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Get a Grip on Those MM-Wave Connectors

Oct. 20, 2016
Coaxial connectors for mm-wave test equipment have extremely fine structures that must be treated gently for optimum performance and operating lifetimes.
It was bound to happen: It was only a matter of time before electronic measurements would become more widely spread at millimeter-wave (mm-wave) frequencies. Test equipment capable of generating or analyzing signals at frequencies to 30 GHz and beyond was once found only in the most well-equipped test labs, and often those of defense contractors. But as automakers embrace mm-wave collision-avoidance and radar systems—and operators of wireless communications networks look for ways to move a growing amount of data from emerging wireless applications, such as Internet-of-Things (IoT) sensors—the availability of bandwidth (even at mm-wave frequencies) becomes quite attractive.

Of course, mm-wave components and systems must be tested, and leading test-equipment suppliers have been responding over the past year with some excellent products for the price. For users to get the full value of mm-wave test equipment, they must bear in mind that as frequencies go higher, component dimensions get smaller. This is particularly true of the broadband-but-minute coaxial test connectors that interconnect to a device under test (DUT), and these must be treated with kindness to extract the greatest value from mm-wave test equipment.

For those who may recall the evolution of coaxial connectors towards mm-wave frequencies—including the development of the “K” (2.92-mm) connector by Bill Oldfield at Anritsu when it was still known as Wiltron Co., or the work of Julius Botka at Hewlett-Packard Co. before it became known as Agilent Technologies and then Keysight Technologies, or Mario Maury at Maury Microwave—the chase for higher frequencies with coaxial connectors has been on for several decades. There has always been a certain blend of mechanical engineering and magic involved in realizing connectors capable of operating with low forward loss and minimal reflections at frequencies past 60 GHz.

Although not approaching the dimensions of semiconductors, fabrication of mm-wave connectors with their fine metal structures and the air spacings necessary for low-loss electrical performance at those wavelengths is an impressive achievement in itself. For test equipment and measurement applications, such connectors must not simply be secured once in a “set-and-forget” hookup, but must survive thousands of connect-and-disconnect operations; they must maintain tight phase and amplitude characteristics within the limits required by the test equipment and measurements.

The 50-Ω coaxial connectors used on mm-wave signal analyzers and generators, such as 2.9-mm connectors in earlier instruments and 1-mm connectors in more recent mm-wave test instruments, are themselves precision instruments requiring great care and handling for maximum longevity. Inevitably, any coaxial connector will wear out with enough use (or abuse), but it is the minute dimensions of mm-wave connectors that makes them susceptible to damage from even routine connect and disconnect operations. The very interface dimensions that enable low forward and return losses at mm-wave frequencies render mm-wave connectors more delicate than lower-frequency connectors, such as Type-N and SMA connectors with their larger dimensions.

Working at mm-wave frequencies requires a special kind of attention to detail, whether in circuit design, fabrication, or in testing. Not that mm-wave testing is about to become routine or commonplace overnight, but the use of mm-wave frequencies is growing steadily. Signal frequencies at 60 GHz and beyond are being used or planned for use in such applications as automotive safety (collision-avoidance) systems and backhaul links for coming Fifth-Generation (5G) cellular communications systems. The components within those systems and the systems themselves will require development and production testing, which will put regular stress on those small-featured coaxial connectors.

For the sake of measurement accuracy and repeatability, those fine-featured coaxial connectors must be cleaned regularly and treated with care. They are oft-overlooked components in a test instrument, but the connectors are as important as any other component in a test signal source or analyzer—especially at these higher test frequencies. Those early K and V connectors enabled coaxial connections to 60 GHz and beyond for vector network analyzers (VNAs), providing new insights into components and devices at those higher frequencies. Years later, coaxial connectors are being counted upon to make mm-wave production-line components (that have been tested) possible. With proper care, those connectors can be long-term, repeatable, and reliable parts of mm-wave test system solutions.