Large signal circuit characterization of solid-state microwave oscillator devices

A method of direct measurement of the admittance in X band seen by an encapsulated solid-state oscillator embedded in microwave hardware is described. It is applicable to many two-terminal devices in waveguide, coaxial line or microstrip circuit configurations. The criterion for oscillation in negative conductance oscillators is used in conjunction with these admittance measurements to determine the complete frequency-dependent 'device-plane' of Gunn and IMPATT devices in S4 (pill-prong) encapsulations, and the forms of negative conductance and susceptance variation with RF voltage amplitude and frequency have been extracted for each of these devices. It is found that for transferred-electron devices in particular, the frequency dependence of these parameters over X band is very marked. The use of the admittance measuring technique to characterize the microwave circuit is discussed with reference to the improvement of AM and FM noise by the controlled insertion of perturbations in the admittance locus, and an example is given of its use in fault-finding in a mechanically tunable waveguide oscillator.