Filter structure

According to the implementation of the resonator, it is divided into two types: lumped element and distributed parameter filter. The distribution parameters are further divided into printed circuit (microstrip, suspended microstrip line), mechanical cavity, dielectric resonator and so on.

1. Lumped (LC) filter : suitable for applications below 3 GHz. This type of filter uses a microwave lumped component design method to realize the resonant circuit, so that the filter has a small volume, a structure that is easy to install, and has no parasitic passband ( Or very far), at the same time has the advantages of flexible design and short development cycle. However, due to the low Q value of the lumped inductance component, the insertion loss of this type of filter is large and the relative bandwidth cannot be made very narrow (generally >3%), which limits its low insertion loss, high rectangularity, narrow band, large Applications such as power requirements.

2. Dielectric filter : The Q value of the dielectric resonator is generally 2~3 times (TEMModular) or 10~20 times (TE01 δ mode, Dielectric Loaded Cavity) of the lumped component, so that the filter can be realized. Narrowband filtering (1% to 1‰). However, the parasitic passband is relatively close (the TEM mode is about 2~3f0, and the TE01 δ mode is about 1.12f0). This type of filter is mainly used in the case where both the near-end hybrid suppression of the passband and the small volume are required.

3. Cavity filter : The resonator is composed entirely of mechanical structure, which has a relatively high Q value (thousands or even tens of thousands), which is very suitable for low insertion loss (<1dB), narrow band (<1%), Applications such as high power (up to 300W or higher) transmission. This type of filter has a large volume and has a parasitic passband, which has high processing cost and long production cycle.

4. Crystal Filter : Quartz material has a piezoelectric effect. The crystal resonator made with unique chamfer has a very high quality factor (105 orders). The constructed filter has excellent selectivity and fractional bandwidth. Available in 0.1~5‰

5. Microstrip circuit filters : These filters are widely used at low cost and without excessive volume requirements. Especially above 3GHz, the overall performance is better than LC filter. The suspended microstrip line combined with the printed board process has a small dispersion effect, and is smaller and more repeatable than the metal cavity, making it widely used in wideband filters and multiplexers.

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