Millimeter Measurements: They’ll Test You

Blog Post created by benz on Oct 20, 2016

  Don’t sacrifice the benefits of improved equipment by missing the basics

Just a decade ago I would have found it hard to believe, but millimeter-frequency applications above 50 GHz are going genuinely mainstream. Wireless HD, 802.11ad wireless networking, 5G cellular, and automotive radar are high-profile examples of an important trend, supported by remarkable advances in semiconductor technology.

I’m not surprised at the existence of applications at these frequencies, but I am surprised that retail prices begin under $200 for Wireless HD, and radar options are available on fairly pedestrian cars, albeit the higher-end versions. Bringing this technology to the mass market at the price points required for large-scale adoption poses tremendous challenges in design and manufacturing.

The engineering challenges have been a little less daunting as more millimeter test equipment has become available with single-box direct-connection coverage to 67 GHz. Keysight’s recent introduction of the N9041B UXA X-Series signal analyzer breaks new ground, increasing direct coverage in coax to 110 GHz. The new signal analyzer provides low noise, good accuracy, and wide bandwidth—1 GHz for internal sampling and 5 GHz for external—to allow engineers to focus on their designs and measurement results instead of pulling together multi-part test solutions where calibration and repeatability may be in question.

However, all this hard-won (and rather expensive) performance can be compromised if you miss even one of the fundamental practices for good measurements at these very high frequencies. The millimeter range is generally defined as 30–300 GHz, with wavelengths down to 1 mm, and these tiny wavelengths are the heart of many problems and challenges.

Connectors are a good place to start because they exemplify so many of the ways that millimeter measurements will test you. Here’s a close-up of female and male 1 mm connectors, which are mode-free to more than 110 GHz.

1 mm millimeter connectors male female

Female and male 1 mm connectors. The center pin in the jack on the right is only one-quarter of 1 mm in diameter, generally too small for me to see without magnification. (Image from Wikimedia Commons)

The small size and precise geometry of millimeter connectors and cables demand special machining and fabrication. They are necessarily somewhat expensive and inescapably more delicate as frequencies increase and dimensions decrease. In a previous post I described their frequency ranges and intermating possibilities, and here I’ll note that, despite mechanical compatibility, all intermating and even same-connector mating still produces impedance problems that should be avoided wherever possible.

Perhaps the most important connection is the one at the front panel of the test equipment. Despite their delicacy, male connectors are better than the alternative at millimeter frequencies. The usual practice is to attach a female-to-female “connector saver” at the instrument, but this choice is complicated by the fact that impedance problems and loss through cables and connectors or adapters also get worse as frequencies increase. In some cases, it’s worth the cost and trouble of acquiring custom cabling with correct gender at each end, especially considering how precious power and performance are at these frequencies. Custom cabling also allows the cables to be as short as possible. Indeed, one tactic that is sometimes overlooked is to simply move the DUT and instrument as close to each other as practical.

The picture above suggests another area of best practices: connector care. These connectors do not appear obviously damaged or displaced, but some contamination is clearly present. Because of their tiny dimensions, special cleaning materials and techniques are needed for microwave and millimeter connectors. Connector gauges are also important to ensure that mechanical dimensions are within the tight tolerances that provide a reasonable impedance match. For more detail on torque and other coaxial connection issues and practices, see the classic (old!) application note Principles of Microwave Connector Care (AN-326), Keysight literature number 5954-1566.

Proper connector torque is another fundamental for good millimeter connections, and in a previous post on torque   I discussed the mechanical essentials and ways to avoid damaging these pricey little parts.

Finally, given the connection losses in coax and the sometimes cumbersome physical implications of waveguide, you may want to consider external mixers. Keysight’s Smart Harmonic Mixers cover 50-110 GHz and make this approach much more convenient and accurate than previous mixers. They allow you to create a remote test head, placing the measurement plane right at the DUT. While they lack the IF bandwidth of the new UXA signal analyzer, they do allow non-millimeter signal analyzers to cover these high frequencies.

These fundamentals are hardly “basics,” but they’re straightforward practices to implement—and they’ll help you pass the tests you’ll face as your designs take you well into millimeter territory.