Cavity filters are microwave filters that use resonant cavity structures to achieve selective signal filtering. They are typically made of conductive metals, with common types including coaxial cavities and waveguide cavities. Cavity filters offer high Q-factors, high power handling, and low insertion loss, making them suitable for high-power applications such as communication base stations, satellite communications, and radar systems. Many distributors offer a wide range of electronic components to cater to diverse application needs, like electronic components distributor
Working Principle of Cavity Filters
The core principle of a cavity filter is to use a metal cavity as a resonator to implement filtering. Inside the metal cavity, the conductor walls (with nearly zero resistivity) form what is called an electric wall, where the tangential component of the electric field and the normal component of the magnetic field are zero.
When an electromagnetic wave enters the cavity, it is fully reflected by the walls. Except for the input and output ports, the cavity is closed. When the input wave frequency matches the cavity’s resonant frequency, the wave undergoes multiple reflections inside the cavity, forming a stable electromagnetic standing wave—this is electromagnetic resonance. By combining multiple resonators with different resonant frequencies, the desired filtering function can be achieved.
Types of Cavity Filters
Cavity filters can be classified into coaxial cavity filters and waveguide cavity filters. The coupling between cavities can be either direct coupling or cross coupling. In direct coupling, each resonator is only coupled to its adjacent cavity, so the electromagnetic wave passes sequentially through all resonators along a single path to the load. Cross coupling introduces coupling between non-adjacent cavities, creating transmission zeros outside the passband and improving out-of-band rejection. Common coupling methods include diaphragm coupling, probe coupling, and spatial coupling, each with different coupling strengths and manufacturing complexities, suitable for different scenarios.
Design Process of Cavity Filters
The design process of cavity filters is similar to other filters. A low-pass prototype is first determined based on specifications, followed by frequency and impedance transformations to build a microwave equivalent circuit, which then guides the physical fabrication. Electromagnetic simulation software such as HFSS and CST can optimize the topology and coupling matrix during design. Through simulation, suitable cavity dimensions and coupling structures are determined, and fabrication drawings are generated.
Cavity filters are usually made of steel or aluminum alloy, with surfaces treated by conductive oxidation, gold plating, or silver plating to ensure conductivity and corrosion resistance. Silver offers good conductivity but is prone to oxidation, while gold provides excellent stability but at a higher cost. After fabrication, the resonant frequency is usually fine-tuned using tuning screws, which are fixed with adhesive to prevent vibration from affecting performance.
Features of Cavity Filters
Cavity filters feature low insertion loss within the passband, steep out-of-band rejection, and high frequency selectivity. Metal cavities have large volume, good heat dissipation, and uniform electric field distribution, allowing them to handle kilowatt-level average and peak power, making them suitable for high-power scenarios such as radar and broadcast transmission. Using low thermal expansion materials like steel or aluminum ensures structural strength, minimal frequency drift due to temperature, and reliable performance in extreme environments such as outdoor base stations or aerospace applications.
However, cavity filters also have disadvantages, including large size, heavy weight, and high manufacturing costs. While high Q-factors provide excellent selectivity, they are unsuitable for wideband applications. Their dimensions depend on signal wavelength, making them less suitable for low-frequency applications. Therefore, cavity filters are ideal when high performance, low loss, and high stability are required, and size and cost are less critical. For applications requiring miniaturization, low cost, or wideband operation, other types of filters may be more appropriate.