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Designing At High Frequencies: 60 GHz And Beyond (Even THz!)

Designing At High Frequencies: 60 GHz And Beyond (Even THz!)

The trend of moving toward higher frequencies in the electronics design industry is not new, but the pace is accelerating. Whether it’s the increasing number of users at the lower frequencies of the spectrum in the communications industry or the need for higher resolution in the aerospace and defense industry, wider bandwidth availability is the key driver for moving up the frequency ladder.

Who’s Doing it?

While traditionally the drive to higher frequencies has come from the defense industry, the interest of the communications industry has increased significantly, as users have become bandwidth hogs. There is significant activity around the 80-GHz bands for point-to-point radios and cellular system backhaul.

Down to the 60s, data rates are also increasing for short-range communications, with notably personal area networking and video networking in the 60-GHz industrial, scientific, and medical (ISM) band. For short-range communications, there is now interest in 120 GHz. Commercial wireless developments such as WiGig with wireless HDMI are areas of interest due to the requirement for HD video over air.  

“The ability to see quality, high-speed HD video on a mobile device without a cable is a significant driver,” says C.J. Meurell, vice president and general manager at Aeroflex. “I expect the 60-GHz market to mature over the next five years.” Companies have indeed already adopted the technology on large appliances such as TV sets, and Panasonic even put it on a smart phone.

The need for wider bandwidth availability, driven by the need for better resolution, is also driving the move to higher frequencies in the homeland security, radar, and imaging industries. “There has been talk about speeding up flows through airports by installing more sophisticated imaging systems in place. Security screening at millimeter waves is a growing area,” says Bob Buxton, product marketing manager for the General Purpose Test business unit at Anritsu. On the imaging front, the strive for finer-resolution images is driving the need to move to higher frequencies such as 94 GHz, but that can extend to 220 GHz. On the radar front, finer resolution is required to detect small targets. 

“In the automotive industry, companies are also trying to design products at higher frequencies such as collision avoidance radars at around 77 GHz,” points out Roger Stancliff, chief technology officer for the Component Test division at Agilent Technologies. Other industries affected by the trend include the food industry, which uses microwave techniques to detect foreign bodies in food, and the medical industry, which is looking at how the human body responds to microwaves or millimeter waves for the purpose of medical diagnostics and treatment.

Cost, Power Consumption, And Phase Noise

Challenges vary by industry, although cost is or will be a significant issue for all over time. To build short-range wireless communications into consumer devices, the cost challenge, besides power consumption issues, needs to be overcome, which in turn impacts technical considerations.

While other industries haven’t been under the same price pressure as the communications industry, it becomes more of an issue when the size of the rollout increases. For example, the medical industry may aim to deploy devices more generally, making them available at doctor offices. Similarly, when it comes to homeland security, expanding security into facilities such as airports or to other points of interest such as railway stations or shopping malls would increase the cost challenge’s importance.

The cost challenge also trickles down to the test and measurement industry. As highlighted by Justin Stallings, product marketing manager for signal generators and power meters at Rohde & Schwarz, “while it is difficult to design a chip at a high frequency and bandwidth that is low cost, it is as hard to test it at low cost.”

In areas like homeland security and defense electronics, however, the greater challenge is to drive to certain degrees of sensitivity rather than tackle the cost challenge, at least for now. Phase noise is particularly critical in the radar industry, as it tries to detect smaller and/or slow-moving targets.

Device Model Availability And Measurement Accuracy

To make a design work in the broadband millimeter or terahertz area, good models in the basic building blocks are required. Without device models, microwave is a black art. However, designing at high frequencies requires different modeling tools than designing at lower frequencies due to parasitics in circuits and other factors. Also, path loss and attenuation in the test setup is an issue. At 60 GHz, engineers have a much higher loss to contend with than at 6 GHz, requiring, for instance, the generation of much higher powers out of their signal generators.

In addition, the measurement accuracy that is possible at high frequencies is a challenge. Test setup at such frequencies is sort of a science experiment and ensuring the accuracy and reliability of the design is difficult. Even interconnecting the devices at high frequencies is a challenge. The measurement technique is difficult too. Engineers working at very high frequencies have to take a lot of care to make the measurements properly.

Test companies provide tools that help accurately measure and characterize subsystems and devices at high frequencies. As an example, Anritsu has a new broadband system that operates up to 125 GHz. The system is an improvement in both the dynamic range and stability of the measurement, which enables customers to perform more accurate characterization and hence develop a better model of both linear and nonlinear devices. In addition, Anritsu expanded the dynamic range of the system so users can power sweep down to lower levels and developed the connectors for this system.

Another example is Rohde & Schwarz’s SMZ frequency multipliers, which contribute to alleviating the burden on engineers for the test setup. With the SMZ frequency multipliers, the user will get the power level and frequency set on the front panel of the microwave signal generator as calibrated output at the end of that multiplier. In addition, one option enables attenuation of the multiplier output (up to 15 to 25 dB) to give the user some dynamic range compared to a traditional multiplier without any program ability. This built-in attenuator/multiplier enables the user to do that freely on the front panel of the microwave signal generator.

