Generally speaking, the mobile communication base station includes the baseband processing unit, the radio frequency processing unit and the antenna feed system. It is a radio transceiver station that transmits information with the mobile phone terminal through the mobile communication switching center.
In mobile communication base station and antenna feeder system, radio frequency device is one of the core components. RF devices mainly include various types of filters, duplexers, combiners, tower amplifiers, feed units, power splitters, couplers, antenna control units and other products. The filter is located at the antenna side. For the 5g base station antenna construction arrangement, in the new massivemimo type antenna, it can be basically determined that each antenna channel will correspond to one filter. The duplexer is located at the base station side and essentially consists of a transmitting filter and a receiving filter.
As an important part of RF devices, the filter undertakes the important task of helping the base station select frequencies. Generally speaking, different base stations have their own specific operating frequency bands. Therefore, the base station must have the ability to select various frequency signals to transmit and receive. The main function of the filter is to filter the transmission and reception signals, which can eliminate the signals that do not need the frequency band, so as to ensure the accuracy of the transmission and reception signals.
Base station filters mainly include metal coaxial cavity, ceramic dielectric resonance and ceramic dielectric filter.
With the arrival of 5g era, a new round of base station construction will be inevitable, and the filter will also usher in a new wave of demand peak. The previous cavity filter will gradually transform to ceramic dielectric filter.
2,Characteristics of ceramic dielectric filter:2.1 the dielectric filter has high dielectric constant, low loss, small volume and large power, which is convenient for large-scale integration; Small size and convenient installation.
2.2 the coverage band has wider bandwidth and higher bearing power.
2.3 good reliability: various properties of ceramics are not sensitive to temperature changes, which is important outdoors. Unlike cavity filters, the performance changes greatly at high and low temperatures, and the ceramic has good corrosion resistance;
2.4 low cost of dielectric ceramic filter.
2.5 the performance of the dielectric filter depends on the microwave ceramic powder: the electromagnetic wave resonance in the 5g ceramic dielectric filter occurs inside the ceramic dielectric material, and the key material for producing the ceramic dielectric filter is the microwave dielectric ceramic powder. Ceramic powder determines the performance of dielectric filter. The formulation and preparation of ceramic powder are very difficult. The main performance indexes of microwave dielectric ceramic powder are dielectric constant（ ε r) , quality factor Q, low dielectric loss, resonance frequency temperature coefficient, etc., which are specifically shown in:
2.5.1 dielectric constant: high dielectric constant can realize the miniaturization design of filter size, but the higher the dielectric constant is, the better. Too high dielectric constant will affect the transmission loss. Therefore, it is necessary to consider the design requirements of the filter to select the appropriate dielectric constant. The dielectric constant of microwave dielectric ceramics mainly depends on the crystal phase in the material structure and the preparation process.
2.5.2 quality factor Q, low dielectric loss: the higher the quality factor Q, the narrower the pass band, the better the circuit selectivity and the better the filtering function. Q value and dielectric loss (tan) δ The larger the Q value, the lower the insertion loss of the filter. The material has uniform structure, high density, uniform grain growth, and the reduction of impurities and defects can improve the Q value.
2.5.3 temperature coefficient of resonance frequency: when the temperature coefficient of resonance frequency is close to zero, high stability and high reliability of the filter can be realized. The temperature coefficient of resonance frequency is mainly determined by the linear expansion coefficient and dielectric constant of the material.
Additional note: Currently, the powder materials used to prepare ceramic dielectric filters mainly include magnesium hydroxide, magnesium titanate, calcium carbonate, magnesium oxide, barium carbonate, samarium oxide, zinc oxide, strontium carbonate, aluminum oxide, lanthanum oxide, etc.
3.Advantages of ceramic dielectric filter
The ceramic dielectric filter is sintered by dry pressing, and then metallized with silver coated on both sides as electrodes. After DC high-voltage polarization, the piezoelectric effect is generated. The electric field generated by the deformation of the two surfaces of the ceramic sheet has the series resonance characteristic. Since the above electromagnetic wave resonance occurs inside the pressure ceramic medium, there is no need to fix the resonator, so the dielectric filter has a small volume.
On the other hand, because the microwave ceramic powder has higher Q-value material characteristics than the metal cavity, it greatly reduces the insertion loss, has high out of band suppression, good temperature drift characteristics, wide temperature application range, various types of packaging structures and output interface forms, meets the development trend of the miniaturization of the base station filter, and becomes a new type of filter with the 5g technology framework Higher requirements for the Q value and power capacity of the high-frequency resonator.
4.Difficulties and disadvantages of ceramic dielectric filter:
4.1 consistency of microwave ceramic powder formulation and preparation batch production: during the production of dielectric filter, it is necessary to try to control the process to produce ceramics with less impurities, fewer defects and uniform grain distribution.
4.2 the production process of ceramic filter is complex, and metallization automation is also difficult: the specific process flow includes powder manufacturing, press molding, sintering, grinding, metallization, assembly and debugging. Due to the irreversibility of ceramic grinding, product debugging is the key to improve production capacity. In addition, metallization automation is also the technical difficulty.
5. In the 5g era, the ceramic dielectric filter of the base station will gradually become the mainstream of the filter industry
5.1 compared with metal cavity filter, ceramic dielectric filter has the advantages of small volume, light weight, small loss, large Q value, etc. it has an absolute advantage in the application of 5g. At the same time, with the continuous development of scientific research and technology, the problems of dielectric filter are being gradually solved, and its practicability is continuously enhanced. With the development of 5g technology, ceramic dielectric filters are expected to keep up with the tide of the times, better meet the needs of base stations in the 5g era by using their own advantages, gradually surpass cavity filters, and become the mainstream of the industry.
5.2 with the advent of the 5g era, the 5g base station is developing towards miniaturization, lightweight and high integration. The number of antennas used in MIMO technology will double, and it is expected that 64 or even 128 antennas will be required. Since each antenna needs to be equipped with a corresponding duplexer, and the signal frequency is selected and processed by the corresponding filter, the demand for the filter will increase greatly, so there are higher requirements for the device size and heat generation performance of the filter. Cavity filter is difficult to be widely used in highly dense antennas because of its large volume and large heat generation, and faces great development pressure. Therefore, 5g ceramic dielectric filter will give play to its advantages such as small size and light weight, take the lead in the broad 5g market in the future, and will gradually become the mainstream of the filter industry.
