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.
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.