Gil Wong Choi

High efficiency Class-E tuned Doherty amplifier using GaN HEMT

This paper describes the design and fabrication of a highly efficient switching-mode Class-E Doherty power amplifier using gallium nitride (GaN) high-electron mobility transistor (HEMT) for S-band radar applications. Measured results of the Doherty amplifier show power-added efficiency (PAE) and drain efficiency of 62.6% and 73.1% at 37 dBm of 6 dB output back-off point from saturated output power at 2.85 GHz, compared with PAE and drain efficiency of 42.9% and 44.7% for the case of balanced amplifier. It was found that PAE was improved by 19.7% by adopting the Doherty efficiency enhancement technique.

Experimental Observation of Frequency Locking and Noise Reduction in a Self-Injection-Locked Magnetron

We report an experimental observation of frequency locking and noise reduction in a self-injection-locked magnetron oscillator operating at 2.455 GHz. Part of the magnetron output signal is injected into the magnetron through a feedback loop to lock the oscillation frequency. A frequency-tunable high- cavity is placed in the feedback loop to tune the locked frequency. Experimental results show a noise reduction by more than 13 dB at 1 MHz offset and a frequency tuning bandwidth of 0.12% at a center frequency of 2.455 GHz.

High-efficiency power oscillator using harmonic-tuned matching network

This paper presents a high-efficiency power oscillator, which uses LDMOS (Laterally Diffused Metal Oxide Semiconductor) transistor for high output power and a harmonic-tuned matching network to obtain high efficiency characteristic for RF power source application. A hair-pin resonator was designed and employed into the high efficiency power oscillator in order to improve phase noise characteristic and frequency selectivity. Measurement results show that output power of the power oscillator is 38.1 dBm, corresponding to a conversion efficiency of 73% at 981 MHz which is the highest efficiency reported in microwave power oscillator. Experiments performed with the hair-pin resonator show that output power is 36.9 dBm and conversion efficiency is 65%. Phase noises are measured to be 78.9 dBc/Hz and 103 dBc/Hz at 10 kHz offset without and with the hair-pin resonator, respectively.

Note: Picosecond impulse generator driven by cascaded step recovery diode pulse shaping circuit

In this paper, a picosecond impulse generator using step recovery diodes (SRDs) is presented. In order to reduce the pulse width of an impulse generator, we employed a cascaded SRD pulse-shaping circuit. A short impulse generation is confirmed in numerical simulation of a time-transient circuit simulator. Measurements show that the measured pulse width of the cascaded SRD impulse generator is 250 ps at 10% of the peak amplitude, which is improved by 85 ps compared with a conventional SRD impulse generator.

1.5 kW solid-state pulsed microwave power amplifier for S-band radar application

This paper describes the design and performance of a 1.5 kW solid-state pulsed power amplifier, operating over 2.7-2.9 GHz at a duty of 10 percent and with a pulse width of 100 us for radar application. The solid-state pulsed power amplifier configures a series of 8-stage cascaded power amplifiers with different RF output power levels. Low loss Wilkinson combiners are used to combine output powers of six 300 W high power solid state modules. Tests show peak output power of 1.61 kW, corresponding to PAE of 26.2 % over 2.7-2.9 GHz with a pulse width of 100 us and a PRF of 1 kHz.

Frequency-tunable high-efficiency power oscillator using GaN HEMT

In this paper, a frequency tunable, high-efficiency power oscillator using Gallium Nitride High Electron Mobility Transistor (GaN HEMT) for the RF power source applications is designed and fabricated. The steady state oscillation occurs in a delay line feedback loop which length is adjusted to tune oscillation frequency. A harmonic-tuned matching network technique is employed to obtain high conversion efficiency characteristic of the oscillator. The measured output power and conversion efficiency of the fabricated oscillator are 44.63±0.2 dBm and better than 62%, respectively, across the 890–950 MHz band with a drain bias voltage of 40 V. Then a hair-pin resonator is designed and employed into the oscillator in order to improve phase noise characteristics and frequency selectivity. The experimental results of the oscillator with the hair-pin resonator exhibit the output power of 43.55 dBm, corresponding to the conversion efficiency of 61% at 920 MHz. The measured phase noise characteristics are −64 dBc/Hz and −81.24 dBc/Hz at 10 kHz offset without and with the hair-pin resonator, respectively.

Modeling of multistage depressed collectors using a 3D conformal finite-difference time-domain particle-in-cell code

Summary form only given. The feasibility of modeling a multistage depressed collector using a conformal finite-difference time-domain particle-incell code has been studied. A feedback mechanism is implemented to provide stable time-dependent voltages for each stage of the depressed collector. An arbitrary space-time dependent spent beam distribution can be given in our time domain simulations. We demonstrate the design of a five stage depressed collector recovering a triangular spent beam distribution achieving an energy recovery efficiency of 70%. Detail modeling and code capabilities will be presented.

The self-injection-locked magnetron

This paper presents a new method of noise reduction in magnetron using self-injection-locking loop. The self-injection-locked magnetron divides into two cases in accordance to filter type: waveguide cavity filter and DR filter. This self-injection-locked magnetron consists of the coupler, high-Q filter (DR filter or waveguide cavity filter), circulator, and a phase-shifter without external source. Experimental results show that the self-injection-locked magnetron using waveguide cavity filter and DR filter is improved by 11.1 dB and 12.6 dB compared to the noise level of free oscillation magnetron at the 1 MHz offset frequency.

Simulations of a 1 MW, 700 MHz klystron using MAGIC PIC-code

A KOMAC (Korea Multipurpose Accelerator Complex) klystron tube operating with a maximum output power of 1 MW at 700 MHz is analyzed by the use of 2-D particle-in-cell code, MAGIC in order to validate the design parameters.

7.4: Development of a compact and lightweight S-band traveling-wave tube for microwave power module

A compact and lightweight S-band 1 kW traveling-wave tube (TWT) is being developed for microwave power module (MPM) radar applications. The S-band MPM provides a tenfold peak power increase compared to the currently available ones. Circuit characteristics including interaction impedance, phase velocity, power, and gain of the TWT were predicted using large signal code. Design of electron gun, periodic permanent magnet, and collector is complete. The helix circuit has been fabricated and is ready to be tested.