Review of Some Types of Varactor Tuned DROs

Dielectric resonator oscillators (DROs) have a number of characteristics that distinguish them from other microwave generators, making them very popular in this field of technology. Only two characteristics, frequency stability and phase noise, are found to be better with crystal multiplier oscillators (Figure 1), however, phase noise of a DRO is lower at narrow band frequencies around the center frequency (<0.5 MHz) [1]. Generally, DROs are generators of stable frequency, but their center frequency is tunable within narrow limits (up to 1 percent fc) by means of circuits that influence the electromagnetic field of a dielectric resonator (DR). Circuits with varactor diodes are most frequently used for center frequency tuning because they are simple and reasonably priced. There are, however, some other methods to obtain this tuning. One method uses ferrite materials to control the magnetic field of a DR by the outer magnetic field [2, 3]. Another method uses a light source to change the conductivity of a photosensitive layer deposited on the top surface of a dielectric resonator, affecting field distribution of a DR [4]. A third method uses the oscillator characteristic to change its operating frequency with biasing voltage variation (pushing) [5, 6]. These methods are not frequently used in practice, because of specific disadvantages limiting their applications. Circuits with varactor diodes are widely used in various commercial and military applications of voltage-controlled dielectric resonator oscillators. Such applications include temperature stabilization and modulation (FM) of voltage controlled oscillators used in PLL circuits for synchronization with referent oscillators. The main function of the diodes is the influence of their own electromagnetic field on the electromagnetic field of a dielectric resonator. As it is known, the electromagnetic field of a DR is mainly concentrated in a dielectric, but a small part is propagated into the immediate vicinity. This part of the electromagnetic field is