JP2006135921A - Ladder type filter and device using same - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To obtain a means for reducing an insertion loss and sharpening an attenuation inclination when narrow-banding a ladder type filter. <P>SOLUTION: In a ladder type filter in which a first resonator having a resonant frequency (frp) and an anti-resonant frequency (fap) is disposed on a parallel arm and a second resonator having a resonant frequency (frs) and an anti-resonant frequency (fas) is disposed on a serial arm and which comprises input wiring connecting an input and one outer resonator and output wiring connecting an output and another outer resonator, when Δf=(frs-fap) and fo=((frs+fap)/2) are defined, Δf/fo satisfies the relation of -0.00635≤Δf/fo<0 and ground capacitance that occurs in each of input wiring and output wiring is within the range of 0.2 to 1.8 pF. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention relates to a ladder type filter, and more particularly to a narrow band ladder type filter with improved insertion loss at a center frequency and guaranteed attenuation in the vicinity of a pass band.

In recent years, SAW filters (surface acoustic wave filters) have been widely used in the communication field, and are widely used particularly for cellular phones and the like because they have excellent characteristics such as high performance, small size, and mass productivity. For example, in the US 1.9 GHz band cellular phone system (PCS), an RF filter having a bandwidth of 60 MHz (approximately 3% in specific bandwidth) is used for both transmission and reception. The RF SAW filter for mobile phone systems uses a rotating Y-cut X-propagation lithium tantalate (LiTaO ₃ ) substrate with relatively good temperature characteristics, and a cut angle of 36 ° or 42 ° is often used. ing. When using LiTaO ₃ substrates with these cut angles, a large electromechanical coupling coefficient is obtained, and the guaranteed bandwidth required for the RF filter of a mobile phone (about 1.6% to about 3% in specific band) is satisfied. Can do.

  On the other hand, a GPS receiver that receives signal radio waves from a plurality of navigation satellites and calculates ground position information with high accuracy based on this signal has a center frequency of 1575.42 MHz and a bandwidth of 2.046 MHz (specific bandwidth). Narrowband RF filters with a width of about 0.13% are required. In the vicinity of this RF band, there are bands used for Inmarsat satellite communication systems (downlink 1525 to 1559 MHz, uplink is 1626.5 to 1660.5 MHz) and bands used for Iridium mobile phone systems (1616). ~ 1626.5MHz), and there are bands (1501 ~ 1525MHz) used in Japanese MCA commercial radio systems. In order to avoid mutual interference with these systems, RF SAW filters for GPS receivers A narrow band and a steep attenuation characteristic in the vicinity of the pass band are required. In addition, signal radio waves coming from GPS satellites are weak, and recently, cellular phones equipped with GPS receivers are becoming widespread. Therefore, low-loss transmission characteristics are required for RF filters of GPS receivers. Yes.

As a SAW filter having a low loss and a steep attenuation characteristic, for example, a ladder-type SAW filter described in detail in JP-A-10-126212 is well known.
FIG. 16A is a schematic diagram showing the configuration of the basic section of a ladder-type SAW filter, which is composed of SAW resonators Xp of parallel arms and SAW resonators Xs of series arms, and the reactance curves of the respective arms are It is set as shown in FIG. That is, when the anti-resonance frequency of the parallel arm SAW resonator Xp (broken line) and the resonance frequency of the series arm SAW resonator Xs (solid line) are set to substantially coincide with each other, the frequency is set as the center frequency. ) Is formed as indicated by F (thick solid line). Then, attenuation poles are respectively formed at the resonance frequency of the parallel arm SAW resonator Xp and the anti-resonance frequency of the series arm SAW resonator Xs, and a filter having a low loss and a steep attenuation gradient is obtained. Furthermore, if a filter with a steep attenuation slope or a filter with a large guaranteed attenuation is required, a higher-order filter can be configured by cascading a plurality of ladder-type basic section filters so that their impedances are matched. Good.

As apparent from FIG. 16B, the bandwidth of the ladder-type SAW filter depends on the difference df = fa−fs between the resonance frequency fs of the SAW resonator and the antiresonance frequency fa. The resonance frequency difference df is expressed by the following equation by the capacitance ratio γ of the SAW resonator (ratio γ = C0 / C1 of the capacitance C0 to the motional capacitance C1).
df = fs ((1 + 1 / γ) ^1/2 −1)
Therefore, the bandwidth of the ladder type SAW filter is determined by the capacitance ratio γ of the SAW resonator.

