JP2006279602A - Branching filter - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To increase the steepness of a low-frequency side shoulder of a pass band of a high-frequency side filter constituting a branching filter while satisfying an attenuation quantity of an attenuation band without making design complicated. <P>SOLUTION: The branching filter includes two SAW filters which are connected to a common terminal and has different center frequencies, and the high-frequency side filter at least partially includes a ladder type circuit part wherein a plurality of SAW resonators are connected in a ladder type; and at least one parallel arm resonator included in the ladder type circuit part has both higher resonance and antiresonance frequencies than other parallel arm resonators included in the high-frequency side filter and the resonance frequency is in the vicinity of a frequency position where the large/small relation between the impedance of a series arm resonator included in the high-frequency side filter and impedance of other parallel arm resonators is inverted. At least the one parallel arm resonator has resonance resistance 3 to 24 times as large as the resonance resistance of the other parallel arm resonators and is arranged preferably at a final-stage position far away from the common terminal. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention relates to a duplexer, and more particularly to a duplexer using a surface acoustic wave filter as a transmission / reception filter.

  In mobile communication devices such as cellular phones, a duplexer using a surface acoustic wave (hereinafter referred to as SAW) filter has been widely used in recent years because it is small and suitable for high functionality. As a configuration, for example, a multiple coupling circuit, a ladder circuit, or a circuit in which these are mixed is used in order to obtain desired characteristics.

  On the other hand, with the progress of high functionality and high performance of mobile phones in recent years, depending on the communication system, for example, the insertion loss amount and attenuation amount (required value / In addition to the transition band between the pass band and the attenuation band (particularly between the pass band of the high-pass filter and the low-pass attenuation band of the high-pass filter) in addition to the reference numerals 110 and 112 described below. In some cases, a predetermined attenuation amount (see reference numeral 111 in the figure) may be required, and in order to satisfy such an insertion loss amount / attenuation amount (required value), the low band shoulder of the pass band is required for the high band filter. It is necessary to improve the characteristics by increasing the steepness of the part.

  Further, there are the following Patent Documents 1 to 3 that disclose such a SAW filter. In the inventions described in these documents, a notch filter is provided in parallel or in series with a multi-coupled circuit, so that the vicinity of the low band side of the passband is provided. Improves damping characteristics.

JP-A-8-65095 JP-A-8-65096 JP-A-8-65097

  By the way, in the method described in the above document, although the attenuation of the transition band can be obtained by adding the notch filter, the insertion loss in the pass band is maintained at the current state, and cannot be improved.

  Moreover, depending on the communication system (frequency band used), there are cases where the transition band for transmission and reception is wide but the specific bandwidth is narrow, and the standard for the pass band and the standard for the transition band are very close. When the method described in the above-mentioned document providing a filter is applied, there is a trade-off relationship between the attenuation of the transition band and the insertion loss of the pass band, and thus there arises a problem that the insertion loss of the pass band increases and the filter characteristics deteriorate.

  On the other hand, if a substrate having a small coupling coefficient is used, the steepness of the edge portion of the passband can be improved to some extent. However, according to such a method, although a relatively large attenuation is obtained in the vicinity of the pass band, a desired attenuation cannot be obtained as the distance from the pass band increases. In particular, when the transition band is wide, the attenuation amount of the attenuation band is small. It may not be possible to satisfy, and the filter design becomes very difficult.

  Further, the problem may be solved by improving the Q value of the resonator itself (quality factor Q = fr / Δf fr: resonance frequency, Δf: half-value width of resonance characteristics). In order to obtain the Q value, for example, various improvements such as improvement of physical properties of the piezoelectric body itself, change of IDT (Interdigital Transducer / interdigital finger electrode) structure, improvement of physical properties of IDT, and the like are required, which is not easy.

  Accordingly, an object of the present invention is to provide a steepness for a low-frequency side shoulder of a pass band with respect to a high-frequency side filter constituting a duplexer without complicating the filter design and satisfying the attenuation amount of the attenuation band. Is to improve the characteristics.

