A filter includes a top panel, a bottom panel, two first side panels located between the top panel and the bottom panel, and at least one diaphragm that has a metal surface. The two first side panels, the top panel and the bottom panel form a rectangular waveguide. The at least one diaphragm is connected to both the top panel and the bottom panel and separates a cavity of the rectangular waveguide into several cavity chambers that extend along an input/output direction of the rectangular waveguide. At least one of the first side panels is an adjustable side panel whose position relative to the diaphragm is adjustable.