Theoretical investigation of an ultra-low phase noise microwave oscillator based on an IF crystal resonator-amplifier and a microwave photonic frequency transposer

Abstract

Resonator Q-factor and amplifier flicker noise are the most important parameters that determine the phase noise of microwave oscillators. The Q-factor of intermediate frequency (IF) crystal resonators at IF frequencies can be larger than that of microwave resonators at microwave frequencies. In addition, flicker noise of IF amplifiers can be less than that of microwave amplifiers. One of the well-known usual ways to implement low phase noise microwave oscillators is to use ultra-low phase noise IF crystal oscillators and frequency multipliers. In this paper a new method to implement such oscillators based on IF crystal resonators and amplifiers, called a transposed-frequency microwave oscillator, is proposed. In the proposed structure, a microwave photonic frequency transposer is used to downconvert RF to IF followed by a high-Q IF crystal filter and low flicker noise IF amplifier and then upconverting IF to RF frequency. This novel method in comparison to the well-known multiplier-based microwave oscillators presents significant advantages. In this paper, the proposed new structure is introduced, and a theoretical model is developed to study its performance. Oscillation frequency and amplitude, equivalent Q-factor, phase noise, etc., are theoretically studied and compared with the performances of the well-known microwave oscillators based on frequency multipliers and optoelectronic oscillators. It is shown that the phase noise of the proposed microwave oscillator can be less than −130 dBc/Hz at 100 Hz from a 10 GHz carrier with a noise floor less than −165 dBc/Hz.