Dual passband radio frequency filter and communications device

1. A communications device, comprising:

a multi-band filter having a lower pass-band, a middle pass-band, and an upper pass-band, the lower pass-band and the middle pass band separated by an intervening lower stop-band, and the middle pass-band and the upper pass-band separated by an intervening upper stop-band, the multi-band filter comprising:

a first ladder network and a second ladder network coupled in series between a first port and a second port, wherein

the first ladder network is configured to provide transmission zeros at frequencies below a lower edge of the lower pass-band and transmission zeros at frequencies above an upper edge of the upper pass-band,

the second ladder network comprises two or more second-network shunt resonators and two or more second-network series resonators, and

respective motional resonant frequencies of each of the second-network shunt resonators and respective anti-resonant frequencies of each of the second network series resonators fall within one of the lower stop-band and the upper stop band.

2. The communications device of claim 1 , wherein the first ladder network comprises:

two or more first-network shunt resonators having respective motional resonant frequencies below the lower edge of the lower pass-band, and

two or more first-network series resonators having respective anti-resonant frequencies above the upper edge of the upper pass-band.

3. The communications device of claim 1 , wherein

the first ladder network comprises three shunt resonators and three series resonators, and

the second ladder network comprises four shunt resonators and four series resonators.

4. The communications device of claim 1 , further comprising:

an antenna coupled to the first port of the multi-band filter; and

a low noise amplifier coupled to the second port of the multi-band filter.

5. The communications device of claim 4 , wherein the antenna, the multi-band filter, and the low noise amplifier are a portion of a diversity receiver within a cellular telephone.

6. A method of designing a multi-band filter having a lower pass-band, a middle pass-band, and an upper pass-band, the lower pass-band and the middle pass band separated by an intervening lower stop-band, and the middle pass-band and the upper pass-band separated by an intervening upper stop-band, the method comprising:

defining a first ladder network and a second ladder network coupled in series between a first port and a second port, wherein

the first ladder network is defined to provide transmission zeros at frequencies below a lower edge of the lower pass-band and transmission zeros at frequencies above an upper edge of the upper pass-band, and

the second ladder network is defined to comprise two or more second-network shunt resonators and two or more second-network series resonators, the method further comprising:

selecting respective motional resonant frequencies of each of the second-network shunt resonators and respective anti-resonant frequencies of each of the second network series resonators to fall within one of the lower stop-band and the upper stop band.

7. The method of designing a multi-band filter of claim 6 , wherein

the first ladder network includes two or more first-network shunt resonators and two or more first-network series resonators, and

the method further comprises:

selecting respective motional resonant frequencies of the first-network shunt resonators below the lower edge of the lower pass-band, and

selecting respective anti-resonant frequencies for the first-network series resonators above the upper edge of the upper pass-band.