Ladder filter

1. A ladder filter including a serial arm and a plurality of parallel arms, the serial arm connecting a first end to a second end, the plurality of parallel arms being connected between the serial arm and a ground potential, the ladder filter comprising:

a plurality of serial arm resonators disposed along the serial arm; and

a plurality of parallel arm resonators each disposed along a respective parallel arm of the plurality of parallel arms; wherein

a plurality of ladder circuit units are disposed from the first end to the second end;

each of the plurality of ladder circuit units includes a corresponding serial arm resonator of the plurality of serial arm resonators and a corresponding parallel arm resonator of the plurality of parallel arm resonators, and the plurality of ladder circuit units are mirror-connected to one another; and

an impedance at the first end is different from an impedance at the second end.

2. The ladder filter according to claim 1 , wherein

in the plurality of ladder circuit units, when impedances of adjacent ladder circuit units are Xa and Xb, the adjacent ladder circuit units are mirror-connected to one another; and

when the ladder circuit unit having the impedance Xa is positioned on the first end relative to the ladder circuit unit having the impedance Xb, Xa≤Xb or Xa≥Xb is satisfied, and not all impedances of all of the plurality of the ladder circuit units are equal or substantially equal to one another.

3. The ladder filter according to claim 1 , wherein, when impedances of each of the plurality of ladder circuit units are Xa, Xb, Xc, . . . , Xn from the first end to the second end, Xa≤Xb≤Xc≤ . . . ≤Xn or Xa≥Xb≥Xc≥ . . . ≥Xn is satisfied, and Xa=Xb=Xc= . . . =Xn is not satisfied.

4. The ladder filter according to claim 1 , further comprising an inductor connected between at least one of first and second ends and the ground potential.

5. The ladder filter according to claim 1 , wherein the impedance at the first end or the second end is equal to or substantially equal to about 50Ω.

6. The ladder filter according to claim 1 , wherein the plurality of serial arm resonators and the plurality of parallel arm resonators are defined by elastic wave resonators.

7. A ladder filter comprising:

a serial arm connecting a first end to a second end;

a plurality of parallel arms connected between the serial arm and a ground potential;

a plurality of serial arm resonators disposed along the serial arm;

a plurality of parallel arm resonators each disposed along a respective parallel arm of the plurality of parallel arms; wherein

a plurality of ladder circuit units are disposed from the first end to the second end;

each of the plurality of ladder circuit units includes a corresponding serial arm resonator of the plurality of serial arm resonators and a corresponding parallel arm resonator of the plurality of parallel arm resonators, the plurality of ladder circuit units being mirror-connected to one another, and

a first impedance at the first end is different from a second impedance at the second end.

8. The ladder filter according to claim 7 , wherein

a first ladder circuit unit of the plurality of ladder circuit units has a first impedance Xa;

a second ladder circuit unit of the plurality of ladder circuit units has a second impedance Xb;

the first ladder circuit unit and the second ladder circuit unit are positioned adjacent to each other and are mirror-connected to one another;

when the first ladder circuit unit is positioned closer to the first end than the second ladder circuit unit, Xa≤Xb or Xa≥Xb, is satisfied, and not all impedances of all of the plurality of the ladder circuit units are equal or substantially equal to one another.

9. The ladder filter according to claim 7 , wherein, when impedances of each of the plurality of ladder circuit units are Xa, Xb, Xc, . . . , Xn from the first end to the second end, Xa≤Xb≤Xc≤ . . . ≤Xn or Xa≥Xb≥Xc≥ . . . ≥Xn is satisfied, and Xa=Xb=Xc= . . . =Xn is not satisfied.

10. The ladder filter according to claim 7 , further comprising an inductor that is connected between at least one of the first and second ends and the ground potential.

11. The ladder filter according to claim 7 , wherein the first impedance at the first end or the second impedance at the second end is equal to or substantially equal to about 50Ω.

12. The ladder filter according to claim 7 , wherein the plurality of serial arm resonators and the plurality of parallel arm resonators are defined by elastic wave resonators.

13. A ladder filter comprising:

a serial arm connecting a first end to a second end;

a plurality of parallel arms connected between the serial arm and a ground potential; and

a plurality of ladder circuit units each including a serial arm resonator and a parallel arm resonator; wherein

the serial arm resonator is disposed on the serial arm, and the parallel arm resonator is disposed on one of the plurality of parallel arms;

adjacent units of the plurality of ladder circuit units are mirror-connected to one another;

impedances of each of the plurality of ladder circuit units sequentially increase or decrease in a direction from the first end to the second end.

14. The ladder filter according to claim 13 , wherein at least one impedance of the impedances of each of the plurality of ladder circuit units is different from another impedance of the impedances of each of the plurality of ladder circuit units.

15. The ladder filter according to claim 13 , further comprising an inductor connected between at least one of the first and second ends and the ground potential.