Experimental study of a double-circuit optoelectronic generator of radio-frequency signals
Authors: Grachev V. A. , Kapustin S. A. , Kozhevnikova T. V. , Kulyaba D. G. , Novoselova N. A. , Olkhova M. S. , Shustov D. M. 
Published: November 2021

An urgent issue is the development of radio frequency generators operating in the microwave range, with high frequency stability, wide tuning band, as well as small size and weight. Microwave signal generators are used as part of radio electronic devices and devices for telecommunication, radio navigation and metrological purposes; therefore, improving their main characteristics is an important goal for the further development of radio electronic devices. A technological feature of optoelectronic generators is the combined use of solid-state components of modern HF and UHF optoelectronics, fiber optics and traditional components of UHF/EHF technology. According to the principle of operation and construction scheme, the optoelectronic generator is similar to the classic radio-frequency self-oscillator with a delay line in the feedback circuit. A feature of its work is the multifrequency nature of the generated oscillations, for which the conditions for amplitude balance and phase balance are satisfied. In the case of an optoelectronic generator, a feedback ring with a resonant system and a delay element is made using a single-mode fiber of a certain length. The signal delay time in the feedback loop is a significant parameter of the optoelectronic generator, which affects the noise and spectral characteristics of the generator. With an increase in the length of the fiber-optic path, the delay time in the feedback loop increases, thereby reducing the level of frequency noise. However, with an increase in the length of the fiber-optic path, the distance between adjacent modes of the optoelectronic oscillator also decreases, increasing the requirements for the bandwidth of the band-pass filter. The solution to this problem was the use of a multi-circuit optoelectronic generator. The paper considers a model created by the authors for a two-circuit optoelectronic generator with fiber-optic paths 1000 m and 30 m long. The available element base did not allow to reduce the generator's weight and dimensions. On this model, the characteristics of the circuit elements have been experimentally studied. The spectral characteristics of the generator have been estimated theoretically, which are in good agreement with the results of practical research. Phase noise is an important parameter in generator evaluation. Oscillators used as reference and measuring oscillators must certainly have a high frequency stability of the generated signal. Oscillations in autogenerators arise due to the effect of intrinsic noise in the circuit elements, which is one of the reasons for the occurrence of noise in the spectrum of the generator output signal. Intrinsic noise includes thermal, shot and flicker noises in semiconductor elements of generators. The article calculates the power spectral density of the phase noise of a single-loop oscillator and a dual-loop oscillator, which confirmed the assumption about a compromise between the noise level and the requirement for the filter bandwidth in a single-loop optoelectronic oscillator. An important advantage of optoelectronic generators, in comparison with generators built according to classical circuitry, is the absence of a compromise between the frequency tuning band of the device generation and the level of frequency noise.