Filter for inhibiting second harmonics

The invention provides a filter for inhibiting second harmonics. The filter comprises a third-order resonant coupling band-pass filtering circuit, a third-order low-pass filtering circuit and a third-order high-pass filtering circuit, wherein the third-order high-pass filtering circuit, the third-order resonant coupling band-pass filtering circuit and the third-order low-pass filtering circuit aresequentially connected. Compared with an ordinary LC filter, the filter provided by the invention has the beneficial effects that the cost is lower; the environment-friendly effect is better; the damage to the environment is small; the performance is excellent; the band-pass difference consumption is less than 2dB; the flatness degree in the band is better, and is smaller than 0.5dB; the rectangular coefficient is higher; the out-of-band inhibition, particularly the inhibition on second harmonics can reach 57dB; when the filter is applied to a power amplifier system, the filter is positionedat a power amplifier or a