JP3259459B2 - Duplexer - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a duplexer in which at least two filters having different pass bands are connected, and in particular, at least a band pass filter having a relatively lower pass band is constituted by a SAW filter. About the duplexer that is being used.

[0002]

2. Description of the Related Art SAW (Surface Acoustic Wave) filters have the advantages of being small in size and having high selectivity, and have been used as filters for duplexers in cordless telephones and mobile communications.

FIG. 1 is a schematic plan view showing an example of a SAW filter used in the above-described duplexer.
The SAW filter 1 is a three-electrode type SAW resonator filter, and has a structure in which three interdigital transducers (hereinafter abbreviated as IDTs) 3 to 5 are arranged on a rectangular piezoelectric substrate 2. Reference numerals 6 and 7 indicate reflectors.

FIG. 2 shows a two-electrode SAW as another example of a SAW filter conventionally used in a duplexer.
3 shows a resonator filter. The SAW filter 8 has a structure in which two IDTs 9 and 10 are arranged on the upper surface of the piezoelectric substrate 2 along the surface wave propagation direction. Reflectors 6 and 7 are arranged on both sides of the IDTs 9 and 10, respectively.

FIG. 3 shows a multi-electrode SA as still another example of a SAW filter conventionally used in a duplexer.
It is a schematic plan view which shows a W filter. Multi-electrode SAW
The filter 11 has a structure in which seven IDTs 12 to 18 are arranged on the upper surface of the rectangular piezoelectric substrate 2 along the surface wave propagation direction. One of the IDTs 12, 14, 16, and 18 is commonly connected and used as an input terminal.
One of the comb electrodes 3, 15, 17 is commonly connected and used as an output terminal. The other comb electrodes of the IDTs 12 to 18 are respectively connected to the ground potential.

The above SAW filters 1, 8, 11
Is suitable as a filter for a duplexer in mobile communication having a narrow reception frequency interval because of a small insertion loss and a steep filter characteristic.

[0007]

However, when a duplexer is constituted by using the SAW filters 1, 8, and 11 as described above, there are the following problems.

That is, the conventional SAW filter 1
When the filters 8 and 11 are used as the filters for the duplexer, the attenuation in the vicinity of the pass band, particularly the attenuation in the higher frequency side than the pass band cannot be increased.
When the filters 1, 8, and 11 are used as filters having a high-frequency side as a stop band, the amount of attenuation may be insufficient.

When a duplexer is formed by combining two bandpass filters for transmission and reception, the impedance outside the pass band of each bandpass filter is as follows:
It must be close to open within the pass band of the bandpass filter of the partner type. However, SA
In the W filters 1, 8, and 11, the impedance is generally capacitive. Therefore, in order to make the impedance close to open, as shown in FIG. 4, the transmission filter 20 and the reception filter 21 are connected to the transmission line 22 and the transmission filter 22, respectively, as external circuits for phase matching. 23 has to be provided externally, and there has been a problem that the external circuit becomes complicated and the size becomes large.

SUMMARY OF THE INVENTION It is an object of the present invention to provide a duplexer comprising a SAW filter for a bandpass filter having a relatively lower pass band, which can simplify an external circuit for impedance matching and provide a SAW filter. It is an object of the present invention to provide a duplexer having a configuration capable of sufficiently increasing the amount of attenuation in a band higher than a pass band of a filter.

[0011]

According to the present invention, at least a first band-pass filter having a relatively low pass-band frequency and a second filter having a relatively high pass-band frequency are connected. A duplexer in which one band-pass filter is constituted by a SAW filter;
The filter includes a two-port SAW resonator filter or a multi-electrode SAW filter, and the SAW filter has an anti-resonance frequency within the pass band of the other filter or between the pass bands of the first and second filters. , Wherein at least one one-port SAW resonator is connected in series.

[0012]

According to the present invention, at least one one-port SAW resonator is connected to a SAW filter. 1
The port-type SAW resonator exhibits a very high impedance near the anti-resonance frequency, and the anti-resonance frequency is located in the pass band of the other filter or between the pass bands of the first and second filters. Selected. Therefore, since the one-port SAW resonator exhibits high impedance near the stop band of the SAW filter, the phase of the reflection coefficient in the stop band can be adjusted to near the open band. Therefore, in the SAW filter, it is not necessary to connect an external circuit for phase matching as in the related art when configuring the duplexer, so that the number of external circuits in the duplexer can be reduced.

