Ensuring Real Results at 110 GHz and Beyond

1. Introduction
2. Working at the Intersection of Technology and Demand
3. Weighing the Challenges and Advantages of Millimeter-wave Frequencies
4. Focusing on the Measurement Challenges
5. Building on Our History of Leadership
6. Summary
7. Related Information

As developers and designers reach toward terahertz frequencies, it's easy to underestimate the challenges that arise in design, simulation, measurement, and analysis. Compared to signals at baseband, RF or microwave, those at 30 GHz, 300 GHz or 1 THz behave quite differently. At the respective wavelengths of 10 mm, 1 mm or 0.3 mm, propagation losses in the atmosphere are high, especially at the resonant frequencies of oxygen, water and carbon dioxide molecules. The differences also make it difficult to generate power, and it becomes increasingly challenging to make calibrated measurements and get useful results.

Engineers working at the leading edge count on Keysight to give them easier access to accurate, repeatable measurements at ever-higher frequencies and wider bandwidths. Today, we are the industry's leading innovator in the commercialization of tools for simulation, test and analysis at millimeter-wave frequencies.

The ability to develop off-the-shelf tools for extremely high frequencies follows from our proven blend of measurement science and millimeter-wave expertise. Our hardware and software products put those capabilities at our customers' fingertips, and our application engineers are available to work side-by-side onsite. Helping the industry reach higher is our heritage—and we're ready to help it succeed at 110 GHz and beyond.

Millimeter-wave technology has been in use for decades, primarily in aerospace, defense and backhaul applications where the benefits have justified the high costs of development, manufacturing and support. In recent years, advancements in the fabrication of millimeter-wave devices have been pushing down the cost of extremely high frequency (EHF) devices, making them more viable in commercial and consumer applications. For example, CMOS developers have produced devices with ft greater than 500 GHz, and some are aiming to push this cost-effective technology into the 1.0 to 1.5 THz range.

Keysight is also pursuing groundbreaking component-level R&D at extremely high frequencies. Specifically, our in-house expertise in microwave semiconductor technology has allowed us to develop a next-generation indium phosphide (InP) process that supports transistor switching frequencies above 300 GHz. This is opening up higher bandwidths in ICs and end products such as our upcoming oscilloscope that will deliver breakthroughs in real-time and equivalent-time performance.

In the marketplace, millimeter-wave technologies such as 802.11ad are available today and consumer-grade WiGig routers currently sell for about US$350.00. Looking ahead to 2020, development of 5G wireless communication is underway. The ability to meet the 5G vision of "everything everywhere always connected" will depend on successful utilization of wider bandwidths in the recently allocated spectrum at 28 GHz, 37 GHz, 39 GHz, and in the 64-71 GHz band. Other communications applications include millimeter-wave line-of-sight backhaul systems and satellite-to-satellite links.

Leveraging the resolution made possible by 1 mm wavelengths, imaging is another emerging application. Examples include the examination of pill coatings in pharmaceutical production, physical measurements of product content and texture in food production, and medical imaging that produces distinct spectral signatures for healthy and diseased tissues.

Developers of millimeter-wave-based systems may encounter a number of difficulties. Losses through the atmosphere are high, especially at the resonant frequencies of oxygen, water and carbon dioxide molecules. Signal losses are also greater through transmission lines such as coaxial cable and waveguide.

As frequencies increase, physical dimensions decrease. Thus, all the associated hardware becomes smaller and more fragile, and manufacturing tolerances become much tighter. This also means it is more difficult to fabricate and assemble delicate millimeter-wave devices.

All the same, EHF signals have a number of attractive properties. For example, antenna dimensions can be very small compared to microwave antennas and the resulting transmitter and receiver systems can be very compact. In addition, the antennas can be highly directional with small beam widths.

With wavelengths that range from 10 to 1 mm, millimeter-wave signals exhibit absorption properties that, at first, may seem problematic but can instead be turned into advantages. For example, in terrestrial applications these signals are rapidly absorbed as they propagate through the atmosphere.

Given these attributes, millimeter signals are most useful for short-range communications. Some of these rely on areas of low absorption: automotive radar (77 to 81 GHz), point-to-point radios, wireless backhaul links, and high-altitude radio-astronomy arrays.

Others utilize high absorption as a way to reduce interference between users. For example, the 802.11ad (WiGig) standard for high-speed audio and video links operates in the unregulated 60-GHz region. Unlike typical Wi-Fi signals, this frequency has a range of about 40 feet (about 12 meters) and is attenuated by wood, stone and glass, making it a good choice for home-entertainment installations in apartment buildings, condominiums or townhomes. Coupling high-absorption properties with highly directional antennas also enables creation of secure communication systems that minimize the chances of unauthorized eavesdropping.

The two key issues mentioned above, guiding signals and generating power, are even more challenging in the creation of commercial, off-the-shelf test equipment that produces accurate, repeatable results.