As shown in FIG. 17, the serial arm SAW resonator Xs is the outermost resonator for both input and output, and parallel arms having a structure in which two SAW resonators Xp are connected in series are arranged inside these, and these two parallel devices are arranged in parallel. The transmission characteristics of a ladder-type SAW filter having a ladder structure in which two arm arm resonators having a structure in which two SAW resonators Xs are connected in series are arranged between arms are determined by simulation.
FIG. 18 is a schematic cross-sectional view showing the structure of a ladder-type SAW filter. A so-called chip size package (CSP) structure is adopted as an analysis model. A ladder-type SAW filter element (SAW chip) T in which an IDT electrode 2 and a pad electrode 3 for connection are formed on the main surface of the piezoelectric substrate 1 includes a connection electrode 5 formed on the upper surface of an alumina ceramic substrate 4 and gold. Flip chip mounting is performed via the bumps 6. Then, if the sealing resin 7 is applied and cured on the SAW chip T, the SAW chip T is sealed to form a chip size package (CSP) having a space 8 inside. The alumina ceramic substrate 34 has a multilayer structure, and the upper electrode 5 and the lower electrode 9 are connected by the internal wiring 10 of the alumina ceramic substrate 4.

In the simulation of the RF filter for GPS, a multi-port S parameter representing the electrical characteristics between the mounting substrate and the wiring of the SAW chip is obtained by electromagnetic field analysis, and this is obtained by actually measuring the SAW resonator. And the entire S parameter is calculated to determine the transmission characteristics of the ladder-type SAW filter having the structure as shown in FIG. 18 (hereinafter, this method is referred to as simulation by electromagnetic field analysis).

A so-called regular IDT in which a 48-degree rotated Y-cut X-propagating LiTaO ₃ substrate is used as a piezoelectric substrate, and the cross width W of IDT electrodes is uniform for all electrode fingers as shown in FIG. By using electrodes and providing a dummy electrode with a large line occupancy (= electrode finger line width / (electrode finger line width + electrode finger space width) × 100 [%]) at the connection between the IDT electrode and the bus bar It has a waveguide structure. The parameters of the series arm SAW resonator Xs are as follows: wavelength λ = Lt is 2.4543 μm, IDT electrode pair Ns is 81 pairs, reflector number Ms is 60, crossing width W is 49 μm, and electrode finger tip gap G0 is 0.4 μm. , Dummy electrode length D0 is 2.5 μm, dummy electrode line occupancy is 60%, Lt / Lr is 0.99 (Lt is wavelength, Lr is twice the distance between reflector electrode fingers), and between adjacent electrode finger centers The distance Ltr was set to 0.46λ, and no insulating film (SiO ₂ film or the like) was deposited on the IDT electrode.

The parallel arm SAW resonator Xp also has a SAW waveguide structure, but the IDT electrode was subjected to elliptical apodization weighting as shown in FIG. The parameters of the parallel arm SAW resonator Xp are as follows: wavelength λ = Lt is 2.5396 μm, IDT electrode pair number Np is 266, reflector number Mp is 40, cross width W is 95 μm, and electrode finger tip gap G0 is 0. 4 μm, dummy electrode length D0 is 2.5 μm, dummy electrode line occupancy is 60%, Lt / Lr (Lt is wavelength, Lr is twice the distance between electrode fingers of reflectors) is 1.0, and between adjacent electrode finger centers The distance Ltr was set to 0.5λ, and an insulating film (SiO ₂ film or the like) did not adhere on the IDT electrode.
For both the serial arm and the parallel arm, the line occupation ratio of the IDT electrode was 50.0%, and the electrode film thickness H was 0.247 μm. The resonance frequency frs of the series arm SAW resonator Xs was 1576.419 MHz, and the anti-resonance frequency fap of the parallel arm SAW resonator Xp was 1574.415 MHz. Here, if Δf = frs−fap and fo = ((frs + fap) / 2) are defined, Δf / fo is 0.00127.

A simulation by electromagnetic field analysis is performed using the parameters set as described above, and the obtained transmission characteristics and impedance characteristics are shown in FIGS. 21A shows the passband characteristics, FIG. 21B shows the characteristics of the attenuation region, FIG. 22A shows a Smith chart showing the impedance characteristics as viewed from the input side, and FIG. 21B shows the characteristics from the output side. It is a Smith chart showing an impedance characteristic.
FIGS. 21A and 21B show a band G (1575.42 ± 1.2 MHz, hereinafter referred to as GPS band) slightly wider than the band used in GPS, and a band used in the MCA commercial radio system in Japan. M (1501-1525 MHz, hereinafter referred to as MCA band) and band S (1626.5-1660.5 MHz, hereinafter referred to as Inmarsat band) used worldwide in the Inmarsat satellite communication system are entered. .
Note that the GPS band insertion loss, which is the GPS pass band, is 1.5 dB or less, and the attenuation amount of the MCA band and Inmarsat band that are the stop band is 35 dB or more.

