JP2004140508A - Demultiplexer - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To improve attenuating characteristics between a passing band and a blocking band of a surface acoustic wave filter in a demultiplexer. <P>SOLUTION: This demultiplexer includes a common signal terminal C, and a surface acoustic wave resonator. The demultiplexer further includes a transmitting side filter BF1 connected at one side terminal to the terminal C via a branching point J and at the other side terminal to a first signal terminal 1; a receiving side filter BF2 having the surface acoustic wave resonator and connected at one side terminal to the terminal C via a branching point and at the other side terminal to a second signal terminal 2 and having a passing band higher than that of the filter BF1; and a terminal pair type surface acoustic wave resonator 4 having an antiresonance frequency fa between a lower end frequency fu of the passing band and an upper end frequency fl of the blocking band of the filter BF2, and connected at one side terminal to the filter BF2 side and at the other side terminal to the branching point J side, on a piezoelectric substrate. <P>COPYRIGHT: (C)2004,JPO

Description

[0001]
TECHNICAL FIELD OF THE INVENTION
The present invention relates to a duplexer using a surface acoustic wave filter.
[0002]
[Prior art]
2. Description of the Related Art In recent years, mobile communication terminals such as mobile phones are rapidly developing. It is desired that the terminal be particularly small and lightweight because of its easy portability. In order to reduce the size and weight of the terminal, it is essential that the electronic components used in the terminal are also small and lightweight. A surface acoustic wave device, that is, a surface acoustic wave filter is frequently used.
[0003]
Important characteristics required for a surface acoustic wave filter include insertion loss and out-of-passband attenuation. The insertion loss affects the power consumption of the device, and the lower the loss, the longer the life of the battery. Therefore, the capacity of the battery can be reduced, which contributes to reduction in size and weight. In addition, if high attenuation outside the band can be obtained with a single surface acoustic wave filter, this contributes to a reduction in the size and weight of the device.
[0004]
Hereinafter, a conventional surface acoustic wave filter will be described.
[0005]
As a filter that satisfies the low-loss and high-attenuation characteristics, for example, there is a ladder-type filter described in JP-A-5-183380.
[0006]
The ladder type filter has a multi-stage surface acoustic wave resonator connected in series to input / output electrodes and a surface acoustic wave resonator connected in parallel. This ladder-type filter is frequently used for a high-frequency filter of a mobile phone because of its low loss and excellent attenuation characteristics near a pass band.
[0007]
FIG. 5 shows a configuration of a conventional duplexer using such a ladder-type filter.
[0008]
The illustrated duplexer has a common signal terminal C, and two surface acoustic wave filters BF1 and BF2 having different pass bands from each other are connected in parallel via a branch point J. Each of the surface acoustic wave filters BF1 and BF2 has signal terminals 1 and 2, respectively.
[0009]
The surface acoustic wave filters BF1 and BF2 have different pass bands from each other, and the signal input from the common signal terminal C passes only through the surface acoustic wave filter BF2 which is the receiving filter and is output from the signal terminal 2. The signal input from the signal terminal 1 passes through a surface acoustic wave filter BF1, which is a transmission-side filter, and is output to a common signal terminal C.
[0010]
Here, signals passing through the surface acoustic wave filters BF1 and BF2 need to pass through without affecting each other. Therefore, in the pass band of one surface acoustic wave filter, it matches the characteristic impedance Z ₀ of the circuit, and in the stop band corresponding to the other pass band, the branch point J becomes much larger than the characteristic impedance Z _{0 of the} circuit. An impedance matching circuit 3 is inserted between the filter and the surface acoustic wave filter BF2.
[0011]
A duplexer using such a ladder-type filter is disclosed in Japanese Patent Application Laid-Open No. 2000-315936.
[0012]
[Problems to be solved by the invention]
However, in the above-described conventional duplexer, when the pass band and the stop band of the surface acoustic wave filter are largely separated from each other, there is a problem that the attenuation characteristic is deteriorated at the frequency between the pass band and the stop band. there were.
