Prospects and advantages of microwave dielectric ceramics

Microwave dielectric ceramics (MWDC) are ceramics used as dielectric materials in microwave frequency band (mainly UHF, SHF band, 300 MHz - 300 GHz) circuits and have one or more functions. It is a new functional ceramic material developed in the last two decades. It is the key material for manufacturing microwave dielectric resonators and filters, which has been widely studied in recent years. On the basis of the original microwave ferrite, its formulation and manufacturing process have been significantly upgraded to make it have excellent properties such as high dielectric constant, low microwave loss and small temperature coefficient. Suitable for the manufacture of electronic countermeasures, navigation, communications, radar, domestic satellite live TV receivers, cell phones in the stabilization of frequency oscillators, filters, discriminators and other modern microwave devices, can meet the requirements of microwave circuit miniaturization, integration, high reliability, low cost. With the development of mobile communications and modern electronic equipment, the study of microwave dielectric ceramics is receiving more and more attention, which carries the infinite hope of future microwave devices.
One: Why use dielectric filters?
With the rapid development of modern communication technology, the special nature of the use of communication equipment requirements make the weight and size of the communication system equipment requirements are increasingly high, especially for mobile communication systems in the filter miniaturization, portability, high frequency and low power requirements. However, the current common filters have their own disadvantages, for example: the miniaturization of filters and duplexers composed of microstrip structures and metal resonators is too difficult to achieve; SAW filters can reduce the circuit size, but due to the small power capacity and insertion loss, the scope of application is limited. Fortunately, with the cross-pollination of modern material science and electronic information science and technology, the development of new materials and manufacturing technologies such as thin-film technology, monolithic integrated circuits, microelectromechanical systems, LTCC, etc. has greatly promoted the rapid development of microstrip and filter design, processing and application based on thin-film technology. Therefore, a variety of all-solid-state micro (chip) IF, HF and microwave filters are moving toward high performance, low cost, miniaturization and high frequency direction
Second: the advantages of microwave dielectric resonators
Microwave dielectric resonators and metal cavity resonators compared to some of the following advantages.
(1) miniaturization (high dielectric constant). It is well known that the way to achieve miniaturization, high stability and low price of microwave devices is the integration of microwave circuits. In the process of microwave circuit integration, metal waveguides to achieve the integration of planar microstrip, microwave tube miniaturization. However, the various metal resonant cavities in microwave circuits are difficult to integrate with microstrip circuits due to their large size and heavy weight. The solution to this challenge lies in the use of microwave dielectric ceramic materials to produce resonators. It is known that the size of the resonator is inversely proportional to the square root of the dielectric constant of the dielectric material. Therefore, the larger the dielectric constant of the dielectric material, the smaller the required dielectric ceramic block, the smaller the size of the resonator. Therefore, the high dielectric constant of the microwave dielectric ceramic material is conducive to the miniaturization of microwave dielectric filters, which can realize the integration of microwave circuits with microwave tubes and microstrip lines in the mix, so that the device size to millimeter level, the price is much lower than the metal resonant cavity. Generally require a dielectric constant > 10.
(2) high stability (frequency temperature coefficient close to zero). The operating environment temperature of communication equipment can not be constant. If the resonant frequency of the microwave dielectric material varies greatly with temperature, the carrier signal of the filter will drift at different temperatures, thus affecting the performance of the equipment. This requires that the resonant frequency of the material does not change too much with temperature. The actual temperature range required is about -40 ℃ ~ +100 ℃, in this range, the frequency temperature coefficient of the material is not greater than l0pm / ℃. At present, the practical microwave dielectric ceramic materials frequency temperature coefficient of up to 0 ppm / ℃, can achieve high stability and high reliability of the device.
(3) Low loss (high quality factor q). Low insertion loss is an important requirement for filters, the dielectric loss of microwave dielectric materials is the main factor affecting the insertion loss of dielectric filters. The q value of the microwave dielectric material is inversely proportional to the dielectric loss and resistance. the larger the q value, the lower the insertion loss of the filter. In summary, dielectric ceramic materials are sweeping the microwave market at a very fast pace of development, mastering the formulation of new ceramic materials has become the new favorite of all microwave manufacturers.
