Radio-frequency filter circuit, multiplexer, radio-frequency front-end circuit, and communication device

A radio-frequency filter circuit comprising:

• a series arm circuit having a path connected between first and second input-output terminals;
• and
• a parallel arm circuit that is connected between ground and the path, wherein:
• the parallel arm circuit comprises:
• a first circuit connected between ground and the path and comprising a first parallel arm resonator, and
• a second circuit connected in parallel with the first circuit between ground and the path and comprising a second parallel arm resonator,
• the first circuit, the second circuit, or the first and second circuits further comprise:
• a variable frequency circuit comprising an impedance element and a switch element connected in parallel with each other, and
• the second parallel arm resonator has:
• a resonant frequency different from a resonant frequency of the first parallel arm resonator, and
• an anti-resonant frequency different from an anti-resonant frequency of the first parallel arm resonator, wherein:
• the series arm circuit comprises a series arm resonator;
• the resonant frequency of the first parallel arm resonator is lower than a resonant frequency of the series arm circuit;
• and
• the resonant frequency of the second parallel arm resonator is higher than the resonant frequency of the series arm circuit, and wherein:
• a bandwidth ratio of the series arm resonator is less than a bandwidth ratio of the first parallel arm resonator or the second parallel arm resonator,
• the bandwidth ratio of the series arm resonator is equal to a frequency difference between the anti-resonant frequency and the resonant frequency of the series arm resonator divided by the resonant frequency of the series arm resonator,
• the bandwidth ratio of the first parallel arm resonator is equal to a frequency difference between the anti-resonant frequency and the resonant frequency of the first parallel arm resonator divided by the resonant frequency of the first parallel arm resonator, and
• the bandwidth ratio of the second parallel arm resonator is equal to a frequency difference between the anti-resonant frequency and the resonant frequency of the second parallel arm resonator divided by the resonant frequency of the second parallel arm resonator.

A multiplexer comprising:

• a plurality of radio-frequency filter circuits, each radio-frequency filter circuit comprising:
• a series arm circuit having a path connected between first and second input-output terminals, and
• a parallel arm circuit that is connected between ground and the path, wherein:
• the parallel arm circuit comprises:
• a first circuit connected between ground and the path and comprising a first parallel arm resonator, and
• a second circuit connected in parallel with the first circuit between ground and the path and comprising a second parallel arm resonator,
• the first circuit, the second circuit, or the first and second circuits further comprise:
• a variable frequency circuit comprising an impedance element and a switch element connected in parallel with each other, and
• the second parallel arm resonator has:
• a resonant frequency different from a resonant frequency of the first parallel arm resonator, and
• an anti-resonant frequency different from an anti-resonant frequency of the first parallel arm resonator;
• and
• and a second radio-frequency filter circuit, wherein:
• the second radio-frequency filter circuit does not include a variable frequency circuit,
• the second radio-frequency filter circuit comprises an elastic wave resonator, and
• a bandwidth ratio of at least one of the first and second parallel arm resonators is greater than a bandwidth ratio of the elastic wave resonator,
• the bandwidth ratio of the first parallel arm resonator is equal to a frequency difference between the anti-resonant frequency and the resonant frequency of the first parallel arm resonator divided by the resonant frequency of the first parallel arm resonator, and
• the bandwidth ratio of the second parallel arm resonator is equal to a frequency difference between the anti-resonant frequency and the resonant frequency of the second parallel arm resonator divided by the resonant frequency of the second parallel arm resonator, and
• the bandwidth ratio of the elastic wave resonator is equal to a frequency difference between an anti-resonant frequency and a resonant frequency of the elastic wave resonator divided by the resonant frequency of the elastic wave resonator.