[0013]
That is, as shown in FIG. 6, in such a case, the conventional duplexer causes deterioration of the attenuation characteristic at a frequency between the pass band and the stop band of the surface acoustic wave filter BF2 having a higher pass band. Ripple (symbol E2) occurs.
[0014]
Accordingly, an object of the present invention is to provide a duplexer having excellent attenuation characteristics between a pass band and a stop band of a surface acoustic wave filter.
[0015]
[Means for Solving the Problems]
In order to solve the above problems, a duplexer according to the present invention includes a common signal terminal and a surface acoustic wave resonator, and an input terminal is connected to a common signal terminal via a branch point and an output terminal is connected. A terminal is connected to the first signal terminal, the terminal includes a first surface acoustic wave filter having a predetermined pass band, and a surface acoustic wave resonator, and an input terminal is connected to a common signal terminal via a branch point. A second surface acoustic wave filter having a passband higher than the passband of the first surface acoustic wave filter, the output side terminal being connected to the second signal terminal; It has an anti-resonance frequency between the lower end frequency of the pass band and the upper end frequency of the stop band, one terminal is connected to the second surface acoustic wave filter side, and the other terminal is connected to the branch point side. One port pair type surface acoustic wave resonator Characterized by comprising a on a piezoelectric substrate.
[0016]
According to this invention, the one-port pair surface acoustic wave resonator having an anti-resonance frequency between the lower end frequency of the pass band of the second surface acoustic wave filter and the upper end frequency of the stop band is provided by the second surface acoustic wave filter. Is connected in series to the surface acoustic wave filter, so that the attenuation characteristics between the pass band and the stop band of the surface acoustic wave filter can be improved.
[0017]
BEST MODE FOR CARRYING OUT THE INVENTION
Hereinafter, embodiments of the present invention will be described more specifically with reference to the drawings. Here, in the attached drawings, the same members are denoted by the same reference numerals, and duplicate description is omitted. The embodiment of the present invention is a particularly useful embodiment in which the present invention is implemented, and the present invention is not limited to the embodiment.
[0018]
FIG. 1 is a block diagram showing a duplexer according to an embodiment of the present invention. FIG. 2 is a diagram conceptually showing a configuration of a one-port pair type surface acoustic wave resonator provided in the duplexer shown in FIG. FIG. 3 and FIG. 3 are graphs showing frequency characteristics at different capacitances in the one-port pair surface acoustic wave resonator of FIG. 2, and FIG. 4 is a graph showing frequency characteristics of a duplexer according to an embodiment of the present invention. It is.
[0019]
The duplexer according to the present embodiment has a structure in which predetermined elements are formed on a piezoelectric substrate made of, for example, a 39 ° rotation Y-cut X-propagation LiTaO ₃ . That is, in FIG. 1, a common signal terminal C for inputting / outputting a signal and a terminal on one side are connected to the common signal terminal C via a branch point J and a terminal on the other side is a first signal terminal 1 And a terminal on one side is connected to a common signal terminal C via a branch point J, and a terminal on the other side is connected to a second signal terminal 2. And a receiving-side filter (a second surface acoustic wave filter) BF2 connected to the receiving side.
[0020]
Each of the transmission-side filter BF1 and the reception-side filter BF2 is configured by a surface acoustic wave resonator that is a resonator having interdigital electrodes and a reflector. The transmitting filter BF1 and the receiving filter BF2 have frequency characteristics in which one pass band becomes the other stop band, and in the case of the present embodiment, the receiving filter BF2 is the transmitting filter. The pass band is higher than BF1 (see FIG. 4).
[0021]
In the present embodiment, the transmission-side filter BF1 and the reception-side filter BF2 are formed by connecting a surface acoustic wave resonator connected in series to an input / output electrode and a surface acoustic wave resonator connected in parallel in multiple stages. This is a so-called ladder filter.
