At present, there are usually three types of magnetic loop inductors: chip inductors, air-core inductors, and magnetic loop inductors. However, chip inductors are rarely used in passive liquid crystal filters due to their low Q value and large temperature drift. However, air-core inductors are mainly used in high-frequency applications because of their small size and light weight. , There is enough gap between the coils, so its reliability is relatively high.

The toroidal inductor mentioned in the topic is widely used, especially at lower frequencies. The magnetic ring inductor is composed of a magnetic ring body and an enameled wire wound on the inductor. The coils are close to each other without gaps.

A passive filter is a filter composed of passive components. Its important branch is an LC filter. The LC filter includes two passive components: a capacitor and an inductor. The reactance of capacitance and inductance components changes with frequency. Therefore, the LC filter circuit composed of inductors and capacitors can realize low/high-pass filtering, band-pass filtering and band-stop filtering. Therefore, the frequency of the conventional liquid crystal filter can reach 1-6000 MHz, and the circuit forms are flexible and diverse. Single-function circuits can be combined in cascade, parallel and other forms to achieve filter characteristics and functions of duplexers and multiplexers.