A filter is widely used in the modern communications field, and a basic function thereof is: making useful signals pass on a signal link to the greatest extent, and restraining harmful signals to the greatest extent.
There are a wide variety of common filters, which mainly include: microstrip filter, strip line filter, and coaxial cavity filter.
The microstrip filter is formed by microstrips, where the microstrips are printed wires separated by dielectrics on a ground plane, that is, printed wires laid on a side of the dielectrics, and grounding metal is disposed at a position on the other side corresponding to the printed wires. Since the microstrip filter is simple in structure and manufacturing process and small in volume, it is widely used in various communication circuits, but it has defects of large insertion loss and small power capacity.
The coaxial cavity filter is widely applied to systems of communication and radar, and generally includes standard coaxial and square cavity coaxial based on different cavity structures. The coaxial cavity filter has features such as high Q value, easy implementation, small insertion loss, and large power capacity. This type of filter is very suitable for mass production, and therefore, the cost is very low. However, when the coaxial cavity filter is used above 10 GHz, it is hard to achieve manufacturing precision because of its tiny physical size, resulting in difficulty of batch consistency of indexes such as filter standing wave, phase, and group delay.