Generally speaking, mobile communication base station includes baseband processing unit, RF processing unit and antenna feeder system. It is a radio transceiver station for information transmission with mobile phone terminals through mobile communication switching center.

In mobile communication base station and antenna feeder system, RF device is one of the core components. RF devices mainly include various types of filters, duplexers, combiners, tower amplifiers, feed units, power dividers, couplers, antenna control units and other products. The filter is located on the antenna side. For the construction arrangement of 5g base station antenna, in the new massivemimo antenna, it can be basically determined that each antenna channel will correspond to a filter. The duplexer is located on the base station side and is essentially composed of a transmit filter and a receive filter.

As an important part of RF devices, filter undertakes the important task of helping base station frequency selection. Generally speaking, different base stations have their own clear working frequency bands, so the base station must have the ability to select various frequency signals for transmission and reception. The main function of the filter is to filter the transmitted and received signals, which can eliminate the signals without frequency band, so as to ensure the accuracy of transmitted and received signals.

 

       

     Application of base station filter        

 

Comparison of three filters
 

With the advent of 5g era, a new round of base station construction tide 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.

1. Production process flow of ceramic dielectric filter

The production process flow of ceramic dielectric filter mainly includes powder manufacturing, press molding, sintering, grinding, metallization, assembly and commissioning, as shown in the figure:

 

           
     Production process flow chart of ceramic dielectric filter  

two point oneDielectric filter has the advantages of high dielectric constant, low loss, small volume and power, which is convenient for large-scale integration;Small size and convenient installation。

two point two It can cover a wider band bandwidth and bear higher power。

two point threeGood reliability: the properties of ceramics are not sensitive to the change of temperature, which is more important outdoors. Unlike cavity filter, the performance changes greatly at high and low temperatures, and the corrosion resistance of ceramics is good;

two point fourLow cost dielectric ceramic filter。

two point fivePerformance of dielectric filterDepends on microwave ceramic powder: the electromagnetic wave resonance in 5g ceramic dielectric filter occurs in the ceramic dielectric material, and the key material for producing ceramic dielectric filter is microwave dielectric ceramic powder. Ceramic powder determines the performance of dielectric filter, and the formulation and preparation of powder are very difficult.Main performance indexes of microwave dielectric ceramic powderDielectric constantCount（ ε r) , quality factor Q, low dielectric loss, resonant frequency temperature coefficient, etc., which are specifically shown in:

    2.5.1 dielectric constantnumber: 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 filter sizeTherefore, the design requirements of the filter should be considered 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 factorsNumber Q, low dielectric loss: the higher the quality factor Q, the narrower the passband, the better the circuit selectivity, and the better filtering function can be realized. Q value and dielectric loss Tan δ Inversely proportional, the larger the Q value, the lower the insertion loss of the filter. The material has uniform structure, high density and uniform grain growth. Reducing impurities and defects can improve Q value.

    2.5.3 resonanceFrequency temperature coefficient: the resonant frequency temperature coefficient is close to zero, which can achieve high stability and high reliability of the filter. The frequency temperature coefficient is mainly determined by the linear expansion coefficient and dielectric constant of the material.

    Additional notes:At present, the powder materials used to prepare ceramic dielectric filters mainly includeMagnesium hydroxide, magnesium titanate, calcium carbonate, magnesium oxide, barium carbonate, samarium oxide, zinc oxide, strontium carbonate, aluminum oxide, lanthanum oxideWait.

Ceramic dielectric filter is sintered by dry pressing powder, metallized with silver coated electrodes on both sides, and produced piezoelectric effect after DC high voltage polarization.The electric field generated by the deformation of both sides of the ceramic sheet has the characteristics of series resonance.Since the electromagnetic wave resonance occurs inside the pressure ceramic medium, there is no need to fix the resonator, so the volume of the dielectric filter is small.

On the other hand, microwave ceramic powder has higher Q-value material characteristics than metal cavity, greatly reduces insertion loss, has the characteristics of high out of band suppression, good temperature drift characteristics, wide temperature application range, various forms of packaging structure and output interface forms, meets the development trend of miniaturization of base station filter, and becomes an adaptive 5g technology framework, Due to the increase of frequency band, the demand for equipment filters increases, and higher requirements for Q value and power capacity of high-frequency resonators.

4. Difficulties and disadvantages of ceramic dielectric filter:

 

     Filter debugging instrument > > > network analyzer  

5.1 compared with metal cavity filter, ceramic dielectric filter has the advantages of small volume, light weight, small loss and large Q value, and has absolute advantages in 5g field. At the same time, with the continuous breakthrough of scientific research and technology, the problems of dielectric filter are being gradually solved and its practicability is increasing. With the development of 5g technology, ceramic dielectric filter is expected to keep up with the tide of the times, make use of its own advantages to better meet the needs of base stations in the 5g era, and gradually surpass the cavity filter, so as to become the mainstream of the industry.

5.2 with the advent of the 5g era, 5g base stations are developing towards miniaturization, lightweight and high integration. The number of antennas used in MIMO technology will grow exponentially. 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 corresponding filter is used to select and process the signal frequency, the demand for the filter will increase greatly, so there are higher requirements for the device size and heating performance of the filter. Cavity filter is difficult to be widely used in highly dense antennas because of its large volume and large calorific value.Therefore, 5g ceramic dielectric filter will give full play to its advantages such as small volume and light weight, take the lead in the broad 5g market in the future, and willGradually become the main player in the filter industryFlow.

At present, Huawei and Ericsson, the world's four major mobile communication base station equipment manufacturers, prefer ceramic dielectric filters. ZTE and Nokia still focus on miniaturized metal cavity filters. In the future, they will transition to ceramic dielectric filters, and the market share of ceramic filters will continue to increase. According to the prediction of the Research Institute, there are nearly 8.5 million global base stations in the 5g era. According to the estimation of three antennas per base station and 64 filters per antenna, the demand for dielectric filters is about 1.6 billion, and the global market capacity of ceramic dielectric filters is about 56.6 billion yuan.

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