Solving the issue of reflected waves at D band

Wireless communication is often the focal point, but commercial applications include high-precision sensing, radio astronomy, and airport security detectors. For the military, the D band will open the door to the next generation of inter-satellite links, imaging radars, and stand-off detection. 

However, as research and development teams attempt to deliver on the much-anticipated move into the frequency range between 110–170GHz, they are running into a common problem of signal reflections, also known as mismatches. These undesirable waves, or ripples, can attenuate power output, distort the digital information on the carrier and, in extreme cases, damage internal components.

To offset the issue of mismatches at lower microwave frequencies, engineers rely on Faraday rotation isolators but at higher frequencies, like those within the D band, traditional isolators often struggle to deliver the same results.  

“When you get above 110GHz it’s sort of uncharted territory,” explains Ed Loewenstein, Chief Architect at NI. “Your connectivity gets a little strange and cables don't work very well anymore. So, most things are done in waveguides.”

Suppressing signal reflections

Until recently, there has been limited test and measurement equipment available at D band frequencies, with few standards and little-to-no traceability to NIST (National Institute of Standards and Technology).

“What we often hear is that once someone sets up their equipment to make a test or measurement at these frequencies and something doesn't go quite right, they spend most of their time trying to figure out whether it's their test equipment or the device that they’re testing,” adds Loewenstein.

Therefore, the development of the D band is dependent on companies like NI, formerly known as National Instruments, which create the equipment that engineers rely upon to efficiently and accurately perform comprehensive research, testing, and validation. However, recently, as the company was looking to create new 6G sub-THz reference architecture they ran into the issue of reflected waves themselves in the waveguides.

“We were really struggling with bad mismatches in our waveguide system and kept getting these really bad frequency ripples,” adds Loewenstein.

In millimetre wave (mmW) systems, the distance between components is often much larger than a wavelength. As you sweep frequencies, the phase changes and there are nulls, dips, and degraded performance. To resolve that in microwave frequencies, engineers simply insert an isolator between components and the reflected signal gets absorbed.

However, as you move up the electromagnetic spectrum shorter wavelengths require smaller constituent parts. At mmW frequencies, the parts are tiny and even the smallest misalignment can significantly degrade performance.

As demand for D band systems increases, so does the number of isolator options available. Unfortunately, their performance still lags behind what engineers have become accustomed to at lower frequencies.

Advances in mmW isolator design

“We evaluated the isolators on the market from what data was available and it was pretty easy arithmetic to see that Micro Harmonics was the most appropriate thing for us,” adds Lowenstein.

Micro Harmonics Corporation is a Virginia-based manufacturer specialising in the design of mmW components. Under a NASA contract, Micro Harmonics essentially reinvented the isolator so that it can operate well into THz frequencies.

The traditional method to manufacture an isolator has been to use ferrites that are substantially longer than required, and then tune the magnetic bias field to achieve optimal performance. This delivers good isolation but at a much higher insertion loss.

What Micro Harmonics did to minimise loss was reduce the ferrite length by as much as possible. The design developed for NASA saturates the ferrite with a strong magnetic bias field, which allows for the shortest possible length of ferrite to achieve the ideal 45° of rotation. This lowers the insertion loss to less than 1dB at 75–110 GHz and only 2dB at 220–330 GHz.

The only way to confirm such precision is to fully characterise each isolator on a vector network analyser. This validates total compliance, as opposed to just spot-checking at a couple of frequencies in the band. The test data are then supplied with each isolator. 

“They are one of the vendors that we’ve encountered that actually supplies full S-parameter files, which include magnitude and phase,” says Lowenstein. “When dealing with a high-value component, whose performance you are counting on, I would say this type of information is essential.”

Meeting deadlines for D band

NI needed to deliver its new mmW test and measurement equipment as well as its advanced software-defined radios which can greatly accelerate prototyping and next-gen wireless innovation.

Loewenstein says it was important that the isolator could cover the entire D band which the isolators designed for NASA could do.

“It was a nice bonus that it is designed in a small cube, so it fits directly onto a flush surface without a rear accessible flange, explains Loewenstein. “You don't need extra waveguide sections or anything.”

NI measured their systems before and then after inserting the isolator and saw a noticeable improvement in the ripple which they said was due to cleaning up the mismatch reflections. This allowed them to announce the 6G Sub-THz reference architecture that provides calibrated measurements with up to four gigahertz of modulation bandwidth.

“We can now receive and transmit at D band. We can also use software-defined radio techniques to prototype and make measurements that are traceable to a power meter,” says Loewenstein. 

The insatiable demand for data bandwidth, especially for communications, and the existing spectrum available within D band has companies charging ahead at lightspeeds. As part of that allure, the FCC recently allowed some experimental licenses in D band and up to 220 gigahertz so that people can begin to develop new broadband communication methods that use those frequencies.

“Right now, people are looking to do research in sub-terahertz communication. They are designing future chipsets, radio links, or the protocols,” explains Loewenstein. “So, we are looking to allow them to make tests and measurements as well as utilise our software-defined radios to enable future growth within the D band and beyond.”