[0022]
In this embodiment, the receiving filter BF2 has a higher passband than the transmitting filter BF1 as described above, but may be reversed.
[0023]
Also, the logarithm of the interdigital electrodes and the arrangement pattern of the surface acoustic wave resonators in the transmission-side filter BF1 and the reception-side filter BF2 employ various forms as appropriate as long as they have predetermined passbands and stopbands. It is possible.
[0024]
Further, the piezoelectric substrate is not limited to the one described above, and various other piezoelectric substrates such as a piezoelectric substrate made of 41 ° rotation Y-cut X-propagation LiNbO ₃ can be used.
[0025]
In the duplexer of the present embodiment, a one-port pair type surface acoustic wave resonator 4 having one terminal connected to the receiving filter BF2 and the other terminal connected to the branch point J is arranged. ing.
[0026]
As shown in FIG. 2, the one-terminal pair type surface acoustic wave resonator 4 includes a pair of finger-like electrodes 4a and 4b, which are terminals crossing each other, that is, a cross-finger electrode. In the present embodiment, the electrode period λ of the finger electrodes 4a and 4b is 1.96 μm, the logarithm N is 260 pairs, and the aperture length Ap is 30λ. However, the present invention is not limited to these numerical values.
[0027]
When the lower end frequency of the pass band of the receiving side wave filter BF2 is fu and the upper end frequency of the stop band is fl, the anti-resonance frequency fa of the one-port surface acoustic wave resonator 4 is fl <fa <fu. (See FIG. 4).
[0028]
Furthermore, in the one-terminal pair type surface acoustic wave resonator 4 of the present embodiment, the capacitance determined by the logarithm N of the interdigital electrodes and the intersection width Ap (λ) is 2 pF or more. However, it may be 2 pF or less. FIG. 3 shows frequency characteristics (relationship between capacitance and anti-resonance frequency) at different capacitances in the one-port pair type surface acoustic wave resonator 4.
[0029]
Between the branch point J and the one-port pair type surface acoustic wave resonator 4, the impedance of the transmission-side filter BF1 and the reception-side BF2 changes according to the frequency. In the pass band of the filter (for example, the transmitting filter BF1), the impedance is matched to a value close to the impedance of the entire circuit, and in the stop band that is the pass band of the other filter (for example, the receiving filter BF2), the entire circuit is The impedance matching circuit 3 is arranged so that the impedance Z1 becomes a value much larger than the impedance Z1 and is mismatched. In this embodiment, the impedance matching circuit 3 is constituted by a strip line provided inside a package.
[0030]
Next, the frequency characteristics of the duplexer configured as described above will be described with reference to FIG.
[0031]
As described with reference to FIG. 6, in the configuration of the conventional duplexer, when the pass band and the stop band of the reception-side filter BF2 are largely separated, the attenuation characteristics in the frequency characteristics between the pass band and the stop band deteriorate. .
[0032]
On the other hand, in the duplexer of the present application, the one-port pair surface acoustic wave resonance having the anti-resonance frequency fa between the frequency fu at the lower end of the pass band of the receiving filter BF2 and the frequency fl at the upper end of the stop band is used. Since the device 4 is connected in series to the receiving filter BF2, as shown by the symbol D2 in FIG. 4, the frequency is near the attenuation pole of the one-port surface acoustic wave resonator 4, that is, the reception The attenuation characteristic is improved between the pass band and the stop band of the side filter BF2 (see E6 in FIG. 6). At this time, the insertion loss of the pass band of the receiving filter BF2 is hardly degraded, and the attenuation characteristic between the pass band and the stop band of the transmitting filter BF1 as shown by reference numeral D1 in FIG. Has also been improved (see FIG. 6, symbol E1).
