Design and application of dielectric filters

fish001

Design and application of dielectric filters [Copy link]

Dielectric filter is a microwave filter that uses dielectric resonant cavity through multi-stage coupling to achieve frequency selection. After entering the 21st century, dielectric filter has gradually moved from laboratory to production line after long-term accumulation of theory and practice.

Dielectric filters have been widely used in mobile communication and microwave communication systems due to their advantages of miniaturization, low loss and good temperature characteristics. They are characterized by low insertion loss, good power resistance and narrow bandwidth, and are particularly suitable for CT1, CT2, 900MHz, 1.8GHz, 2.4GHz, 5.8GHz, 5G, portable phones, car phones, wireless headphones, wireless microphones, radio stations, cordless phones and integrated transceiver duplexers.

The surface of the dielectric filter is covered with a metal layer with zero tangential electric field. The electromagnetic wave is confined in the dielectric, forming a standing wave oscillation, and its geometric size is about half of the waveguide wavelength. The material is generally made of ceramics with a relative dielectric constant of 60 to 80. The size of the dielectric ceramic filter actually used in wireless communications is in the centimeter level.

The main advantages of dielectric filters are large power capacity and low insertion loss, but there are two major disadvantages: first, they are large in size, at the centimeter level, and occupy a large volume of the system compared to integrated circuits; second, dielectric filters are generally discrete devices and cannot be integrated with signal processing circuits. In addition, a non-negligible transmission line must be passed from the filter to the signal processing chip, and impedance matching must be performed, which not only complicates the structure but also causes a certain amount of signal attenuation.

How it works

The dielectric filter is formed by coupling several dielectric resonators. The main loss of the metal cavity resonator comes from the loss of the conductor. The dielectric filter replaces the metal conductor with a dielectric (such as microwave ceramics), which can confine the electromagnetic field within the resonant cavity, so it has a higher Q value.

According to the propagation characteristics of electromagnetic waves, when electromagnetic waves enter a medium with a low dielectric constant from a medium with a high dielectric constant, they will be emitted and refracted at the interface of the medium. When the angle of incidence is greater than or equal to the critical angle, the electromagnetic wave will be totally reflected. The higher the dielectric constant of the medium and the smaller the critical angle, the easier it is for total reflection to occur. It is also easier for a magnetic wall to form on the surface of the medium. The dielectric block surrounded by the magnetic wall constitutes a dielectric resonator. This microwave resonator formed by a high dielectric constant and low loss dielectric material has its electromagnetic field energy basically concentrated in the resonant cavity, and the radiation loss is very small. The loss of the dielectric itself determines the Q value of the resonator, that is, Q=1/tanδ. The loss tangent of some commonly used dielectric materials is usually 0.0001~0.0002, and its Q value can reach 500~10,000. It is precisely because the quality factor of the medium is very high that most of the electromagnetic energy is concentrated in the dielectric resonator, so the electromagnetic oscillation is very easy to maintain. Therefore, the dielectric resonator can be used as a filter. At present, the relative dielectric constant of ceramic dielectric materials is about 39, and the maximum can be above 90. Therefore, using dielectric materials as resonators can greatly reduce the volume and mass of the filter without reducing the performance of the filter.