Nonreciprocal circuit device and communications apparatus incorporating the same

Abstract

A compact nonreciprocal circuit device in which a large amount of attenuation can be obtained at a predetermined frequency band without increasing cost. In the nonreciprocal circuit device, three central conductors are arranged to cross each other on a ferrite member to which a direct-current magnetic field is applied. An inductor is disposed at the bottom of a capacitor connected to a port of a first central conductor. The capacitor and the inductor constitute a series resonant circuit, by which a trap filter for signals passing through an isolator as the nonreciprocal circuit device is constituted. A resonance frequency of the series resonant circuit is set to be approximately twice the central frequency of a pass bandwidth of the isolator so that the second harmonic of a fundamental wave is attenuated. In addition, at the basic wave frequency, the series resonant circuit acts as capacitive impedance. Thus, combination of the capacitive impedance with an equivalent inductance of the first central conductor acts as a matching capacitance with respect to a basic wave frequency.