Further, when the high band side of the SAW filter is a stop band, the anti-resonance frequency of the one-port SAW resonator is within the pass band of the counterpart filter or the first and second filters as described above. Between the passbands of
The attenuation on the high frequency side can be increased.

[0014]

DESCRIPTION OF THE PREFERRED EMBODIMENTS FIG. 5 is a schematic plan view showing a SAW filter used in a duplexer according to an embodiment of the present invention and a one-port SAW resonator connected to the SAW filter.

In this embodiment, the SAW device 30 is configured using a rectangular piezoelectric substrate 31. On the piezoelectric substrate 31, three IDTs 32 to 34 are arranged in parallel.
Each of the IDTs 32 to 34 is constituted by a pair of comb-shaped electrodes having a plurality of electrode fingers interposed therebetween.

Reflectors 35 and 36 are arranged outside the IDTs 32 to 34 in the surface wave propagation direction. IDT3 above
The SAW filter 37 is configured by 2 to 24 and the reflectors 35 and 36. In the present embodiment, the SAW filter 37 constitutes a first band-pass filter of which the pass band is on the low frequency side among the first and second filters.

One of the IDTs 32 and 34 of the SAW filter 37 is commonly connected to an input terminal 38. The other comb electrodes of the IDTs 32 and 34 are connected to the ground potential. In the center IDT 33, one comb electrode is connected to the ground potential, and the other comb electrode is connected to the one-port SAW resonator 39. The SAW resonator 39 is constituted by a pair of comb electrodes having a plurality of electrode fingers interposed between each other. One of the comb electrodes is electrically connected to the IDT 33, and the other comb electrode. Is the output terminal 40.

In the duplexer of the present embodiment, the SAW filter 37 constituting the first band-pass filter is combined with a second filter (not shown) having a relatively high pass band. Is configured. By the way, the above 1
The anti-resonance frequency of the port-type SAW resonator 39 is selected so as to be positioned higher than the stop band, in particular, the pass band of the SAW filter 37.

FIG. 6 is a schematic circuit diagram for explaining the duplexer according to the present embodiment. The receiving filter 41 is constituted by the SAW device 30 described above, and the transmitting filter as the second filter is used. The trust filter 42 is composed of another filter such as a two-electrode, three-electrode or multi-electrode SAW resonator filter or a ladder filter. Further, the second filter need not be a band-pass filter, but may be a band-rejection type filter.

The reception band-pass filter 41 is based on the S
It is configured using the AW filter device 30, and the SA
In the W filter device 30, since the one-port SAW resonator 39 is connected to the SAW filter 37, no external circuit for impedance matching is required as is clear from FIG. That is, only on the transmission filter 42 side,
An impedance matching circuit 43 composed of a transmission line is connected.

The reason why an external circuit can be omitted by using the SAW filter device 30 as described above will be described. In general, when a duplexer is configured by combining two filters, the impedance of the stop band frequency of each filter, that is, the impedance of the pass band frequency of the other filter, may be infinite (the reflectivity is 1 and the phase Is preferably zero). As the reflection coefficient deviates from the above value, the insertion loss when a duplexer is configured increases.

FIG. 7 shows the result obtained by calculating the relationship between the reflection coefficient and the insertion loss. As is clear from FIG. 7, when the phase angle exceeds 60 °, the insertion loss rapidly deteriorates, so that it is desirable that the phase angle be within ± 60 ° from the open point. However, 2 port SAW
In a resonator filter or a multi-electrode SAW filter, the impedance is capacitive as described above. Therefore, the impedance had to be separated from the open side.

On the other hand, a one-port SAW resonator like the SAW resonator 39 has an equivalent circuit shown in FIG.
Accordingly, the impedance-frequency characteristics are as shown in FIG. That is, the impedance becomes extremely high near the anti-resonance frequency. Therefore, if a one-port SAW resonator having an anti-resonance frequency near the stop band of the first band-pass filter is connected to the first band-pass filter, the phase of the reflection coefficient of the stop band can be adjusted to near the open band. it can.

