A STUDY OF SECOND HARMONIC INFLUENCE ON SINGLE DOMAIN RESONANT MODE OPERATIONS OF GUNN DIODE IN X-BAND WAVEGUIDE GUNN OSCILLATOR

Abstract

This work comprises a study of the influence of second harmonics on the output power and oscillation frequency of X-band waveguide Gunn oscillators. Only the resonant modes of operation of the Gunn diodes have been considered, with the assumption that not more than one high field domain exists in the n-GaAs sample at any instant of time. Besides, a critical review of the existing literature on the second harmonic influence on Gunn oscillator performance is presented and an up-to-date bibliography is appended. The study begins with a theoretical analysis of the process of harmonic generation by Gunn diodes operating in the resonant modes. It is found that oscillations at any particular frequency is accompanied by oscillations at a number of its harmonics. Next, the physics of the Gunn diodes of X-band dimensions and operating in the resonant modes have been studied for an applied voltage consisting of a component at a X-band frequency and another having a frequency twice as large as that of the former. From this theoretical consideration of the device physics it is found that, owing to its influence on the parameters of the high field domain and the r. f. current through the device, the second harmonic voltage affects the susceptance and conduc tance of the device chip at the fundamental frequency. As a consequence the fundamental frequency, power output and d. c. to r. f. conversion efficiency into the fundamental are also affected by the second harmonics. In order to study the circuit properties of a X-band post-mounted waveguide cavity at the second harmonic frequency, an equivalent circuit representing the waveguide structure is worked out and the properties of the circuit analysed. It is found that the circuit behaviour largely depends on the dimensions of the mounting post, positions of the planes reflecting the waveguide modes carrying second harmonic power and the order of the propagating modes. Also it is shown that the phase relation between the fundamental and the second harmonic voltages is a function of the circuit parameters at both the frequencies. Finally, to obtain a complete picture of the second harmonic influence on X-band waveguide Gunn oscillators, experiments are conducted which indicate that the second harmonics in waveguide Gunn oscillators can be rendered practically ineffective by using a thin mounting post and a circular iris of proper dimensions between the oscillator and the load. These findings are supported by a theoretical analysis of the Gunn device - waveguide circuit interaction. This analysis finds its basis on the outcomes of the analysis of the device physics and the circuit behaviour of the waveguide mount and the cavity. It is found that the second harmonic influence in the X-band waveguide Gunn oscillator is more pronounced in the case of Quenched mode operation of the diode. However, operation in this mode can be avoided by mounting the diode with a thin post. Another important point brought out by the analysis is that the power and (v) frequency fluctuations produced by second harmonic load variations, to a great extent, are dependent on the device capacitance at the second harmonic frequency.