  In order to solve the above problems and achieve the object, a duplexer according to the present invention is a duplexer including two surface acoustic wave filters having different band center frequencies connected in parallel to a common terminal, Of the two surface acoustic wave filters, the high-pass filter includes at least part of a ladder type circuit unit in which a plurality of surface acoustic wave resonators are connected in a ladder type, and at least included in the ladder type circuit unit. One parallel arm resonator has both a resonance frequency and an antiresonance frequency higher than those of the other parallel arm resonators included in the high-pass filter, and the resonance frequency of the series-arm resonator included in the high-pass filter. The impedance and the impedance of the other parallel arm resonator are in the vicinity of the frequency position where the impedance is reversed.

  In the duplexer of the present invention, a SAW (surface acoustic wave) filter is provided as a transmission filter and a reception filter. However, a high frequency filter (a high frequency filter with a high center frequency is a transmission filter and a center frequency is a high frequency filter). Even if the low-pass filter is a reception-side filter, the high-pass filter having a high center frequency may be a reception-side filter and the low-pass filter having a low center frequency may be a transmission-side filter. A ladder type circuit unit in which a plurality of SAW resonators are connected in a ladder type is included at least in part. And about one or more parallel arm resonators which comprise this ladder type circuit part, both the resonance frequency and the anti-resonance frequency are higher than the other parallel arm resonators included in the high band side filter, and the high band side Setting is made so that the resonance frequency is positioned in the vicinity of the frequency position where the magnitude of the impedance of the series arm resonator included in the filter and the impedance of the other parallel arm resonator are reversed.

  If such a setting is performed for one or more parallel arm resonators constituting the ladder-type circuit unit, the passband is obtained without substantially degrading the attenuation characteristics in the attenuation band for the high-pass filter constituting the duplexer. It becomes possible to increase the steepness of the low-frequency side shoulder of the. This point will be described in detail in the following description of the embodiment with reference to the drawings. The high-pass filter may be a ladder type filter in which all of the plurality of surface acoustic wave resonators are connected in a ladder type, or a part of them is connected in a ladder type (ladder type circuit unit). May be included).

  As a specific method for increasing the resonance frequency and antiresonance frequency of the parallel arm resonator of the high-pass filter, for example, the pitch of the IDT crossing fingers constituting the resonator may be narrowed.

  Further, in order to obtain the effect of improving the characteristics more favorably, the resonance resistance of the at least one parallel arm resonator constituting the ladder circuit unit is made larger than the resonance resistance of the other parallel arm resonator. Preferably, the resonance resistance of the at least one parallel arm resonator is preferably 3 to 24 times the resonance resistance of the other parallel arm resonator. If the resonance resistance of the at least one parallel arm resonator is less than three times the resonance resistance of the other parallel arm resonator, the steepness of the low-side shoulder of the passband of the high-pass filter is degraded. This is because if the ratio is larger than 24 times, the attenuation characteristic of the high-pass filter deteriorates. Therefore, it is desirable to set the resonance resistance within such a range of 3 to 24 times.

  As a specific method for increasing the resonance resistance, for example, the crossing length of IDTs constituting the resonator may be reduced, or the number of crossing fingers may be reduced.

  Furthermore, it is preferable that the at least one parallel arm resonator constituting the ladder-type circuit unit is a parallel arm resonator connected to a final stage position separated from the common terminal (for example, farthest).

  Since the at least one parallel arm resonator has a higher impedance than other resonators, if it is provided on the common terminal side, the input impedance on the common terminal side becomes high, which affects the characteristics of the low-pass filter. At the same time, the design of the entire duplexer becomes complicated.

  According to the duplexer structure of the present invention, the low-frequency side shoulder of the passband is used for the high-frequency filter constituting the duplexer without complicating the filter design and satisfying the attenuation amount of the attenuation band. The characteristics can be improved by increasing the steepness.

  Other objects, features and advantages of the present invention will become apparent from the following description of embodiments of the present invention.

  FIG. 1 shows a duplexer according to a first embodiment of the present invention. As shown in the figure, this duplexer has a SAW filter having a center frequency f1 as the transmission side filter 11 and a center frequency f2 (f2> f1) larger than the center frequency f1 of the transmission side filter as the reception side filter 12. In this embodiment, the transmission-side filter 11 includes series arm resonators 41 to 43 and parallel arm resonators 51 and 52 connected in four stages in a ladder form in this embodiment. In this embodiment, the filter 12 includes series arm resonators 21 to 23 and parallel arm resonators 31 to 34 connected in a ladder shape over six stages.

