Microwave Transistor Oscillator Design and Analysis

Abstract

At radio and microwave frequencies, transistor oscillators are mainly used to provide carrier signal source for a generator. Transistor oscillator is a device which provides RF energy at its output by converting a DC energy applied at the input. This characteristic of transistor oscillators has made them to be used widely in most of systems applied at high frequencies, such as radar and wireless communication systems. The purpose of this thesis work was to design and experiment the process of designing an oscillator according to theoretical approaches at microwave frequency and finally analyse its characteristics.

Generally, a high frequency oscillator consists of a nonlinear active device in conjunction with a resonant circuit. A commonly used method at microwave frequency is to design a transistor operating at negative resistance in the port where a lossless passive circuit is connected and that leads the device in an unstable region. In this thesis work, an RF bipolar transistor was biased to minimize the RF signal losses and decouple the transistor from biasing in common base configuration. Next, the input of the biased transistor was designed to have a large negative resistance. Finally, a passive lumped element resonant circuit was used to determine the fundamental frequency of the oscillation. The design was done based on theoretical approaches and linear analysis in frequency domain was used to verify the frequency of oscillation in the process of designing.

This thesis presents the design method used to create a microwave transistor oscillator according to theoretical approaches and computer simulation AWR software. A couple of negative resistance transistor circuits as well as resonant circuits were implemented. The circuits were simulated and manufactured. Finally, the analysis of their characteristics was carried out using spectrum and network analyzer. The final design in which the output port of the biased transistor is coupled to a network created by transmission lines, oscillates at the desired frequency selected by its resonator.

The results showed that the oscillation occurs at a point, which is determined by the resonant circuit, in a frequency range where the input port of a negative resistance transistor circuit has negative resistance value which implies that the reflection coefficient is larger than unity at the mentioned frequency range. This practical approach can be used for any further design of microwave transistor oscillator as well as voltage controlled oscillator.