Filter, antenna duplexer, and communication apparatus incorporating the same

摘要

A filter with which it is possible to control the relationship between a pass band and an attenuation band. In this filter, for example, a single-stage trap filter is electrically connected to a three-stage band pass filter via a coupling capacitor. The trap filter has a serial resonance section composed of a resonator and a resonance capacitor. The serial resonance section is connected in parallel to a capacitive reactance element (capacitor), and a serial circuit composed of an inductive reactance element (inductor) and a PIN diode as a switching element. The capacitive reactance element and the inductive reactance element both serve to make an admittance of the trap circuit substantially zero.