Dielectric resonator

Saturday, 20 January 2007

A dielectric resonator is an electronic component that exhibits resonance for a narrow range of frequencies, generally in the microwave band. Resonators are important components in microwave communication circuits. Resonators are useful in RF communication equipment in connection with filters, low noise oscillators, and other circuits. With the increase in demand for exchanges of information via wireless communications, needs for communications systems using microwaves are increasing. Devices used in a wireless communications field tend to have a smaller size and a higher capacity. Furthermore, working frequencies thereof are changed to a high frequency band, and thus the GHz frequency band is being utilized. Dielectric resonators are used in many circuits, particularly microwave circuits, for concentrating electric fields. They can be used to form filters, oscillators, triplexers, and other circuits. Dielectric resonators are frequently employed in microwave circuits such as microwave oscillators and filters because of their relatively high quality factor (Q) values and good frequency stability. Various types of dielectric resonators are used for governing the frequency in oscillator circuits for microwave systems such as radar, communication links, navigation and electronic warfare, by way of example. In the microwave communications, where the microwave frequency spectrum has become severely crowded and has been sub-divided into many different frequency bands, there as an increasing need for microwave filters to divide the microwave signals into these various frequency bands. Accordingly, various waveguide and resonator filters have been employed to perform band pass and band reject functions in order to divide up the frequency spectrum into these different frequency bands. The use of dielectric resonators in microwave filters results in a significant reduction in size and mass while maintaining a performance comparable to that of waveguide filters without dielectric resonators. The higher the dielectric constant of the dielectric material out of which the resonator is formed, the smaller the space within which the electric fields are concentrated.

Dielectric resonators possess resonator quality factors (Q) comparable to cavity resonators, strong linearity at high power levels, weak temperature coefficients, high mechanical stability and small size. Dielectric resonators may be used to determine and stabilise the frequency of a microwave oscillator or as a resonant element in a microwave filter. A typical dielectric resonator for use in the microwave band is formed using a rectangular or cylindrical dielectric block having a coaxial through-hole wherein an inner conductor is formed on the inner surface of the through-hole and an outer conductor is formed on the outer surface of the dielectric block. Dielectric resonators are smaller than air cavity resonators having equivalent resonant frequencies because wavelengths in the dielectric resonator are divided by the square root of the resonator's dielectric constant. A dielectric material is widely used in a resonator, which is a principal device constituting a part of such a communications system used in such a high frequency band, employing microwaves with a range of 300 MHz to 300 GHz. Dielectric resonators utilizing a dielectric as a material for constructing the resonator have been widely used so as to miniaturize the resonant system of an electric circuit which handles high-frequency waves such as microwaves. Dielectric ceramic materials are utilized in resonator materials, MIC dielectric substrate materials, dielectric waveguides, dielectric antennas, and any of various electronic components. Ceramic dielectric materials are used to form thermally stable dielectric resonators as key components in a number of microwave subsystems which are used in a range of consumer and commercial market products. The dielectric ceramic materials are generally required to have high dielectric constant to meet a demand for size reduction of devices, a small dielectric loss in high frequency regions (a high Q value) and a small change in resonant frequency with respect to a temperature change, that is, a stably low temperature dependency of dielectric constant. The Q value of a dielectric resonator is defined as the ratio between the energy stored per cycle to the energy dissipated per cycle. The resonance frequency of the dielectric resonator is primarily determined by the dimensions of the resonator body. Another factor affecting the resonance frequency is the environment of the resonator. In order to prevent electromagnetic energy from being scattered and lost, dielectric resonators are usually housed in a metallic casing, or alternatively, metal electrodes are formed on the dielectric surface.

Owing to the superior performance characteristics of dielectric resonators, the use of dielectric resonators has become widespread, particularly in highly selective bandpass filters. Dielectric resonator filters are a class of stable microwave filters that are frequently used in radar and communications systems. A typical dielectric resonator filter consists of a ceramic resonator disc mounted in a particular way inside a metal cavity. Dielectric resonators are often utilized in filter circuits because of an intrinsically high Q value. The dielectric resonator, operable at a particular frequency, is tunable over a narrow bandwidth and frequency fine tuning must be accomplished without affecting the high Q of the resonator. These characteristics allow a filter employing a dielectric resonator to have excellent frequency stability with only a small amount of frequency drift over a wide range of temperatures and environmental conditions. Unlike metallic resonators, dielectric resonators yield little external high impedance electric fields when they are operated in desired operating modes. The electric field of a dielectric resonator is contained substantially within the resonator structure in the desired mode of operation. The magnetic fields yielded by dielectric resonators do extend beyond the confines of the resonator structures and into a cavity of a filter in which the resonators are contained. These magnetic fields can be used to provide proximity magnetic coupling between a pair of adjacent dielectric resonators. Dielectric resonators employed in filters could be utilized in a variety of modes, such as TE, TM, and HEM (hybrid electromagnetic) modes. A mode is a field configuration corresponding to a resonant frequency of the system, as determined by Maxwell's equations. In a typical dielectric resonator circuit, the fundamental resonant mode, i.e., the field having the lowest frequency, is the transverse electric field mode (TE). Filters using resonators employing dual hybrid mode exhibit a symmetric bandpass response. For dielectric resonator filters, the size of the cavity can be substantially reduced by mounting the dielectric resonator along a base wall of the cavity rather than mounting the resonator in a centre of the cavity.

Microwave oscillators are used in transmission systems and more particularly close to the antenna in order to carry out a frequency transposition between an intermediate frequency band and a transmission frequency band. In communication circuits, the local oscillator (LO) signal must exhibit low phase noise, corresponding to the random phase instability of a signal, to meet the signal requirements of the digital modulation scheme used in a communications system. The signal also must maintain acceptable bit error rate (BER) requirements. Dielectric resonator oscillators (DROs) are very popular devices in the radio frequency (RF) or microwave electronic field. These oscillators are typically employed in communication systems, radar systems, navigation systems and other signal receiving and/or transmitting systems. Their popularity has been attributed to their high-Q, low loss, and conveniently sized devices for various applications in the RF and microwave fields. The oscillation frequency of a dielectric resonator oscillator depends on its dimensions and on the electromagnetic properties of its environment. Dielectric resonator oscillators have been used in radars, transponders, and communication systems, among other systems, to generate microwave signals with extremely low phase noise and good temperature stability. Generally, in these systems, the DRO is used to generate a frequency that is locked to a reference oscillator within a phase-locked loop circuit.

Dielectric resonator antennas (DRAs) are miniaturized antennas of ceramics or another dielectric medium for microwave frequencies. Dielectric resonator antennas fabricated entirely from low loss dielectric materials and are typically mounted on ground planes. Their radiation characteristics are a function of the mode of operation excited in the DRA. The mode is generally chosen based upon the operational requirement. Dielectric resonator antennas offer several advantages over other antennas, such as small size, high radiation efficiency, and simplified coupling schemes for various transmission lines. The bandwidth can be controlled over a wide range by the choice of dielectric constant, and the geometric parameters of the resonator. Dielectric resonator antennas can also be made in low profile configurations, making them more aesthetically pleasing than standard whip, helical, or other upright antennas.

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