  The transmission side filter 11 is connected between a common terminal C connected to the antenna and a transmission terminal T to which a transmission signal is input, and the reception side filter 12 outputs the reception signal with the common terminal C. It is connected between the receiving terminal R. Further, the phase line 1 is inserted between the common terminal C and the receiving filter 12 as an impedance matching circuit.

  In addition, with respect to the parallel arm resonator 34 in the final stage (sixth stage in this example) when viewed from the common terminal C, other parallel arm resonators that constitute the reception-side filter 12 with both the resonance frequency and the antiresonance frequency. The resonance frequency is higher than 31 to 33 and near the frequency at which the impedance of the other parallel arm resonators 31 to 33 and the impedance of the series arm resonators 21 to 23 included in the reception filter 12 are reversed. Set to position.

  That is, FIG. 2 is a diagram showing the frequency characteristics of a resonator when a conventional duplexer is configured by a ladder type filter as in the present embodiment, in which the symbol Ls is a series arm resonator and Lp1 is the first. Frequency characteristics of the parallel arm resonators from the first stage to the fifth stage and Lp2 of the parallel arm resonator of the final stage (sixth stage) are shown. On the other hand, FIG. 3 shows the frequency characteristics of each resonator constituting the receiving side filter in the present embodiment. In FIG. 3, the symbol Ls denotes a series arm resonator as in FIG. 2, and Lp1 denotes the first to second stages. Lp2 indicates the frequency characteristics of the parallel arm resonator of the last stage, and Lp2 indicates the frequency characteristics of the parallel arm resonator of the last stage.

  As shown in these figures, in the conventional duplexer, the resonance and antiresonance frequencies of the final-stage parallel arm resonator and the other parallel arm resonators are substantially equal. The frequency (resonance frequency) at the resonance point P21 and the frequency at the antiresonance point P22 (antiresonance frequency) of the parallel arm resonator of the final stage (sixth stage) of the filter are the other (first). The frequency at the resonance point P11 and the antiresonance point P12 of the parallel arm resonator (1-5 stages) is set higher than the frequency at the antiresonance point P12, and the impedance of the other parallel arm resonator and the impedance of the series arm resonator are reversed. The resonance point P21 (resonance frequency) of the parallel arm resonator at the final stage is positioned near the frequency of X. Further, the resonance resistance at the resonance point P21 of the parallel arm resonator in the final stage is set to be 3 to 24 times the resonance resistance at the resonance point of the other parallel arm resonator.

  Note that the resonance frequency and the anti-resonance frequency can be increased by, for example, reducing the pitch of the IDT crossing fingers constituting the resonator as described above. In order to increase the resonance resistance, for example, the crossing length of the IDT may be reduced or the number of crossed fingers may be reduced.

  FIG. 4 shows the frequency characteristics of each of the duplexers (as a whole filter) of the conventional and this embodiment, and FIG. 5 shows the passband of the reception side filter in an enlarged manner. In these drawings, a broken line 100 indicates the reception filter of the conventional duplexer, a solid line 101 indicates the reception filter of the duplexer according to the present embodiment, and a one-dotted line 102 indicates the duplexer according to the present embodiment. For example, reference numeral 110 denotes an insertion loss amount for the pass band of the reception side (high band side) filter, 112 denotes an attenuation amount for the attenuation band of the filter, and 111 denotes attenuation for the transition band of the filter. Each amount is shown. Note that the above-described insertion loss amount and attenuation amount are examples of the required values, and the present invention is not limited thereto.

  As shown in these figures (see particularly the enlarged view of FIG. 5), the duplexer (101) of the present embodiment has a lower shoulder on the low band side of the pass band of the reception filter as compared with the conventional duplexer. The steepness is improved (shoulder 101a protrudes and the inclination is steep), and the insertion loss amount 110 in the pass band and the attenuation amount 111 in the transition band are improved at the same time (in actual use, for example, the temperature The pass band may shift to the high frequency side or the low frequency side due to a change or the like. Further, the same characteristics as those of the conventional duplexer can be obtained for the attenuation band, and the attenuation 112 in the attenuation band can be satisfied.

