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."
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.
This article was first published by EDN and EE Times.