Characterization of Second-Harmonic Effects in IMPATT Diodes

Abstract

We discuss characterization of the tuned-harmonic mode of operation in IMPATT oscillators, and introduce an equivalent circuit which incorporates the large-signal, “single-frequency” oscillator admittances at the fundamental and second-harmonic frequencies. Complete characterization of this mode is equivalent to specifying the behavior of each of the four elements of the equivalent circuit as functions of the oscillation state variables: fundamental voltage and frequency, second-harmonic voltage and relative phase. Using the approximate large-signal analysis of Blue,1 the values of the equivalent circuit elements are presented, as an example, for a 6-GHz IMPATT diode under a variety of oscillation conditions. This equivalent circuit is used to clarify the role played by the fundamental and second-harmonic, single-frequency oscillator admittances in the tuned-harmonic mode. Using an approximation to the equivalent circuit, we investigate the criteria for stable oscillation of the tuned-harmonic mode. It is found that the stability criteria are in general quite restrictive. For the same 6-GHz germanium diode, the range of stable phase is investigated, as a function of the RF parameters, for certain special cases. It is found to be possible to satisfy the stability criteria for the phase which gives an optimum enhancement of the fundamental power output if certain conditions on the external RF circuit are satisfied.