RF energy: Measurements improve cooking, lighting, and more

-December 13, 2016

This article is part of EDN and EE TimesHot Technologies: Looking ahead to 2017 feature, where our editors examine some of the hot trends and technologies in 2016 that promise to shape technology news in 2017 and beyond.

A certain person in my household has been known to burn microwave popcorn—fortunately, without starting a fire. What's really needed here is a microwave oven that detects the changes taking place in food as it cooks and makes the appropriate adjustments. Such an oven is now possible, though it might be a few years before it's practical and affordable.

RF energy is on the cusp of bringing changes to cooking, lighting, industrial heating, automotive spark plugs, and a host of other applications. It's made possible through the development of RF transistors that can provide sufficient power at the right frequencies, namely the 2.45 GHz ISM band. Yes, the same band used by Wi-Fi and Bluetooth.

As with any new technology, RF energy applications come with engineering challenges such as thermal dissipation, cost, size, and measurement. At EDICON 2016 in Boston, I met with Klaus Werner, Executive Director of the RF Energy Alliance, who also gave a presentation that day. After the conference, I spoke with Mark Murphy, Senior Director Marketing and Business Development for RF Power at MACOM and with Robin Wesson, Advanced Applications Architect at Ampleon.

"RF energy could change the way we cook food," said Werner, "but it's being used in other applications." He explained that RF energy, generated by RF transistors in power amplifiers, could replace the magnetrons in microwave ovens. By generating energy with semiconductors and more than one antenna (Figure 1), microwave ovens could produce energy sufficient for cooking and adapt to changing conditions as food cooks. That can result in more even cooking than we currently get from our microwave ovens, which essentially operate as on/off, open-loop systems. Instead, the next generation of microwave ovens will have complete closed-loop control. Some of today's ovens have mode-stirrers or turntables to attempt to produce a uniform field inside the cavity while others use humidity sensors that provide some feedback, but not enough, for the kind of control needed.

Figure 1 Power transistors such as these from MACOM and Ampleon can produce 300 W of power at 2.45 GHz. (The Ampleon device is rated at 250 W, but a 300-W version is available.) MACOM uses a GaN-on-silicon process while Ampleon uses laterally diffused metal-oxide-silicon (LDMOS).

Figure 2 shows a block diagram of a microwave oven that uses solid-state amplifiers instead of a magnetron. RF signals are generated by oscillators and can be mixed to provide modulation as well as make adjustments to the output's amplitude, frequency, or phase. An RF switch applies the signal to a high-power amplifier (HPA). To be practical, a microwave oven will need at least two energy sources to produce sufficient heating.

Figure 2 A solid-state cooking system will likely consist of at least two energy sources that produce and amplify RF signals. RF couplers provide the signal for a feedback loop. Courtesy of MACOM.

As with any closed-loop system, this design requires feedback, and that means measurement. Although today's microwave ovens may use moisture sensors to provide some feedback, that's an indirect measurement. Solid-state microwave ovens can get a measurement on the load itself.

"As food cooks," said Murphy, "it absorbs less and less energy. That means more and more energy is reflected back into the cavity." The key to the feedback system lies in measuring the properties of food as it cooks. That causes changes in the cavity's behavior at some frequencies. Thus, the system needs to measure the energy reflected from the food. Wesson's paper, RF Solid State Cooking, provides data showing how reflected energy, and hence return loss, changes as food cooks.

With return-loss (S11) measurements from the couplers, the control system can adjust the RF heating signal's amplitude, frequency (between 2.4 GHz and 2.5 GHz), and phase (to any angle). The system can make adjustments for each antenna, thus altering the energy field in different parts of the cavity. While it's possible to adjust signal phase to any angle, the effects of phase and how to best use them are still under investigation.

RF power amplifiers should provide better consistency from oven-to-oven and from one cooking to the next. Today's magnetron-based ovens are notorious for creating different RF environments depending on the load, with no way to compensate.

The ability to change the frequency, power level, and phase of the RF heating signal means that ovens should be able to spread the power over the optimum settings for each type of food. Eventually, different types of food will have unique cooking profiles. Users should be able to find the settings that work best and store them for future use.

Over time, microwave ovens may even be able to use beamforming—a concept borrowed from wireless communications—to direct energy where it's needed most. Thus, having a uniform energy field in the cavity might not be optimal. Even now, proof-of concept microwave cooking has, according to Murphy, demonstrated the ability to cook a hot dog while not melting a scoop of ice cream placed inside the cavity at the same time. MACOM has demonstrated RF cooking, which you can see in this video. NXP has demonstrated how RF energy can be used to a cook a fish in a block of ice.

Four-port directional RF couplers, shown in the Figure 2 diagram and in Figure 3, provide low-power samples of the forward and reflected energy. "The measurement circuit is essentially a vector-network analyzer (VNA)," said Murphy. The forward and reflected power from the coupler feeds an A/D converter. The digital output goes to a microcontroller that implements the control loop. "At this point," said Wesson, "it's still unclear if the industry will opt to use phase information, but instead may rely on amplitude only."

"At Ampleon," he continued, "we feel that having phase information is important for cooking. In the forward direction, phase control does nothing in single-channel applications but adds a new dimension of field pattern variation in multi-channel applications. More field-affecting variables means better control."

Figure 3 RF directional couplers provide a low-power version of forward and reflected power, used for measurement and control. Source: Marki Microwave.

The ability to measure reflected RF energy and provide tight closed-loop control over cooking should reduce the likelihood of burning popcorn and with it, the possibility of fire.

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