Availability Of Components And Test Equipment

Another significant challenge faced by engineers designing at high frequencies is the lack of components for their design and limited choice in test equipment to test their design. When reaching high frequencies (60 GHz and above), there aren’t many companies involved in manufacturing components and test equipment. Not only is there less choice but the test equipment is also less integrated, which complicates testing. Since it is harder to use and less repeatable, more uncertainties are associated with the measurements.

Challenges Ahead

While engineers face significant challenges in the R&D phase of the product lifecycle for designing at high frequencies, greater challenges lie ahead for these industries as these devices move on through the different phases of the lifecycle.

“Because there is nowhere to connect the cable to make a measurement of these devices to see if they are working correctly, they will have to be tested in production over the air, which is going to be a major challenge for contract manufacturers, ODMs, and OEMs, likely to require some sort of isolation technique per station,” explains Meurell.

Further ahead, once these devices start migrating into the field, there will inevitably be returns. OEMs will have to figure out how to emulate that environment when the device comes back in their return centers or quality departments to try to understand what went wrong. Software platforms such as Averna’s Proligent might help alleviate this challenge for OEMs.

“Averna has designed a manufacturing test and quality management solution that gives OEMs greater visibility into all test, repair, and quality control in their supply chains,” says Pascal Pilon, president and CEO of Averna Inc. “Proligent, which stands for ‘product intelligence,’ contains a product data model to capture critical information so engineering and quality teams can perform analytics metrics at remote sites from a central location. Proligent also controls versions and configurations, both globally and in real time. This enables engineers to make decisions and adjustments so design and manufacturing flaws don’t lead to product recalls.”  

To The Terahertz Region

While the industry is moving toward designing at higher and higher frequencies, and the shatter is increasing over time, designing in the terahertz region is rather nascent (see the figure). Today, the limit is around 100 GHz for any kind of significant application. The work in the terahertz region is emerging, with activity in universities and research institutes.

However, the test and measurement industry always needs to be a few steps ahead of everyone else and vendors such as Agilent Technologies, Anritsu, and Rohde & Schwarz are working with partners to extend the frequency range of their test equipment to the terahertz region. Anritsu’s broadband system is extendable into the terahertz region while Agilent is building network analysis products with partners up to 1.1 THz currently and even foresees its VNA going up to 2 THz. Rohde & Schwarz has been supplying customers with frequency converters in the 500-GHz to terahertz range. Such partners include companies like Virginia Diodes Inc. and Oleson Microwave Ltd.

It will be a while before designing in the terahertz region becomes more developed but it is developing. The design techniques are one key issue. “While the boundaries have moved over time, and the techniques used over 40 years ago for up to 500 MHz are now being used for up to 20 GHz, conventional microwave circuit simulation techniques cannot be used above 100-300 GHz. Multi-wavelength distributed effects in the terahertz region are important, making designing in this region more difficult,” says Stancliff.

The challenges in the terahertz region are severe. Engineers currently use waveguides or free space techniques. A few physical limitations need to be overcome such as the Schottky cutoff frequency of diodes, for example. A key issue in the terahertz region remains the ability to get a good high-power phase controllable signal source. Several technologies are vying to be the source to cover the 1- to 10-THz region, though there is no obvious winner at the moment.

“The key benefit from the electronics side is the direct link it has with microwave technology, so that engineers can simply use the same technology such as VNAs and extend to higher frequencies. However, optical-based systems offer broad frequency coverage, with a single system working from 100 GHz to 2 THz,” says Jeffrey Hesler, chief technology officer at Virginia Diodes Inc.

Having said this, a number of applications could translate into significant business in the future, one of the near-term opportunities being in the semiconductor industry. TeraView recently collaborated with Intel to develop a next-generation time-domain reflectometry system (EOTPR) capable of launching terahertz pulses into integrated circuits and isolating faults in the IC packaging with 10-µm resolution.

‘’Intel launched the EOTPR system at the ECTC trade show in 2010, and since then we have seen considerable interest from fault analysis labs throughout the industry,” says Don Arnone, TeraView’s cofounder and chief executive officer. Other promising applications include paint inspection, explosive detection, portal security imaging, radar imaging, non-destructive testing, gas spectroscopy, and medical imaging.

Companies such as TeraView and Picometrix have built equipment that’s used to identify explosives based on their unique signature in the terahertz part of the spectrum. Similarly, the pharmaceutical industry can leverage the physics of that part of the spectrum to identify counterfeit patent-protected drugs. Research is also ongoing in proteomics to tell proteins apart.

The Last Word

Designing in the terahertz region is rather embryonic, but the drivers behind the trend toward higher frequencies are real. With significant activity occurring in the 60- to 110-GHz range, designing in the terahertz region may be here sooner than we think.

TAGS: Defense
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