phase versus frequency as a function of cable movement. The cables are evaluated by forming them into small loops, measuring performance, and then straightening the cables to see how well they recover their initial amplitude and phase characteristics.

For test applications that may require a fully nonmagnetic construction, United Microwave Products offers its Microflex 165 flexible cable assemblies for use from DC to 26.5 GHz. With a finished diameter of 0.165 in., the cables feature a fused laminated polytetrafluoroethylene (PTFE) dielectric core and passivated stainless-steel SMA connectors. They can achieve better than 90-dB SE and a minimum bend radius of 1 in. The firm also supplies Microflex 150 as a flexible alternative to 0.141-in.-diameter semi-rigid coaxial cable. With an outer diameter of 0.150 in., Microflex 150 has a minimum bend radius of 1 in. and is usable from DC to 26.5 GHz when terminated in various types of SMA coaxial connectors. For extremely high-frequency applications, Microflex 098 flexible cable assemblies operate through 65 GHz with 1.85-mm V connectors. They use a solid PTFE dielectric.

Huber + Suhner supplies raw cables, cable assemblies, and special test cables in the form of its SUCOTEST assemblies, including its SUCOTEST 18A product lines. These armored test assemblies are engineered for outdoor and harsh test environments at frequencies through 18 GHz. They maintain insertion-loss stability of better than 0.05 dB through 18 GHz with phase stability of better than ±4º through 6 GHz and better than ±6º through 18 GHz. The maximum insertion loss is 2.74 dB for a 1500-mm-long cable assembly at 18 GHz. They are suitable for outdoor wireless and military test systems and are waterproof and offer high flexibility in spite of the armbor protection.

Recognizing that high-speed digital test systems require cables with much of the same electrical performance as high-frequency analog test systems, the company also developed its low-loss MXP breakout system (see figure) for use with its high-performance coaxial cables. The MXP interface supports digital testing to 40 Gb/s (analog range of DC to 40 GHz) with low insertion loss . In many cases, the flexible cables will be the frequency-limiting element of an MXP assembly, such as the company’s DC-to-26.5-GHz Sucoflex 404 flexible cable. To support test system engineers, the company offers computer modeling files for its MXP system on its web site, on the specific web page —JB

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