Solving Millimetre Wave Test Challenges
Millimetre wave (mmWave) frequencies have traditionally been dedicated to military applications, with some commercial use for point-to-point microwave links. Octave bandwidth waveguide was the preferred transmission line as mmWave capable coaxial cables and connectors were not available.
That has changed during the past decade, with technology improvements in semiconductors, components, cables and connectors. As mmWave frequencies are now being used for commercial and consumer electronics, design engineers must be aware of the issues encountered using coax cables within a test system. Reducing test equipment size and using fewer interconnections will yield improved measurement accuracy.
Several market segments are adopting mmWave frequencies, each with its own design and test considerations. These include 5G, Automotive Radar, 60 GHz Wi-Fi (WiGig), Point-to-Point Links & Security/Defence applications.
Higher frequency transmission, however, comes with challenges such as higher propagation loss, measurement repeatability and field testing. The loss of a signal propagating at RF and microwave frequencies is proportional to the frequency and distance. At mmWave frequencies, there is additional attenuation from components of the Earth’s atmosphere, such as oxygen. The additional loss presents a testing challenge. Test equipment needs higher output power or improved sensitivity for accurate measurements at these bands.
At 70 GHz, the diameter of the coaxial centre connector is just 0.5 mm. A centre pin diameter is the same size. Connector dimensions are approaching the limits of machine shops, and scratches and dust particles on the connector interface affect the impedance match at mmWave frequencies. mmWave connections require significantly more care than at lower frequencies. Connector interfaces should be inspected with a microscope and cleaned before each use.
Spectrum analysers are often used to measure the path loss of a proposed wireless link. The setup comprises a test source with antenna and spectrum analyser with antenna, placed at realistic locations. At lower frequencies, a bench instrument on a cart with the antenna elevated on a pole and fed with a coaxial cable is used. At mmWave frequencies, a similar approach with a long cable run results in significant loss. For example, at 70 GHz, a 3 m cable has more than 20 dB loss, reducing measurement range and accuracy. Also, the loss and phase characteristics of cables vary with temperature, which adds to the uncertainty. To address this, a portable spectrum analyser can be connected directly to the antenna and elevated above the control PC, using a USB extender cable to interface with the analyser.
Reducing the number of connections in a test system reduces measurement error and the possible points of failure, including dust or dirt affecting the return loss of a connection. It also minimises the chance for imperfections that cause test system impedance to vary from 50 Ω. Each connection in the system (male to female connector pair) will add uncertainty, and mmWave connectors and cables are particularly sensitive. They must be handled carefully to ensure accurate measurements.
Advances in mmWave testing over the years are also enabling more accurate measurements at these frequencies. The introductions of the 40 GHz K connector in 1983, the 70 GHz V connector in 1989 and the 110 GHz W connector in 1997 are examples of such innovations.
Test equipment has also progressed to meet the market need: VNAs are now available to 145 GHz, as well as spectrum analysers in ultra-portable form factors. Some VNAs have very small mmWave frequency extension modules that enable full frequency coverage for on-wafer measurement systems. Using nonlinear transmission line (NLTL) technology allows the probe’s tips to be mounted directly to the modules, enhancing measurement and calibration stability. A handheld spectrum analyser using this same technology is slightly bigger than a smartphone and provides performance similar to a benchtop instrument in a much smaller and more affordable design. The small size allows direct connection to almost any DUT.
In the past decade, technology improvements in semiconductors, components, cables, connectors and test equipment have helped make it possible for mmWave frequencies to be used for low cost commercial and consumer electronics products and systems. The continuing evolution of test instruments will significantly reduce mmWave measurement challenges and improve measurement performance and accuracy.
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