Frequency-saturation effects have been observed in the mechanical tuning characteristics of transferred electron (Gunn) oscillators operating in conventional waveguide cavities at frequencies from 12 to 40 GHz. Their existence leads to a restriction in oscillator tuning range and enhances frequency switching and hysteresis effects in the oscillators. Investigation of oscillator behavior in coaxial cavities as well as in waveguide cavities of varying height has shown that frequency saturation is caused by a coaxial resonance along the mounting post, which in the limit of fully reduced-height waveguide becomes the resonance of the diode with the encapsulation itself, or by a frequency limitation imposed by the diode thickness and operating voltage. The transverse coaxial resonance will occur in any waveguide size at a frequency near to if not within the band of interest. Simulation of the encapsulated diode by a localized reduction in mounting-post diameter shows that the frequency of transverse resonance can be increased by moving the encapsulation to a central position in the waveguide cross section. It is suggested that this technique and that of using cavities with a local reduction of height will make an important contribution to improving the performance characteristics of waveguide-mounted oscillators.

Frequency-Saturation Effects in Transferred Electron Oscillators

A theoretical and experimental investigation into a particular type of tunable microwave circuit commonly used for LSA operation of GaAs layers is reported. It is shown that certain effects which often impose severe limitations on the performance of wide-band transferred-electron oscillators (TEO's) such as frequency saturation, fixed frequency operation, resonance switching, deterioration of efficiency, and high FM noise may, in many instances, be attributed directly to the interaction of the equivalent circuit of the encapsulated device and the microwave circuit. The theoretical result are used to deduce design criteria for a J-band (11 to 19 GHz) waveguide circuit in order to optimize the performance of the oscillator with respect to tunning range, efficiency, and FM noise performance. In particular the results of the investigation indicats that, for continuous tuning combined with low FM noise over the tuning range, the parallel resonant frequency associated with the encapsulated device should be less than the minimum operating frequency, and that, by suitable design of the oscillator, frequency deviation due to fluctuation in the magnitude of the device capacitance can, in principle, be reduced to zero over a narrow bandwidth.

Circuit considerations in the design of wide-band tunable transferred-electron oscillators

A J band Gunn oscillator, tuned by a varactor, has been constructed. It can be mechanically tuned over 3 GHz, and an electronic tuning range of 1 GHz has been achieved with an average power output of 15 mW and less than 1 dB variation.

Microwave Gunn oscillator tuned electronically over 1 GHz

An account is given of the four types of microwave generator made from semiconductors: the transistor, the varactor, the avalanche diode and the transferred electron diode. The design problems for each device are outlined. A more detailed discussion of the physical principles of avalanche diodes and transferred electron diodes is included. The two types of transferred electron device, the Gunn diode and the LSA diode, are contrasted. Finally, comparisons are made between the performances of all the generators, including power output and noise, and some indication is given of future prospects.

https://iopscience.iop.org/article/10.1088/0022-3727/1/3/201/pdf

Semiconductor microwave generators

It has been observed that in many cases the power generated by waveguide-tunable Gunn oscillators varies irregularly and rapidly with the tuning. These power variations, which are not to be confused with those deriving from mode switching, markedly depend on the components connected to the oscillator, and are present in spite of their good matching in the operating band. Theoretical evaluations and an experimental test have been performed which allows one to ascribe this phenomenon to the interaction between the fundamental and harmonic signals due to the diode nonlinearity. The understanding of this phenomenon allows one to design the oscillator in such a way as to reduce its effects.

Influence of the Harmonics on the Power Generated by Waveguide-Tunable Gunn Oscillators (Short Papers)

By using a simple cavity which gives a high rate of change of reactance with frequency, it is shown that the f.m. noise from a Gunn oscillator can be reduced to a much lower level than that reported recently by Faulkner and Meade.

W.P.N. Court

Papers

Reduction of frequency-modulation noise from Gunn oscillators