Band-pass filter with trap circuits having different q factors

Abstract

A band-pass filter device includes a band-pass filter having a characteristic for sufficiently passing signals belonging to a passing frequency band to be passed, a first trap circuit having a Q factor sufficiently higher than that of the BPF and attenuating signals belonging to a frequency band ranging frequencies higher than the passing frequency band, a second trap circuit for attenuating signals belonging to a frequency band ranging frequencies lower than the passing frequency band, a third trap circuit having a Q factor lower than that of the first trap circuit and attenuating an unnecessary component generated in the first trap circuit from the signal, a fourth trap circuit having a Q factor lower than that of the second trap circuit and attenuating an unnecessary component generated in the second trap circuit from the signal. When a first signal belonging to a passing frequency band and a second signal belonging to a frequency band ranging frequencies lower or higher than the passing frequency band are simultaneously supplied to a BPF device having this structure, the BPF device can selectively pass the first signal, and completely attenuate the unnecessary second signal. A structure comprising two BPF having different central passing frequencies, first and third trap circuits having relatively low Q factors, a second trap circuit having a high Q factor and eliminating an unnecessary component which cannot be removed by the first trap circuit can also realize such a good BPF device as described above.