1. Technical Field
The present invention relates to a Surface Acoustic Wave (hereinafter called SAW) filter used in a mobile phone or the like, an antenna duplexer using the same, and a mobile communication terminal using the same.
2. Prior Art
Generally, the SAW filter used in the radio frequency spectrum such as used by a mobile phone typically includes a ladder-type circuit to realize a band-pass filter of small size and high selectivity. Hitherto, these types of ladder-type SAW filter was constituted as shown in FIG. 17 as disclosed in Japanese Laid-open Patent No. 9-102728.
That is, serial-branch SAW resonators 12-1 to 12-4 include reflectors 15 sandwiching an interdigital transducer 14 are arranged at the left side (the side of L0) in the drawing on a piezoelectric substrate 13, and are connected by serial-branch wiring patterns 16-1 to 16-3. On the other hand, parallel-branch SAW resonators 12-5 to 12-7 including reflectors 15 sandwiching an interdigital transducer 14 are arranged at the right side (the side of L1) and are connected to the serial-branch wiring patterns 16-1 to 16-3 by parallel-branch wiring patterns 16-4 to 16-6.
In this constitution, a seven-element ladder-type SAW filter 11 is formed. This SAW filter 11 has a circuit configuration in which the input electrode 16-8 side starts from the serial-branch SAW resonator 12-1. Such circuit configuration is called the serial-branch input type. By contrast, the circuit configuration in which the input side starts from the parallel-branch SAW resonator is called the parallel-branch input type.
The conventional ladder-type SAW filter 11 tends to be less in increase of reflection coefficient at the input electrode 16-8 even outside of the pass band. FIG. 18 is a polar coordinate chart of reflection coefficient as seen from the input electrode 16-8 of a receiving filter e.g. a receiving filter for Advance Mobile Phone Service (hereinafter called AMPS) which is a mobile phone system in the United States. The receiving band, that is, the pass-band is 869 MHz to 894 MHz (marker M1 to M2), and the transmitting band to be rejected at the receiving input terminal is 824 MHz to 849 MHz (marker M3 to M4).
As known from FIG. 18, at the upper end M4 of the transmitting band, the reflection coefficient of only about 0.8 is obtained. The cause is the inductive components of the parallel-branch wiring patterns 16-4 to 16-6 as explained in detail later in the embodiment. In the case of serial-branch input type, in particular, since the SAW resonators 12-1 to 12-7 are arranged as shown in FIG. 17 for the convenience of constitution, the parallel-branch wiring pattern is extended in length, and this tendency appears obviously.
The use of this ladder-type SAW filter 11 in an antenna duplexer is explained below. The antenna duplexer is a branching filter comprising a transmitting terminal, an antenna terminal, and a receiving terminal, and a transmitting filter is placed between the transmitting terminal and the antenna terminal, and a receiving filter is placed between the antenna terminal and the receiving terminal.
Moreover, phase shifters are provided between the antenna terminal and transmitting filter, and between the antenna terminal and receiving filter, in order to heighten the impedance as seen from the antenna terminal in the rejection band, and thereby suppressing leak of a receiving signal toward the transmitting terminal and a transmitting signal toward the receiving terminal, respectively (that is, isolation is assured).
In this constitution, the transmitting signal entering from the transmitting terminal is output to the antenna terminal without leaking to the receiving terminal. Likewise, the receiving signal entering from the antenna terminal is input into the receiving terminal without leaking to the transmitting terminal.
Generally, as the transmitting filter, a band-rejection filter is used because it is enough to reject only the receiving band, while a band-pass filter is used for the receiving filter because it is required to reject not only the transmitting frequency, but also the neighborhood band of local oscillator frequency and image frequency of the receiving signal.
Therefore, the ladder-type SAW filter of small size and high selectivity has optimum features as the receiving filter. However, when the SAW filter is used as the receiving filter, an electric power of about one watt is applied to the transmitting band which is the rejection band allocated in the lower outside of receiving band, and therefore a corresponding withstanding power is needed. As mentioned in detail below in the embodiment, when the circuit is composed in the serial-branch input type, the withstanding power to the transmitting power is enhanced, and problems are practically solved.
However, when the conventional SAW filter 11 is used as the receiving filter for the antenna duplexer, since the reflection coefficient is small at the transmitting frequency to be rejected, the impedance when seeing the receiving filter from the antenna terminal through the phase shifter is not sufficiently high, and the isolation between transmitting terminal and receiving terminal may not be assured.
The invention is intended to solve the above problems, and it is hence an object thereof to realize not only a SAW filter capable of obtaining a high reflection coefficient in rejection band even in the case of the serial-branch input type with high withstanding power, but also a SAW filter capable of obtaining a high withstanding power characteristic against the transmitting power and assuring a sufficient isolation between the transmitting terminal and receiving terminal when used as the receiving filter for an antenna duplexer, and the object also includes to realize an antenna duplexer using the SAW filter and a mobile communication terminal using the same.
To solve the problems, the invention presents a ladder-type SAW filter composed by connecting alternately a serial branch having at least one SAW resonator connected in series to a signal path, and a parallel branch having at least one SAW resonator connected between the signal path and the ground, in which the sizes of wiring patterns of the parallel branch are smaller than wiring patterns of the serial branch.
According to this constitution, even in the case of serial-branch input type, the inductive components of the parallel-branch wiring patterns are small, and hence a large reflection coefficient is obtained in the rejection band, thereby realizing a SAW filter with a high withstanding power characteristics and assuring a sufficient isolation between the transmitting terminal and receiving terminal, and also an antenna duplexer using the same and a mobile communication terminal using the same.