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Better Measurements at Centimeter and Millimeter Bands

Blog Post created by benz on May 20, 2017

  Handling the frequency and bandwidth challenges of 5G, radar, and more

5G technologies and markets are much in the news these days, and for good reasons. The economic potential is large, the opportunities are relatively near-term, and the technological challenges are just the kind of thing RF engineers can get excited about. Whether your focus is on design or test, there is plenty of difficult work ahead in the pursuit of practical ways to fully utilize the potential capacity of the centimeter and millimeter bands.

Unfortunately, much of the analysis and commentary focuses on economic factors and broad-brush coverage of technological challenges. A good overview of a complex subject is essential for resource planning, but it isn’t deep enough for us to see the specific measurement challenges and how we might handle them.

Some measurement experts have a “just right” combination of broad technical knowledge and specific measurement insight to make a contribution here, and I can heartily recommend Keysight’s Pete Cain. He has not only the expertise but also an impressive ability to explain the technical factors and tradeoffs.

Pete recently produced a webcast on millimeter-wave challenges, and it’s a good fit for the needs of the RF/microwave engineer or technical manager who will be dealing with these extreme frequencies and bandwidths. It’s available on-demand now, and I wanted to share a few highlights.

His presentation begins with a discussion of general technology drivers such as the high value of lower-frequency spectrum and the public benefit of shifting existing traffic to higher frequencies to free it up whenever possible. That’s an important issue, and perhaps a matter of future regulation to avoid a tragedy of the commons.

Next, Pete goes on to explain the problem of increased noise that goes along with the wider bandwidths and increased data rates of microwave and millimeter bands. This noise reduces SNR and eventually blunts channel capacity gains, as shown here.

Comparing the maximum spectral efficiency of different channel bandwidths. The S-shaped curve shows how spectral capacity and channel efficiency returns diminish as bandwidths get very wide.

The wide bandwidths available at millimeter-wave frequencies promise dramatic gains in channel capacity. Unfortunately, these bandwidths gather up more noise, and that limits real-world capacity and spectral efficiency.

As shown in the diagram, Pete also discusses spectral efficiency and shows where existing services operate. This is where RF engineers have already turned theory into practical reality, and the landscape of tradeoffs they’ll optimize as millimeter technologies become widespread.

To further inspire the technically inclined, Pete dives deeper into the essentials of high-frequency testing, including the issues of loss and frequency response at microwave and millimeter-wave frequencies. As is often the case, high quality measurements require a combination of hardware, software, and careful measurement technique. In particular, he describes the value of establishing source and analyzer calibration planes right at the DUT, thereby minimizing measurement error.

Diagram shows measurement of a millimeter-wave DUT where the calibration planes of the input and output have been moved to the edges of the DUT itself, for better accuracy.

To optimize accuracy, it’s necessary to move the calibration planes of measurements from the instrument front panels to the DUT signal ports. Software such as the K3101A Signal Optimizer can make this much easier.

Moving the calibration planes to the DUT ports grows more important as frequencies increase. Loss is an issue, of course, but in many cases the thorniest problems are frequency-response effects such as ripple and non-repeatability. Ripple is especially troublesome for very-wideband signals, while repeatability can be compromised by sensitivity to cable movement and routing as well as connector torque and wear.

In the webcast, Pete also compares signal-connection methods, including coax, waveguide, probes, and antennas.

That’s just a quick overview of an enlightening presentation. To see the whole thing, check out the “Millimeter wave Challenges” on-demand webcast—and good luck in the land of very short waves.

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