## 3 Circuit Fabrication and Results  
A photograph of the high power oscillator module is shown in Figure 9. Discrete circuit elements are mounted on the Rogers substrate (4350 B) with \(100\times 50\times 0.76\) mm\({}^{3}\) by soldering. The high power resonator [13] is a quarter-wavelength short-circuited coaxial-type, which employs a ceramic substrate  
Figure 8: Simulated output spectrum of the high-power oscillator.  
Figure 6: Simulated S\({}_{31}\) and S\({}_{21}\) of the power oscillator.  
Figure 7: Precise schematic diagram of the high power oscillator connecting Ports 1 and 3.  
having a dielectric constant of 38 and has a dimension of \(4\times 4\times 4.35\) mm\({}^{3}\). A surface mount type of the GaN-HFET was directly die-attached to the case by soldering. The parasitic inductances and capacitances included in the circuit elements of Figure 9 have a serious effect on the performance, which are carefully taken into account the circuit simulation. The circulator was employed at the output port of the high power oscillator in order not to be affected by load impedance changes. In Figure 9, T\({}_{1}\), T\({}_{2}\) and T\({}_{4}\) are transmission lines. T\({}_{3}\) is a transmission line of the phase shifter. C\({}_{1}\) and C\({}_{2}\) are coupling capacitors. C\({}_{3}\) are capacitances of the phase shifter. V\({}_{\rm g}\) and V\({}_{\rm d}\) are a gate and drain voltages, respectively. M\({}_{\rm O}\) is an output port of a module. Water cooling was employed to dissipate heat.  
Measured output power and efficiency of the power amplifier are plotted in Figure 10. A small-signal gain of 17 dB, a P\({}_{\rm 2 dB}\) of 54.4 dBm (275 W) and a drain efficiency of 62.7% have been obtained at 2.45 GHz. In comparison with the simulated results of Figure 3, almost the same values have been successfully achieved. The magnitude and phase of the S\({}_{31}\) of Figure 5 were measured and plotted in Figure 11. Mag(S\({}_  
The measured output power and efficiency of the module with the circulator are plotted in Figure 12. V\({}_{\rm d}\) was varied from +45 to +50 V. As V\({}_{\rm d}\) increases, the output power, and the oscillation frequency increase. The efficiency, however, keeps around constant for V\({}_{\rm d}\) of +45 to +50 V. In the case with the circulator of Figure 12, an output of 54.2 dBm (263 W), an efficiency of 61.3%, and an oscillation frequency of 2.44 GHz were obtained at a supply voltage of +50 V. Now it is assumed that the insertion loss of the circulator is 0.15 dB. Then the output power and drain efficiency before the circulator become 54.4 dBm (275 W) and 63.5% at V\({}_{\rm O}\) in Figure 9, respectively, which are in good agreement with the measured power amplifier results.  
The measured output spectrum was plotted in Figure 13. The output power of 54.2 dBm (263 W) and a drain efficiency of 61.3% have been accomplished at 2.44 GHz. Vd was +50 V. The measured C/N was \(-\)100 dBc/Hz at 100 kHz offset frequency. Spurious responses appear in Figure 13. It is mainly due to the switching noise of 250 kHz for use in the generation of the negative gate voltage of V\({}_{\rm g}\) from the positive drain voltage of V\({}_{\rm d}\). This noise was amplified and then appeared in the output spectrum of Figure 13.  
Figure 11: Measured magnitude and phase of the S\({}_{\rm 31}\) of Figure 5.  
Figure 12: Measured output power and efficiency of the module with the circulator.  
A PWM (Pulse Width Modulation) becomes necessary in the actual microwave oven. The measured PWM output waveform was displayed in Figure 14, where ON/OFF time is 20 ms, respectively. We realized high speed rise and fall within 1 ms.