Signal Analysis Choices for Millimeter Frequencies

Blog Post created by benz on Dec 1, 2016

  A single analyzer can cover 3 Hz to 110 GHz. Is that the best choice for you?

I haven’t made many measurements at millimeter frequencies, and I suspect that’s true of most who read this blog. All the same, we follow developments in this area—and I can think of several reasons why.

First, these measurements and applications are getting a lot of press coverage, ranging from stories about the technology to opinion pieces discussing the likely applications and their estimated market sizes. That makes sense, given the expansion of wireless and other applications to higher frequencies and wider bandwidths. As a fraction of sales—still comparatively small—more money and engineering effort will be devoted to solving the abundant design and manufacturing problems associated with these frequencies.

I suppose another reason involves a kind of self-imposed challenge or discipline, reminiscent of Kennedy’s speech about going to the moon before 1970, where he said the goal would “serve to organize and measure the best of our energies and skills.” Getting accurate, reliable measurements at millimeter frequencies will certainly challenge us to hone our skills and pay close attention to factors that we treat more casually at RF or even microwave.

Of course, this self-improvement also has a concrete purpose if we accept the inevitability of the march to higher frequencies and wider bandwidths. For some, millimeter skills are just part of the normal process of gathering engineering expertise to stay current and be ready for what’s next.

In an earlier post I mentioned the new N9041B UXA X-Series signal analyzer and focused attention on the 1 mm connectors used to get coaxial coverage of frequencies to 110 GHz. In this post I’ll summarize two common signal analyzer choices at these frequencies and some of their tradeoffs.

Two types of solutions are shown below. The first practical millimeter measurements were made with external mixers, but signal analyzers with direct coverage to the millimeter bands are becoming more common.

M1970 & M1971 Smart external waveguide mixers (left) and N9041B UXA X-Series signal analyzer (right)

External mixing (left) has long been a practical and economical way to make millimeter frequency measurements. Signal analyzers such as the new N9041B UXA (right) bring the performance and convenience of a single-instrument solution with continuous direct coverage from 3 Hz to 100 GHz.

My post on external mixing described how the approach effectively moves the analyzer’s first mixer outside of the analyzer itself. The analyzer supplies an LO drive signal to the mixer and receives—sometimes through the same cable—a downconverted IF signal to process and display. This provides a lower-cost solution, where analyzers with lower frequency coverage handle millimeter signals through a direct waveguide input.

In use, this setup is more complicated than a one-box solution, but innovations such as smart mixers with USB plug-and-play make the connection and calibration process more convenient. An external mixer can be a kind of “remote test head,” extending the analyzer’s input closer to the DUT and simplifying waveguide connections or the location of an antenna for connectorless measurements.

The drawbacks of external mixers include their banded nature (i.e., limited frequency coverage) and lack of input conditioning such as filters, attenuators, and preamps. In addition, their effective measurement bandwidth is limited by the IF bandwidth of the host analyzer, a problem for the very wideband signals used so often at millimeter frequencies. Finally, external mixing often requires some sort of signal-identification process to separate the undesirable mixer products that appear as false or alias signals in the analyzer display. Signal identification is straightforward with narrowband signals, but can be impractical with very wide ones.

Millimeter signal analyzers offer a measurement solution that is better in almost all respects, but is priced accordingly. They provide direct, continuous coverage, calibrated results and full specifications. Their filters and processing eliminate the need for signal identification, and their input conditioning makes it easier to optimize for sensitivity or dynamic range.

The new N9041B UXA improves on current one-box millimeter solutions in several ways. Continuous coverage now extends to 110 GHz and—critically—analysis bandwidths are extended to 1 GHz internally and to 5 GHz or more with external sampling.

Sensitivity is another essential for millimeter frequency measurements. Power is hard to come by at these frequencies, and the wide bandwidths used can gather substantial noise, limiting SNR. The DANL of the UXA is better than -150 dBm/Hz all the way to 110 GHz and, along with careful connections, should yield excellent spurious and emissions measurements.

Millimeter measurements, especially wideband ones, will continue to be demanding, but the tools are in place to handle them as they become a bigger part of our engineering efforts.