In this case, the anti-resonance frequency of the connected one-port SAW resonator is located within the stop band of the first band-pass filter or between the stop band and the pass band of the first band-pass filter. Have to do it. If the anti-resonance frequency is higher than this range, the impedance of the stop band decreases, and the phase does not change. Conversely, when the anti-resonance frequency is lower than the above range, the impedance in the pass band of the first bandpass filter increases, and insertion loss increases due to impedance mismatch.

Therefore, in this embodiment, the SA
The anti-resonance frequency of the W resonator 39 is located within the stop band of the SAW filter 37 as the first band pass filter, that is, within the pass band of the second filter or between the pass bands of the first and second filters. ing. Therefore, as is clear from the above description, in the SAW filter device 30,
Since the impedance in the stop band of the SAW filter 37 can be made close to open without connecting an external circuit, the number of external circuits for impedance matching required when configuring a duplexer can be reduced. That is, as shown in FIG. 6, a transmission line for impedance matching may not be connected to the reception filter 41 side.

As described with reference to FIGS. 10 and 11, in this embodiment, the SAW filter 37 is
Since the port-type SAW resonator 39 is connected, the attenuation in the stop band can be greatly increased.

FIG. 10 shows the attenuation-frequency characteristics of the conventional three-electrode SAW filter, and FIG. 11 shows the attenuation-frequency characteristics of the SAW device 30 shown in FIG. As is apparent from FIGS. 10 and 11, the attenuation in the stop band higher than the pass band is improved by about 20 dB according to the present embodiment as compared with the conventional example.

Therefore, in this embodiment, since the first bandpass filter is constituted by the SAW device 30,
Not only can the number of impedance matching circuits be reduced, but also the amount of attenuation on the higher band side than the pass band of the first band-pass filter can be made sufficiently large.

In the above embodiment, the SAW filter is
Although the one-port SAW resonator 39 is connected using the three-electrode SAW filter 37, the present invention is not limited to the one using the SAW filter and the SAW resonator having such a configuration. FIG. 12 and FIG.
It is each schematic plan view which shows the SAW device used for the Example of FIG.

The SAW device 50 shown in FIG. 12 has a structure in which a two-electrode type SAW resonator filter 52 and a one-port type SAW resonator 53 are formed on a rectangular piezoelectric substrate 51. The SAW filter 52 has an IDT 54,
55, and reflectors 56, 5 on the outside in the surface wave propagation direction.
7 is arranged. One comb electrode of the IDT 54 constitutes the input terminal 58, and one comb electrode of the IDT 55 is connected to the one-port SAW resonator 53. The other comb electrodes of the IDTs 54 and 55 are connected to the ground potential.

The one-port SAW resonator 53 has a pair of IDTs 53A and 53B, each of which has a plurality of electrode fingers interposed therebetween, as shown in FIG. The bus bars 53C are configured back to back so as to be common. And
The other comb electrode of the IDT 53B is an output terminal 59.

In the SAW filter device 60 shown in FIG. 13, a multi-electrode SAW filter 62 is formed at the center on a piezoelectric substrate 61. The SAW filter 62 has many IDTs 63 to 69, similarly to the conventional multi-electrode SAW filter shown in FIG. And IDT63,
One of the comb electrodes 65, 67, 69 is commonly connected, and the other of the IDTs 63, 65, 67, 69 is connected to the ground potential. Similarly, IDTs 64 and 6
One of the comb electrodes 6 and 68 is commonly connected, and the other of the comb electrodes is connected to the ground potential.

The commonly connected comb electrodes of the IDTs 63, 65, 67, and 69 are electrically connected to a one-port SAW resonator 70 as shown. The other comb electrode of the SAW resonator 70 is used as an input terminal 71.

The IDTs 64, 66, and 68 commonly connected to each other have a comb-shaped electrode which is a one-port SAW resonator 72.
The SAW resonator 72 has an output terminal 73 on the other comb electrode side.

