Frequency stability of gunn oscillators with variation of ambient temperature

Abstract

The factors controlling the frequency dependence of Gunn oscillators with ambient temperature variations are determined from an analysis of the equivalent circuit. By considering a simple I/V characteristic and the domain growth time, a model of the electron current waveform is obtained, which includes the temperature dependent properties of the Gunn diode. This leads to an equivalent negative conductance and a parallel susceptance. The domain capacitance contributes an additional susceptance. In the delayed domain mode, the transit time variation with temperature is the dominant factor and df/dT is about — 1 MHz/°C, except for high device temperature where the magnitude increases rapidly. In the quenched domain mode, elimination of the transit frequency variation causes lower df/dT magnitudes which become positive at low device temperatures. The rapid increase in df/dT at high temperatures for both the delayed and quenched domain modes is caused by the reduction in negative mobility, which increases the domain growth time. The rate of voltage change across the device is shown to delay the current drop from threshold for about 0·1 of the oscillator period. Good agreement for the experimental curve of df/dT at maximum power output is obtained with the theoretical delayed domain mode, for delays of less than 0·06 of the oscillator period. With the operating frequency to transit frequency ration of 0·9, the analysis shows that quenched domain operation is not normally possible. However, the rate of change of device voltage is shown to cause domain quenching. Experimental curves of df/dT against temperature for undercoupled oscillators agree qualitatively with the theoretical quenched domain mode curves. These show very low average drift rates of less than 100 kHz/°C. An increasing delay period for current drop is indicated at higher temperatures in agreement with the increasing domain growth time. Experimental results for n+ contacted C.W. devices operating in 50 Ω, λ/2 coaxial cavities and decoupled by 2 dB from maximum power were better than 100 kHz/°C over a temperature range of 100°C.