  Further, FIG. 6 shows frequency characteristics when the resonance resistance of the parallel arm resonator 34 at the final stage of the receiving filter is changed in the duplexer of the present embodiment, and FIG. 7 shows the frequency characteristics of the receiving filter. This is an enlarged view of the passband. In these drawings, a broken line 100 indicates the characteristics of the reception filter of the conventional duplexer. In the above embodiment, the resonance resistance of the parallel arm resonator at the final stage is Zp2, and the resonance resistance of the other parallel arm resonators. When Zp1 is set, the thin solid line 105 is when Zp2 is 2.95 times Zp1 (Zp2 = 2.95Zp1), and the thick solid line 101 is when Zp2 is 7.13 times Zp1 (Zp2 = 7. 13Zp1), a fine broken line 106 indicates a case where Zp2 is 24.76 times Zp1 (Zp2 = 24.76Zp1), and a rough broken line 100 indicates a reception side filter of the conventional duplexer.

  As shown in these figures (see particularly the enlarged view of FIG. 7), in order to satisfactorily satisfy the insertion loss amount of the pass band and the attenuation amount of the transition band, the resonance resistance Zp2 of the parallel arm resonator in the final stage is set to other values. The size is within the range of about 3 to 24 times the resonance resistance Zp1 of the parallel arm resonator (3Zp1 ≦ Zp2 ≦ 24Zp1) (for example, Zp2 is about 5 to 10 times or about 7 to 8 times Zp1). It is desirable to set so that

  Further, FIG. 8 shows the frequency-VSWR (voltage standing wave ratio) characteristics of the reception-side filter of each duplexer according to the above-described conventional and embodiment. In the figure, a solid line 121 indicates the present embodiment, a broken line 122 indicates the characteristics of the conventional duplexer, and a reference numeral 125 indicates an example of a VSWR value required for reflection of input power in the passband.

  As shown in this figure, according to the present embodiment, reflection of input power in the pass band of the reception side filter can be suppressed to be small, and the required VSWR value can be satisfied more satisfactorily. Further, in the transition band (left side of the pass band 875 to 885 MHz in the figure), the amount of reflection is larger than that of the conventional duplexer, and the attenuation can be increased.

  The characteristics of the duplexer of this embodiment are summarized as follows.

  In the duplexer of this embodiment, the capacitance ratio Cp / Cs (Cp: parallel arm resonance) of the entire filter is adjusted by adjusting the resonance and antiresonance frequencies of the parallel arm resonator at the final stage of the receiving filter as described above. (Capacitor capacity, Cs: capacity of the series arm resonator) can be made equivalent to that of the conventional receiving side filter, thereby maintaining the attenuation outside the passband as shown in FIGS. The shoulder 101a on the low frequency side of the pass band is lifted and the inclination is improved, and as a result, the insertion loss and the attenuation of the transition band can be improved.

  The reason why such an improvement effect can be obtained is that the parallel arm resonator in the final stage has a structure (resonance frequency and anti-resonance frequency) peculiar to the present invention, so that the resonance frequency of the parallel arm resonator is within the transition band. In addition to this, a notch is formed in the transition band of the reception side filter, and in addition, the input impedance in the pass band of the reception side filter is more easily matched than the conventional filter as shown in FIG. This is because loss is reduced.

  Further, as shown in FIGS. 6 and 7, the attenuation of the transition band and the insertion loss are adjusted by changing the resonance resistance of the parallel arm resonator in the final stage while maintaining the capacitance ratio Cp / Cs of the entire filter. It becomes possible. At this time, since the attenuation of the transition band and the insertion loss are in a trade-off relationship with each other, the resonance resistance Zp2 of the parallel arm resonator at the final stage is set to another parallel arm resonator in order to realize characteristics higher than those of the conventional filter. Greater than the resonance resistance Zp1.

  As mentioned above, although embodiment of this invention was described, this invention is not limited to these, It is clear to those skilled in the art that a various change can be made within the range as described in a claim. .

