Non-transit time limited mode of operation of the Gunn effect oscillator

The object of the work described in this paper was to obtain the LSA mode of oscillation and to study its physical mechanism. Gunn diodes have been operated in waveguide circuits at frequencies from 3 to 30 times their transit time frequency, the longest devices giving pulsed output powers as high as 750 watts at 2 GHz and 300 watts at 3 GHz with conversion efficiencies of 5 and 2 1/2%, respectively. As the output power, impedance (frequency), product of these samples is considerably in excess of the values normally obtained for transit time oscillators, it was initially assumed that the LSA mode was operating. However, dh'ect observation of the diode RF current indicates that the major component of current, though higher than the transit time frequency, is in fact at a subharmonic of the output frequency. The high frequency current builds up relatively slowly during the pulse and its final amplitude is much smaller than that of the subharmonic component. Thus although this mode of operation is potentially useful it is not the pure LSA mode. The significance of these results is discussed with particular reference to the findings of other workers in this field.