Third: the use of dielectric resonators
Microwave dielectric ceramics are mainly used as microwave components, such as resonators, filters, dielectric antennas and dielectric waveguide circuits. The advantages of microwave dielectric filters are miniaturization, low loss, small frequency temperature coefficient, high dielectric constant, and low cost. Compared with metal resonant filters, it has the advantage of miniaturization, the volume is only a few tenths of the former; compared with the sound surface filter, it has a higher frequency and lower cost. Microwave filters are widely used in microwave communications, radar navigation, electronic countermeasures, satellite relay, missile guidance, test instruments and other systems, is an indispensable device in microwave and millimeter wave systems. Its performance often directly affects the performance of the entire communication system. It has a wide range of applications. Take cell phones as an example, the annual sales of cell phones in China in 2005 was 64 million, the Chinese cell phone market will grow at 20% per year, and sales will reach 100 million in two or three years. This shows that microwave dielectric ceramics in commercial applications have a lot of room for development and market.
Fourth: foreign research and development and application
At present, foreign companies have produced a large number of microfiltration devices. Famous companies are DLI, TRANS-TECH, MURATA, Japan, FILTRONIC, etc.. They produce a variety of microwave dielectric ceramic filters, duplexers, resonators, dielectric antennas and other products have been used in microwave base stations, cell phones, cordless phones, and achieved significant economic and social benefits. The most mature dielectric filter is the MB series integrated dielectric filter manufactured by Murata Manufacturing Co. Although this filter is made of common ceramic dielectric materials and electrodes and is based on high frequency circuit design techniques, it has greatly contributed to the development of the mobile communication terminal market. In order to reduce the size, Murata developed MB chip dielectric filters consisting of 2-3 coaxial resonators without circuit substrate, coupler, cover, etc. The smallest secondary bandpass filter used in the PHSl900 is only 3.8 x 4.3 x 2.0 mm3, while the corresponding secondary coupling dielectric filter can only reach 7.0 x 8.0 x 3.7 mm3. Typical filters are the common DP series, common FB series and MB series dielectric filters.
(1) Common DP series products are surface mount filters, square dielectric resonators and circuit coupling devices are mounted on the PCB, input, output and ground terminals are used PCB surface electrodes. Compared with the surface mount filter with metal housing, the ground flatness and reflowability of the common DP series filter has been greatly improved. And its input and output terminals no longer require special space on the PCB, reducing its mounting area. This makes the resonator size and coupling circuit design flexible and easy to adapt to performance requirements. However, the biggest drawback is the complex and time-consuming fabrication process. Therefore, the most fundamental issue for cost reduction is to automate the production process.
(2) Common FB series. Common FB series products are integrated filters, consisting of a multi-stage resonator composed of a combination of dielectric, input and output terminals and housing. Compared with DP series products, FP series products require fewer parts and cost less. Coupling between resonant devices is controlled by coupling holes in the dielectric, and coupling between the resonator and external components is achieved by inserting molded resin metal plugs into the coupling holes in the dielectric. This product is popular because of its small size, but its disadvantage is that the coupling is controlled by the coupling holes on the combined dielectric, and the design freedom is inferior to that of the DP series.
(3) MB series dielectric filters. The MB series dielectric filters developed by Murata Manufacturing Co., Ltd. are mainly concerned with reducing size and cost while maintaining the performance of common devices. To achieve this goal, the researchers designed the device as a monolithic (circular coaxial) axisymmetric structure. The entire surface of the device was designed as electrodes, except for the input and output terminals and the resonator conductor. From the performance point of view, the gap of the resonator without any conductor electrodes is used to generate the floating capacitance, while the electrical properties are determined by adjusting the position and width of the gap as the beginning of the resonator. Making the filter variety suitable for automation of the production process. The ceramic material used to manufacture the resonators is a highly insulating material and the electrodes are copper plated. The resonators are electromagnetically coupled in the dielectric and can also be capacitively coupled to external components. The coupling elements between the resonant devices and between the resonator and the external world are separated from the resonator. Thus, the synthesized filter is equipped with the elements necessary for coupling so that a single filter can be achieved. The absence of soldered elements on the common device greatly enhances its reliability. Some or all of the bottom end of the component is made of ceramic material to eliminate any problems associated with a flat bottom. Good reflowability is also achieved, and the coaxial round hole is the loop that guarantees high Q0 values. Compared with ordinary filters, the Q0 value per filter capacity is increased by 40%. In addition, the product's attenuation electrodes (required for mobile communication filters) are designed to be controlled with a multi-stage impedance resonator structure at the low-frequency end and the high-frequency end. Siemens developed a three-stage monolithic integrated chip dielectric filter with size specifications of only 8.8×7.45×2.75 mm3-5.75×4.0×2.95 mm3 for ISM915, GPSl500, PCN/PCSl800, PHS.
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