A PROPOSED METHOD FOR EVALUATING THE OPTIMUM LOAD IMPEDANCE IN NEGATIVE RESISTANCE RF OSCILLATORS

Abstract

Negative resistance radio frequency (RF) and microwave oscillators are widely used in modern wireless communication systems. In these circuits, the active device is imposed to work in the unstable region in order to present a negative resistance at its input or output port. This paper discusses the effect of the load impedance on the large signal behavior of the oscillator circuit throughout a design process of a practical 900 MHz RF oscillator circuit. New closed-form expressions for the optimum load resistance and reactance that maximize the output power have been derived analytically in terms of the transistor Z-parameters, and a design criterion for the RF oscillator is proposed based on the evaluated optimum load impedance. It has been shown through computer simulation that the optimum load resistance and reactance for maximum output power are dependent on each other. Furthermore, it has been verified that the optimum load impedance for maximum negative resistance differs slightly from its value required for maximum output power and this deviation increases with the increase in the RF power level. The designed oscillator circuit has been implemented and tested successfully.

Keywords

• Negative Resistance Oscillators
• Large-Signal S-parameters
• Microwave CAD
• Nonlinear RF Circuits