A STUDY OF RESONANT-CAVITY MULTIPLE-DEVICE OSCILLATOR

Abstract

The solid-state negative conductance devices such as Gunn's IMPATT's, etc. have proved to be potential sources of millimeter and microwave power. However, when singly operated these devices often fail to meet the power requirements of many high-power appli cations. This has led to the development of the multiple-device oscillators. A multiple-device oscillator is essentially constituted by a circuit that efficiently combines :he power outpucs of a number of oscillating devices. A global activity, extending over more than a decade has resulted in several types of multi ple-device oscillators. For narrow-band applications, the most promising of these oscillators is the resonant-cavity multipledevice oscillator. In such an oscillator, a number of osci llating devices are coupled to a single power-combining cavity. This thesis deals with several gaps, which exist in the understanding of the circuit and active device influence on the performance of a resonant-cavity multiple-device oscilla tor. The performance of a multiple-device oscillator is charac terized by its power-combining efficiency, power output, grace ful degradation performance , injection-locking behaviour, noise and the electronic tuning capability. The objective is achieved by analytical treatment of the effects of increasing the number of active devices and the influence of circuit and device parameters on the various

aspects of the performance of the ^oscillator. It is found that,as the active devices are increased in number,the powercombining efficiency increases . This shows that an increase in the number of active devices not only increase the avai lable power, it also makes the system a more efficient power combiner. The power-combining efficiency also increas es with increase in the coupling between an individual active device and the power-combining cavity. The optimum negative conductance of an individual device also plays an important role in determining the power-combining efficiency and power output. An analysis of the graceful degradation performance shows that, the power output degradation with the failure of one or more of the constituent active devices is related to the optimum negative conductance of an individual device. Based on this analysis, a scheme for the improvement of grace ful degradation performance by post-failure optimization of the oscillator load conductance has been proposed. From an analysis of the injection-locking behaviour of a resonantcavity multiple-device oscillator, it is found that, the lock ing range and locking gain of the oscillator are related to the number of active devices, circuit and the device para meters through their relations with the external-Q, Qexfc/ and the.free running power output. Q decreases to a limiting value, when the active devices are increased in number. This limiting value of C jj.i1 to the quality factor of an individual active device. Joyt -^ also influenced' by the coup(

ling between an individual active device and the cavity. A lower Q results, when active devices of higher optimum nega tive conductance are used. Relating the injection-locking gain with the circuit and device parameters it is shown that, both locking range and locking gain improve^ with increase in the number of active devices. The improvement in locking range is more, if the locking gain is maintained constant while increasing the active devices in number. A consideration of the noise in a resonant cavity multiple-device oscillator shows that, for a critical number, Npeak, of the active devices, the FM noise reaches a peak. This critical number of active devices can be controlled by circuit design. When the active devices are increased in number beyond Npeak, FM noise reduces. AM noise also reduces as the devices are increased in number. FM noise improves, if the active devices are loosely coupled to the power-combining cavity. Finally, the possibility of electronic tuning of s resonant-cavity multiple-device oscilla tor is considered. EM : • I it! narrow bandwidth, electronic tuning by coupling a number of varactor diodes to the power combining cavity is suggested. An analysis is presented, which shows that such electronic tuning of a resonant-cavity multi ple-device oscillator is possible. This will, however, have a tradeoff between frequency tuning range and power output. In addition to the above studies, a critical review of the existing literature on the research works carried out in the area of resonant-cavity multiple-device oscillator design and devlopment is presented in the thesis.