Abstract: A signal filter includes a notch filter and a wideband filter. The notch filter is configured to perform a band-rejection filtering operation according to a band-rejection filtering property. The wideband filter is coupled to the notch filter, and is configured to perform a wideband filtering operation according to a wideband filtering property. The band-rejection filtering property includes a first cutoff frequency, a frequency bandwidth, a relatively high quality factor and a relatively low coupling coefficient. The wideband filtering property includes a second cutoff frequency, a relatively low quality factor and a relatively high coupling coefficient. The first and the second cutoff frequencies have a frequency difference therebetween. A ratio of the frequency difference to the frequency bandwidth is within a preset ratio range being from 2.5% to 20%.

Abstract: An acoustic-wave ladder filter has a first port, a second port and a ground terminal, and includes a series resonator and a shunt circuit. The series resonator is coupled to and disposed between the first and the second ports in series. The shunt circuit is coupled to and disposed between the series resonator and the grounding terminal, and includes a shunt resonator and a functional circuit. The functional circuit is connected in series with the shunt resonator. The functional circuit includes a resistor having a resistance value. The resistance value is greater than 5 Ohms and is smaller than 50 ohms. The functional circuit may further have an inductance.

Abstract: A signal filter includes a notch filter and a wideband filter. The notch filter is configured to perform a band-rejection filtering operation according to a band-rejection filtering property. The wideband filter is coupled to the notch filter, and is configured to perform a wideband filtering operation according to a wideband filtering property. The band-rejection filtering property includes a first cutoff frequency, a frequency bandwidth, a relatively high quality factor and a relatively low coupling coefficient. The wideband filtering property includes a second cutoff frequency, a relatively low quality factor and a relatively high coupling coefficient. The first and the second cutoff frequencies have a frequency difference therebetween. A ratio of the frequency difference to the frequency bandwidth is within a preset ratio range being from 2.5% to 20%.

Abstract: An acoustic-wave ladder filter has a first port, a second port and a ground terminal, and includes a series resonator and a shunt circuit. The series resonator is coupled to and disposed between the first and the second ports in series. The shunt circuit is coupled to and disposed between the series resonator and the grounding terminal, and includes a shunt resonator and a functional circuit. The functional circuit is connected in series with the shunt resonator. The functional circuit includes a resistor having a resistance value. The resistance value is greater than 5 Ohms and is smaller than 50 ohms. The functional circuit may further have an inductance.