Transmitting filter including SAW resonators
View Patent ↗A surface acoustic wave filter includes an antenna terminal, a transmitting filter and a variable branching line both of which are coupled to the antenna terminal and a receiving filter coupled to the variable branching line. The transmitting filter has a first end series-arm SAW resonator, a second end series-arm SAW resonator, a middle series-arm SAW resonator coupled between the first and second end series-arm SAW resonators and parallel-arm SAW resonators. Each of the first and second end series-arm SAW resonator has a first resonance frequency. The middle series-arm SAW resonator has a second resonance frequency that is higher than the first resonance frequency.
1. A transmitting filter comprising:
a first SAW resonator coupled to a first node, the first SAW resonator having a first high frequency equivalent resistance value;
a second SAW resonator coupled to the first node and a second node, the second SAW resonator having a second high frequency equivalent resistance value which is higher than the first high frequency equivalent resistance value;
a third SAW resonator coupled to the second node, the third SAW resonator having the first high frequency equivalent resistance value;
a fourth SAW resonator coupled to the first node and a third node, wherein a high frequency equivalent resistance value of the fourth SAW resonator is lower than 15 ohms;
a fifth SAW resonator coupled to the second node and the third node, wherein a high frequency equivalent resistance value of the fifth SAW resonator is lower than 15 ohms; and
an inductor coupled to the third node and a ground node.
2. The transmitting filter according to claim 1 , wherein each of the first through fifth SAW resonators comprise:
a reactance element;
a first capacitance element coupled in series with the reactance element; and
a second capacitance element coupled in parallel with the series connection of the reactance element and the first capacitance element.
3. The transmitting filter according to claim 1 , wherein a distance between electrodes of the first SAW resonator is different than a distance between electrodes of the third SAW resonator.
4. The transmitting filter according to claim 1 , wherein the second high frequency equivalent resistance value of the second SAW resonator is within a range from 1.93 to 3.77 ohm.
5. A transmitting filter comprising:
a first SAW resonator having a first high frequency equivalent resistance value;
a second SAW resonator coupled in series with the first SAW resonator, the second SAW resonator having a second high frequency equivalent resistance value which is higher than the first high frequency equivalent resistance value;
a third SAW resonator coupled in series with the second SAW resonator, the third SAW resonator having the first high frequency equivalent resistance value;
a fourth SAW resonator having a first terminal coupled to a first node between the first and second SAW resonators, and having a second terminal coupled to a second node;
a fifth SAW resonator having a first terminal coupled to a third node between the second and third SAW resonators, and having a second terminal coupled to the second node; and
an inductor having a first terminal coupled to the second node and having a second terminal coupled to a ground node,
wherein respective resistance values of the fourth and fifth SAW resonators are lower than 15 ohms.
6. The transmitting filter according to claim 5 , wherein each of the first through fifth SAW resonators comprise:
a reactance element;
a first capacitance element coupled in series with the reactance element; and
a second capacitance element coupled in parallel with the series connection of the reactance element and the first capacitance element.
7. The transmitting filter according to claim 5 , wherein a distance between electrodes of the first SAW resonator is different than a distance between electrodes of the third SAW resonator.
8. The transmitting filter according to claim 5 , wherein the second high frequency equivalent resistance value of the second SAW resonator is within a range from 1.93 to 3.77 ohm.