The elimination of tuning-induced burnout and bias-circuit oscillations in IMPATT oscillators

Abstract

IMPATT diode microwave oscillators suffer from the effects of low-frequency instabilities, which include excessive up-conversion of bias-circuit noise, bias-circuit oscillations, and diode burnout induced by tuning at the microwave frequency. These instabilities are particularly troublesome in GaAs diodes, although also present in both Ge and Si to a lesser extent. Moreover, these instabilities are more prominent in higher efficiency, higher power diodes, presenting a severe systems problem in the practical utilization of GaAs diodes at their highest power and efficiency levels. In this paper, it is shown that these instabilities may be eliminated in a systematic and well controlled manner with little or no loss in microwave power or efficiency. It is shown that the source of the unstable behavior is a low-frequency RF voltage-induced negative resistance which extends from dc to several tens, and perhaps hundreds, of megahertz, depending on the loaded Q of the microwave circuit. The negative resistance is an unavoidable fact of large-signal avalanche diode operation and is due to the rectification properties of the nonlinear microwave avalanche.