Analysis of Odd-Mode Parametric Instabilities at Fundamental Frequency in an X-Band MMIC Power Amplifier

Odd-mode parametric oscillations are a common cause of malfunctioning in power amplifiers. A very typical example of odd mode oscillation is the showing up of a spurious component at the fundamental frequency divide-by-two for some input power values. In this paper, a less known instability of odd-mode nature is analyzed. It occurs at the fundamental frequency, without the generation of spurious spectral lines at sub-harmonic frequencies. The instability is detected using pole-zero stability analysis and a procedure for determining the oscillation mode is given. The analysis is applied to an X-band MMIC power amplifier that was found to be unstable when measured in the lab. Using the information provided by the stability analysis, a stable second version of the MIMIC has been successfully fabricated and measured.