A low-frequency analog for a Gunn-effect oscillator

After a brief introduction to the Gunn effect and an explanation of the proposed equivalent circuit of the Gunn diode, a low-frequency (5 Hz) lumped-circuit analog is described. The analog is able to simulate the initiation and circuit quenching of the domain, the transit time of a dipole domain, and the dynamic negative-resistance characteristic associated with the domain. Thus the analog can qualitatively account for many properties of the dipole domain modes. A short section also shows how, within certain limitations, the LSA mode can be investigated by means of the analog. The analog enables the behavior of the Gunn diode to be studied under various forms of loading, and the results to be related to a wide range of actual diodes in microwave circuits by a simple scaling process. The sciencies of various modes can be compared. Photographs are presented of an oscilloscope display showing the relevant current and voltage waveforms (including the domain voltage) for a pure resistive load, a parallel resonant load, and a series inductive load. The provision for a direct display of domain voltage means that a particularly clear visual distinction is possible between transit-time and circuit-quenched modes of oscillation. Tuning by variation of the external load is demonstrated; the importance of the semiconductor's current peak-to-valley ratio and of the choice of transit time in determining the efficiency in a given mode is illustrated. A possible explanation for the reported observation of subharmonics is provided by the analog, on the basis of alternate modes, i.e., a cycle of circuit-quenched operation followed by a cycle of transit-time operation. The limitations of the analog are discussed together with ways in which some of them could be overcome.