High-power harmonic-extraction and triggered-amplification with high-efficiency avalanche diodes

Abstract

Princeton, N. J.HIGH-EFFICIENCY-MODE avalanche diodes are normally operated in circuits which in addition to the fundamental output frequency must support higher harmonicsl-4. In a novel way of operating the diodes it has been demonstrated that power can also be extracted at a prescribed harmonic frequency, while the circuit provides reactive termination for the fundamental and the remaining harmonics. The fundamental frequency which shall be referred to as the trapped-plasma frequency, is associated with a plasma formation process516 within the diode.Because of the inherent high nonlinearity associated with the trapped-plasma process, one expects to extract harmonic powers with good efficiency. Using a microstrip circuit, it has been possible to obtain second-harmonic powers of 110 W at 2.67GHz with 22% efficiency from L-band diodes, and 64 W at 4.85 GHz with15% efficiency from S-band diodes. With thirdharmonic extraction we were able to extend the frequency range into X-band with substantial power output. The diodes have also been operated as power amplifiers with gains achieved either at the fundamental or the second-harmonic frequency.The circuit for second-harmonic extraction- Figure l/u)-consists mainly of a 50-ohm microstrip line which has an open-circuit at one end, and a low-pass-filter section toward the other; i.e., the load end. The distance from the open-end to the diode is approximately h1/4, and from the tuning plates to the diode approximately h2/2, where hl and ha refer respectively to the wavelength of the fundamental and the second harmonic frequency. The open circuit and the tuning plates provide reflection planes of opposite polarity from which the plasmaformed traveling voltage pulses are reflected. As a result of multiple reflection, a voltage waveform is formed at the diode which sustains the plasma forming process at the fundamental frequency with enhanced second-harmonic content. Computed impedances at the diode terminals of the circuit have shown an impedance of 5.3-j7.0 ohms at the second-harmonic frequency and nearly a pure reactance at the fundamental and the third-harmonic frequencies. A circuit used for power extraction at the fundamental frequency is shown in Figure l/bJ. Details of the circuit construction are similar to those reported previously3. Figure 2 shows measured powers that were obtained when the circuit configuration was changed to extract either the fundamental or second and third harmonic frequencies. The presence of harmonically-related frequencies inside the circuit was verified for all data points. Notice that the starting level of the harmonically extracted output power is higher than the power of the fundamental osciuation at the same frequency range. Figure 3 shows an example of power output and efficiency versus pulsed current through the diode.The RF voltage and current waveforms at the diode were measured using chip-resistors integrated with the main circuit. The voltage waveform (Figure 4) consists of a positive-going pulse which is immediately followed by ...