[0033]
If the capacitance of the one-port pair surface acoustic wave resonator 4 is set to a very large value of 2 pF or more, the one-port pair surface acoustic wave resonator 4 can be inserted near the center frequency of the pass band of the receiving filter BF2. The effect on the loss is not significant, and the effect can be reduced by changing the design of the receiving filter BF2. However, considering that the larger the capacitance, the larger the size of the one-port pair type surface acoustic wave resonator 4, the space efficiency is deteriorated and it is difficult to manufacture the one-port pair type surface acoustic wave resonator. It is desirable that the capacitance of the surface acoustic wave resonator 4 be about 2 to 6 pF (see FIG. 3).
[0034]
Note that Japanese Patent No. 3175581 discloses a technique in which a surface acoustic wave resonator is connected in series to a filter having a higher pass band to obtain attenuation, but the resonance frequency of the connected surface acoustic wave resonator is disclosed. Is set higher than the center frequency of the filter. As described above, the present invention provides a one-port pair elastic surface having an anti-resonance frequency fa between a frequency fu at the lower end of the pass band and a frequency fl at the upper end of the stop band of the receiving filter BF2 having a higher pass band. The wave resonator 4 is connected in series to the receiving filter BF2, which is different from the technique described in the publication.
[0035]
【The invention's effect】
As is clear from the above description, the present invention has the following effects.
[0036]
In other words, a one-port pair type surface acoustic wave resonator having an anti-resonance frequency between the lower end frequency of the pass band and the upper end frequency of the stop band of the second surface acoustic wave filter is used as the second surface acoustic wave filter. Since they are connected in series, it is possible to improve the attenuation characteristics between the pass band and the stop band of the surface acoustic wave filter.
[Brief description of the drawings]
FIG. 1 is a block diagram showing a duplexer according to an embodiment of the present invention.
FIG. 2 is an explanatory diagram conceptually showing a configuration of a one-port pair type surface acoustic wave resonator provided in the duplexer of FIG.
FIG. 3 is a graph showing frequency characteristics at different capacitances in the one-port SAW resonator of FIG. 2;
FIG. 4 is a graph showing frequency characteristics of the duplexer according to one embodiment of the present invention.
FIG. 5 is a block diagram showing a conventional duplexer.
FIG. 6 is a graph showing frequency characteristics of a conventional duplexer.
[Explanation of symbols]
REFERENCE SIGNS LIST 1 first signal terminal 2 second signal terminal 3 impedance matching circuit 4 one-port pair surface acoustic wave resonator BF1 transmission-side filter (first surface acoustic wave filter)
BF2 Receiver filter (second surface acoustic wave filter)
C Common signal terminal J Branch point

Claims (4)

A common signal terminal,
A first surface terminal comprising a surface acoustic wave resonator, one terminal connected to the common signal terminal via a branch point, and the other terminal connected to the first signal terminal, having a predetermined pass band; A surface acoustic wave filter;
A first surface acoustic wave filter comprising a surface acoustic wave resonator, one terminal connected to the common signal terminal via the branch point, and the other terminal connected to a second signal terminal; A second surface acoustic wave filter having a passband higher than the passband of
The second surface acoustic wave filter has an anti-resonance frequency between a lower end frequency of a pass band and an upper end frequency of a stop band, and one terminal is connected to the second surface acoustic wave filter side and the other terminal is connected to the second surface acoustic wave filter side. And a one-terminal pair type surface acoustic wave resonator having the above-mentioned terminal connected to the branch point side on a piezoelectric substrate. Between the branch point and the one-port surface acoustic wave resonator, the impedance matches the circuit impedance in the pass band of one surface acoustic wave filter, and corresponds to the pass band of the other surface acoustic wave filter. 2. The duplexer according to claim 1, wherein an impedance matching circuit that is mismatched with the impedance of the circuit in the stop band is arranged. 3. The duplexer according to claim 1, wherein the capacitance of the one-port surface acoustic wave resonator is 2 pF or more. 4. It said piezoelectric substrate, the duplexer according to any one of claims 1 to 3, characterized in that it is constituted by a 39 ° rotated Y-cut X-propagation LiTaO _3.

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