Also in the embodiments shown in FIGS. 12 and 13, the one-port SAW resonators 53, 70, 72 are provided within the pass band of the second band-pass filter combined with the SAW filter devices 50, 60, respectively. Alternatively, it is selected such that the anti-resonance frequency is located between the pass bands of the first and second band pass filters. Therefore, as in the case of the first embodiment, the number of external circuits on the side of the first bandpass filter can be reduced, and the passband of the first bandpass filter, that is, the pass band of the SAW filters 52 and 62 can be reduced. In this case, the attenuation on the high frequency side can be sufficiently increased.

[0036]

According to the present invention, the first component constituting the duplexer is provided.
Out of the band pass filter and the second filter, the first band pass filter having a relatively low pass band frequency is a two-port SAW resonator filter or a multi-electrode S filter.
At least one one-port SAW resonator, which is composed of an AW filter and whose antiresonance frequency is within the pass band of the second filter or between the pass bands of the first and second filters, is connected. Therefore, in the first band-pass filter, the impedance is close to open in the stop band. Therefore, it is not necessary to connect an external circuit for impedance matching to the first band-pass filter when configuring the duplexer. Absent. Therefore, the circuit configuration of the duplexer can be simplified.

Further, since the one-port SAW resonator is connected, it is possible to sufficiently increase the stop band of the first band-pass filter, that is, the amount of attenuation in the higher band than the pass band. Therefore, in the duplexer using the SAW filter, it is possible to reduce the size of the duplexer and realize sufficient attenuation characteristics.

[Brief description of the drawings]

FIG. 1 is a schematic plan view showing an example of a conventional SAW filter.

FIG. 2 is a schematic plan view showing another example of a conventional SAW filter.

FIG. 3 is a schematic plan view showing still another example of the conventional SAW filter.

FIG. 4 is a schematic circuit diagram for explaining a conventional duplexer.

FIG. 5 is a schematic plan view showing a SAW filter device used as a first bandpass filter in one embodiment of the present invention.

FIG. 6 is a schematic configuration diagram for explaining a duplexer according to an embodiment of the present invention.

FIG. 7 is a diagram illustrating a relationship between a reflection coefficient of a SAW filter in a duplexer, a phase angle, and an insertion loss.

FIG. 8 is a diagram showing an equivalent circuit of a one-port SAW resonator.

FIG. 9 is a diagram showing impedance-frequency characteristics of a one-port SAW resonator.

FIG. 10 shows a SAW port type S used in a conventional duplexer.
The figure which shows the attenuation-frequency characteristic of an AW resonator filter.

FIG. 11 is an SA used in a duplexer according to one embodiment of the present invention.
The figure which shows the attenuation-frequency characteristic in W apparatus.

FIG. 12 is a schematic plan view showing a SAW device used in the second embodiment.

FIG. 13 is a schematic plan view showing a SAW device used in a third embodiment.

[Explanation of symbols]

Reference Signs List 30 SAW device 37 SAW filter as first band-pass filter 39 One-port SAW resonator 41 Receive filter as first band-pass filter 42 Transmission filter as second filter 50 SAW filter device 52 SAW filter 53 1-port SAW resonator 62 multi-electrode SAW filter 70, 72 1-port SAW resonator

Continuation of the front page (56) References JP-A-6-177697 (JP, A) JP-A-4-94208 (JP, A) JP-A-59-97216 (JP, A) US Patent 4,0062,290 (US, A) Yahei Koyamada et al., Analysis and application of surface acoustic wave resonators using many-pair IDT, Transactions of the Institute of Electronics and Communication Engineers, Japan, Society of Electronics and Communication Engineers, Vol. J60-A No. 9,805-812 (58) Field surveyed (Int. Cl. ⁷ , DB name) H03H 9/72 H03H 9/64

Claims (1)

(57) [Claims] 1. A first bandpass filter having a relatively low passband frequency and a second filter having a relatively high passband frequency are connected, and at least the first bandpass filter is a SAW filter. The SAW filter comprises a two-port SAW resonator filter or a multi-electrode SAW filter, and the SAW
The filter is characterized in that at least one one-port SAW resonator having an anti-resonance frequency within the pass band of the other filter or between the pass bands of the first and second filters is connected in series. Splitter.