RF energy: Measurements improve cooking,
lighting, and more
Martin Rowe - December 13, 2016
This article is part of EDN and EE Times’ Hot 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 lowpower 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.
Beyond cooking
Beyond cooking
Although home cooking RF energy has the potential to become a huge market once the price comes
down, it's by no means the only one where RF energy could intervene. Another potentially large
market comes from lighting.
Figure 4 shows a demonstration of plasma lighting. RF energy excites a quartz "bulb," producing
light. Werner sees plasma lighting using RF energy for home, outdoor, and automotive lighting. RF
energy plasma lighting efficiently produces bright light. According to Werner, plasma lighting can
produce 120 to 130 lumens/watt. Compare that to an LED floodlamp in my office, which produces
about 57 lumens/W (750 lumens/13 W), although more efficient LEDs are now available. Murphy
noted that plasma lights can produce as much as 20,000 lumens, making them applicable for
theaters and sporting venues. The light in Figure 4 is running at a small fraction of its possible
brightness. This RF plasma lighting could replace LEDs much in the way LEDs are replacing the
short-lived compact florescent (CFL) bulbs.
Figure 4 This demonstration of plasma lighting is powered by RF energy. Courtesy of MACOM.
Because lighting can also use couplers, control systems can use the same concepts used in cooking
to vary a bulb's light intensity. Wesson notes that unlike in the cooking application, phase
information isn't important for RF energy plasma lighting. Amplitude is all that's needed. Plasma
lighting can, according to Murphy, come closer to simulating sunlight than is possible with LEDs. He
also noted that plasma lighting can achieve a color-rendering index (CRI) of at least 90. Getting the
color right could improve how dentists make crowns, where they try to match the color of the crown
material to that of the tooth. Better lighting can result in more accurate detection of tooth color.
Another possible advantage of RF energy plasma lighting is that light emits from the quartz "bulb"
itself. Thus, it radiates in all directions, where LEDs are usually mounted on a board, which limits
their ability to spread light all around. Because the light radiates from the blub, it can take less
space than an array of LEDs needed to produce the same amount of light. That can have an
advantage where space is limited.
Automotive
Another application is that of using RF energy to ignite fuel in internal-combustion engines. A paper
by Werner, Prof. H. Heuermann, and Dr. A. Sadeghfam discusses the use of RF energy to replace
today's automotive spark plugs:
The key advantage of this concept, when compared to a conventional ignition system, is
the easy variability of the ignition time and duration, which enables the ignition to burn
any residual gasoline in the chamber and results in significant exhaust reduction and
fuel saving. Continuous electronic control enables the plasma to combust fuel with more
efficiency, potentially up to 15% higher, which significantly reduces the carbon footprint
of the next generation of automotive engines.
Even if using RF energy to ignite fuel results in a smaller increase in efficiency than noted above, it
could still prove significant. Wesson added that unlike the cooking and lighting applications, the RF
energy delivered to spark plugs will be pulsed, as opposed to continuous. But, the same
measurement, feedback, and control concepts that apply to cooking apply to igniting fuel. After all,
it's still RF energy being used to excite a load in a cavity. It's just that load is a different kind of fuel.
The idea of a significant improvement in fuel efficiency is powerful, for the less gasoline we use, the
better. Using RF spark plugs should provide better control than is possible with today's devices, but
engineers will have to deliver sufficient RF power to the plugs. Efficient power supplies will be
needed and with such high RF power comes EMI problems to address.
Medical applications
Murphy explained how RF energy heating can be used to treat cancer. "Using RF energy," he said,
"you can heat cancer cells to 43˚C, the temperature at which cancer calls won't regenerate but good
cells will." RF energy is already being used to treat Barrett's esophagus, a pre-cancerous condition
where the inner wall lining takes on the characteristics of stomach lining, usually caused by
prolonged acid reflux. My father had Barrett's and he was treated using a light-sensitive therapy.
That caused him to avoid direct sunlight for about a month because he had been injected with a dye
that made his entire body sensitive to the sun's ultraviolet rays. RF treatment lacks that
complication.
Industrial heating
RF energy has the potential to change the way materials are processed. The ability to measure how
heating affects the load, combined with microcontrollers or computers, could make for smarter
industrial ovens just as is possible for kitchen microwave ovens. Potential applications include curing
and drying. During our conversation, Murphy mentioned an application were RF energy is being
used to heat wood, killing any pests or their eggs that might be lurking inside.
Conclusion
What will it take for these and other RF energy applications to become mainstream? The answer, of
course, is "it depends." In terms of engineering challenges, dissipating heat in power amplifiers will
be crucial, for heat goes to efficiency. In heating and cooking applications, the ability to tightly
control the output should result in better efficiency because less energy will be wasted. Ovens can
reduce power as food cooks, saving electricity.
Currently, the use of RF energy is in widespread R&D. Costs will need to be reduced to the point
where it will become attractive to consumers. Right now, the RF couplers alone make RF energy
cost prohibitive in consumer applications. "Integration and volume will drive down costs," said
Wesson. "The measurement systems will eventually be integrated into the amplifier in a low-cost
way."
Much of the RF energy technology comes from communications: modulation, beamforming,
measurement, feedback, and control. But, there's a significant difference. Unlike communications
systems where devices must interoperate, the systems described here don't. Thus, engineers are
free to design systems without the restrictions and compliance issues that go with interoperability.
Of course, safety standards will still apply, and new ones may be needed to address this technology.
Further reading:
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Oven Uses Solid State RF to Cook Foods Better, Faster
Ampleon RF Cooking
RF Energy Systems: Realizing New Applications
The Potential of RF Energy for the Ignition of Microplasmas
Long life solid-state RF powered light sources for projection display and general lighting
applications
Also watching:
Connected cars: As more and more software takes over, automotive testing will become more
automated.
Power integrity: High-speed digital signal can compromise the DC power delivered through PCB
traces and planes. Better, faster measurement will be needed.
5G: More and more companies are jumping on the 5G test bandwagon as the next generation of
wireless communications takes shape.
Sensors and data: Engineers will have to learn to interpret the massive amounts of data that will
come from connected devices.
Test automation: Related to sensors and data, test automation will improve yields, but not without
understanding, interpreting, and implementing the test results into actions.
Martin Rowe is a Senior Technical Editor covering test and measurement for EDN and EE Times.
Read more of EDN and EE Times' Hot Technologies: Looking ahead to 2017:
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Appliance autonomy promises life simplicity
VR, AR, and Cognitive Capabilities in Embedded Systems Get Real
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