  For example, the number of connections of the resonators of the series arm and the parallel arm in each filter or ladder type circuit unit can be various other than the above embodiment. In the above embodiment, all the resonators constituting the transmission side and reception side filters are connected in a ladder type. For example, as shown in FIG. 9, a part of the reception side filter 12 is connected to a structure other than the ladder type. 30 (for example, a DMS circuit or a lattice-type circuit configuration) is also possible. Further, the transmission side filter may be partly or entirely connected to a connection structure other than the ladder type.

  Further, the parallel arm resonator for changing the resonance and antiresonance frequencies according to the present invention is not limited to one parallel arm resonator included in the final stage as viewed from the common terminal side. Two or more parallel arm resonators included in more than one stage may be targeted, and the present invention may be applied to a parallel arm resonator that is not the last stage (for example, at an intermediate position). Further, the range of the resonance resistance value of the parallel arm resonator that changes the resonance and antiresonance frequencies according to the present invention is shown in the range of the resonance resistance value of the resonators combined into one, and at least two or more. Even when the resonance resistance value of the resonator is lowered by dividing it in series or in parallel, the resonance resistance value of the resonator when the resonators are combined into one is within the range defined by the present invention. Also applies. Further, the frequency values shown in the respective diagrams are shown as an example, and it goes without saying that various other values can be taken corresponding to various communication systems.

1 is a circuit diagram showing a duplexer according to an embodiment of the present invention. It is a diagram which shows the frequency-impedance characteristic of the serial arm resonator and parallel arm resonator in the conventional splitter. It is a diagram which shows the frequency-impedance characteristic of the series arm resonator and parallel arm resonator in the duplexer which concerns on the said embodiment. It is a diagram which shows the frequency characteristic of each duplexer of the said prior art and embodiment. FIG. 5 is an enlarged diagram showing FIG. 4 (frequency characteristics of the pass band of the high-pass filter). It is a diagram which shows a frequency characteristic when the impedance of the parallel arm resonator of a high-pass filter is changed in the duplexer of the said embodiment. FIG. 7 is an enlarged diagram showing FIG. 6 (frequency characteristics of the pass band of the high-pass filter). It is a diagram which shows the frequency-VSWR (voltage standing wave ratio) characteristic of each branching filter of the said prior art and embodiment. FIG. 6 is a circuit diagram showing a duplexer according to a modification of the embodiment.

Explanation of symbols

1 Phase line (impedance matching circuit)
DESCRIPTION OF SYMBOLS 11 Transmission side filter 12 Reception side filter 21, 22, 23, 41, 42, 43 Series arm resonator 31, 32, 33, 34, 51, 52 Parallel arm resonator 30 Resonator connection structure other than ladder type (DMS circuit) , Lattice type circuit, etc.)
C Common terminal T Transmission signal terminal R Reception signal terminal

Claims (5)

A duplexer comprising two surface acoustic wave filters having different band center frequencies connected in parallel to a common terminal,
Of the two surface acoustic wave filters, the filter on the high frequency side includes at least part of a ladder type circuit unit in which a plurality of surface acoustic wave resonators are connected in a ladder type,
At least one parallel arm resonator included in the ladder-type circuit unit is:
The resonance frequency and anti-resonance frequency are both higher than those of the other parallel arm resonators included in the high-pass filter, and the impedance of the series arm resonator included in the high-pass filter and the other parallel arm resonances. A duplexer characterized in that the impedance of the filter is in the vicinity of the frequency position where the impedance is reversed. The duplexer according to claim 1, wherein the high-pass filter is a ladder filter in which a plurality of surface acoustic wave resonators are connected in a ladder shape. The duplexer according to claim 1 or 2, wherein a resonance resistance of the at least one parallel arm resonator is larger than a resonance resistance of the other parallel arm resonator. 4. The duplexer according to claim 3, wherein a resonance resistance of the at least one parallel arm resonator is 3 to 24 times a resonance resistance of the other parallel arm resonator. 5. The duplexer according to claim 3, wherein the at least one parallel arm resonator is a parallel arm resonator connected to a final stage position away from the common terminal.

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