Radio frequency filter, also known as "radio frequency interference filter", is mainly used in high frequency electronic equipment, which is used to attenuate the high frequency interference signal produced by high frequency electronic equipment.

Background
With the rapid increase of the working frequency of electronic equipment, the frequency of electromagnetic interference is also increasing. The frequency of interference usually reaches hundreds of MHz, or even more than GHz.

The higher the frequency of voltage or current is, the easier radiation will be generated. It is these high frequency interference signals that cause the problem of radiation interference to become more and more serious.

Therefore, there is an urgent need for the emergence of a filter which can attenuate the high frequency signal of radiation interference. This filter is the radio frequency interference filter.

The effective filtering frequencies of common interference filters range from several KHz to MHz, while those of radio frequency interference filters range from several KHz to more than GHz.

RF Filter Classification
There are four basic types of RF filters: low-pass, high-pass, band-pass and band-stop. They perform the following functions according to their own characteristics. Only low-pass signal passes through and high-frequency signal is suppressed.

Contrary to the characteristics of low-pass and high-pass, high-pass is allowed to pass at high frequency, and low-frequency suppression, band-pass and band-stop are allowed to pass or suppress the frequency signals in the required frequency band, respectively. In radio frequency communication, filters are used as basic components, which play a unique role in ensuring the purity of signals, i.e. not being destroyed by interference signals.

There are many kinds of filter structures. In the future, microstrip and multi-layer structure should be the main structure. Small size, high power and low loss are the goals of the filter.