As an example, waveguide must be as close to perfect as possible to ensure proper internal operation of any millimeter-wave instrument. Working within the range of frequencies between 100 GHz to 1 THz requires use of different waveguide bands. At millimeter wavelengths, any skew in a flange connection can cause unwanted reflections that degrade signal quality and power.

Generating adequate signal power is a challenge because it is difficult to simultaneously maintain amplifier efficiency and linearity at these frequencies. This tends to limit the top-end power that can be produced with a signal generator or network analyzer. Related to this, wider bandwidth is an alluring feature of millimeter-wave; however, a wideband measurement introduces more noise into the instrument and thereby raises the noise floor. The net effect: lower maximum power and a higher noise floor will reduce the available dynamic range in wideband spectrum measurements.

Once you get past these difficulties, the next crucial challenge is in calibration of the instrument and the test setup. In addition, it is difficult to accurately calibrate power levels at extremely high frequencies, but precise control of power is essential to ensuring measurement accuracy and avoiding damage to the device under test (DUT).

Measurements themselves are very different at these frequencies—and this may require an engineer to set aside old habits and adjust their expectations. From spectrum analysis and the assessment of distortion or spectrum emission mask (SEM) to network analysis and the characterization of passive (S-parameters) or active devices (X-parameters), every stage—instruments, cables, accessories—must be right: pristine connections, clean upconversion of output signals, precise downconversion of incoming signals, low-level internal spurious signals, well-managed internal harmonics, and more.

Finally, one more critical difference adds to the challenges: in some cases the connection between instrument and DUT must be made over the air (OTA) rather than through cables or waveguide. In an OTA situation, it is necessary to control and calibrate the radiated environment around the test setup. There must also be a way to consistently control or lock down any directional element in the DUT to ensure repeatable measurements.

Although millimeter-wave has recently started to ramp up in commercial applications, Keysight has been advancing the learning curve for decades. Our earliest gigahertz products date back to 1967 and the introduction of the HP 8410 network analyzer, which measured up to 12 GHz and also computed S-parameters.

Our first millimeter-wave equipment followed in the late 1980s with signal generators that reached above 26.5 GHz with upconverters, and broadband network analyzers that covered 45 MHz to 100 GHz. Most recently, we have continued to advance the state of the art with instruments ranging from the 67 GHz PNA network analyzers (2006) to the 50 GHz FieldFox handheld spectrum and combination analyzers (2015). In addition to the new 110 GHz N9041B UXA X-Series signal analyzer (October 2016), several recent products demonstrate our leadership in millimeter technology:

• 90 GHz 86100D Infiniium DCA-X wide-bandwidth oscilloscope
• 68 GHz E7760A wideband transceiver
• 67 GHz PNA-X microwave network analyzers (extendable to 1.1 THz)
• 67 GHz E8257D PSG analog signal generator (extendable to 1.1 THz)
• 63 GHz Infiniium Z-Series oscilloscopes
• 50 GHz M9393A PXIe performance vector signal analyzer

Through the use of frequency-extender products from two of our solution partners, Virginia Diode, Inc. (VDI) and OML, Inc., many of our signal generators, spectrum analyzers and network analyzers can cover frequencies between 50 GHz and 1.5 THz. As an example, a recently deployed solution includes spectrum analysis capability up to 1.5 THz.

Keysight application software extends the capability of these instruments. For example, 89600 VSA software enables detailed analysis of complex modulated signals, and Signal Optimizer software is the industry's only all-in-one application for calibration, signal creation and signal analysis at millimeter-wave frequencies.

Keysight software products address the need to integrate design, simulation, measurement, and analysis at millimeter-wave frequencies. Our software solutions for design and simulation provide an efficient workflow that accelerates development of next-generation devices and systems. The Keysight EEsof EDA suite includes circuit simulators, electromagnetic solvers and device-modeling solutions that help designers from first design through first prototype. In fact, the N9041B R&D team used our Advanced Design System (ADS) software to develop millimeter-wave filters used in the front end of the analyzer.

Current applications of these tools include investigation and development of 5G wireless communications (up to 100 GHz), millimeter-wave backhaul (up to 95 GHz), satellite communications (up to 110 GHz), automotive radar (up to 79 GHz), and fire-control radar. Emerging applications encompass development of devices and systems capable of performing high-resolution materials measurements in manufacturing, pharmaceutical and medical settings.

For more than 75 years, engineers have counted on Keysight to give them easier access to accurate, repeatable measurements at ever-higher frequencies and wider bandwidths. Today, we’re extending the leading edge for R&D engineers performing design, simulation and measurement at millimeter-wave frequencies. New products such as the N9041B UXA signal analyzer and the upcoming oscilloscope based on our next-generation InP process are testament to our ongoing leadership in solutions for millimeter-wave applications.