Japanese Patent Laid-Open No. 10-126212 JP 2004-242281 A

However, the means disclosed in Japanese Patent Application Laid-Open No. 10-126212, Japanese Patent Application Laid-Open No. 2004-242281, or the like is intended for a wide band in the passband. When a ladder-type SAW filter is simulated using these means, characteristics that barely satisfy the attenuation (35 dB or more) in the MCA band and Inmarsat band can be obtained as shown in FIGS. 21 (a) and 21 (b). There is a problem that the insertion loss is almost the limit in the vicinity of the filter center frequency of 1575.42 MHz. This is because the input / output impedance in the GPS band is far away from 50Ω, which is the termination condition, and exhibits capacitance. For example, as described in Japanese Patent Laid-Open No. 10-126212, paragraph number 0050, the factor that causes the input / output impedance to be capacitive is that frs> fap. In addition, the SAW filter structure shown in FIG. 18 is associated with the occurrence of ground capacitance in each of the input wiring and the output wiring.

In order to improve this, if chip inductors are connected on the input and output sides, the loss due to impedance mismatching will be reduced, but there will be disadvantages such as an increase in the number of components, an increase in the component mounting area, and an increase in cost. . Further, it is conceivable that an inductor for impedance matching is built in the mounting substrate of FIG. 18 by a microstrip line or a strip line. However, due to the recent demand for downsizing and thinning of the SAW device, a sufficiently large inductance is required. It has become very difficult to incorporate the in a mounting board.
Further, by reducing the ground capacitance generated in the input wiring and the output wiring, the loss due to the impedance mismatch is reduced. However, according to the study of the present inventor, ground capacitance of about 0.2 pF is generated in the input wiring and the output wiring. It was inevitable, and it was difficult to reduce it further. In particular, when a material having a large relative dielectric constant such as LiTaO ₃ or LiNbO ₃ is used for the piezoelectric substrate, the ground capacitance of the wiring pattern on the SAW chip is not negligible. As described above, the conventional ladder-type SAW filter design method has a problem that a filter that sufficiently satisfies the standard required for the GPS RF filter cannot be realized.

In addition, when such a ladder type filter is used in an RF circuit or a communication device, in addition to the ground capacitance possessed by the ladder type filter alone, the ground capacitance of the circuit board of the RF circuit or communication device in which the ladder type filter is mounted is also increased. As a result, the RF circuit and the device have various problems.

According to the first aspect of the present invention, the first resonator in which the parallel arm has a resonance frequency of frp and the antiresonance frequency is fap, and the series arm has a resonance frequency frs and the antiresonance frequency is fas. In a ladder type filter in which two resonators are arranged,
When defined as Δf = (frs−fap), fo = ((frs + fap) / 2), the relationship of −0.00635 ≦ Δf / fo <0 is satisfied, and between the outermost resonator and the input / output terminal The ladder type filter is configured within the range where the ground capacitance generated in each of the input wiring and the output wiring is 0.2 to 1.8 pF.
According to a second aspect of the present invention, there is provided an insulating substrate provided with an external input electrode for surface mounting, an external output electrode, and an external ground electrode at the bottom, and an upper input electrode, an upper output electrode, and an upper ground disposed on the insulating substrate. A mounting substrate including electrodes, an input pad electrode, an output pad electrode, a ground pad electrode, a first and second SAW resonator on one main surface of the piezoelectric substrate, a SAW chip, and the external input electrode An input wiring connecting the upper input electrode and the input pad electrode; and an output wiring connecting the external output electrode, the upper output electrode and the output pad electrode, and having a resonance frequency (frp) and an anti-resonance frequency ( Ladder-type SAW in which the first SAW resonator having (fap) is arranged in a parallel arm and the second SAW resonator having a resonance frequency (frs) and an anti-resonance frequency (fas) is arranged in a series arm. In the filter,
When defined as Δf = (frs−fap), fo = ((frs + fap) / 2),
A ladder filter is used when Δf / fo satisfies the relationship of −0.00635 ≦ Δf / fo <0 and the ground capacitance generated in each of the input wiring and the output wiring is in the range of 0.2 to 1.8 pF. Constitute.
According to a third aspect of the present invention, in the ladder type filter according to the first or second aspect of the present invention, the ladder type filter is configured such that the total ground capacitance of the connection pattern connecting the resonators is 1.5 pF or less.
According to a fourth aspect of the present invention, in the ladder type filter according to any one of the first to third aspects, which has at least three series arms and at least two parallel arms, the resonance frequency of the resonator disposed in the outermost series arm. The ladder type filter is configured by setting the resonance frequency of the series arm resonator disposed between the two parallel arms higher than the resonance frequency.
The invention according to claim 5 is the ladder type filter according to any one of claims 1 to 3, having at least two series arms and at least three parallel arms, and anti-resonance of a resonator disposed on the outermost parallel arm. The ladder type filter is configured by setting the anti-resonance frequency of the resonator of the parallel arm disposed between the two series arms to be lower than the frequency.
According to a sixth aspect of the present invention, when the piezoelectric substrate is LiTaO ₃ with rotational Y-cut X propagation and the insertion loss in the pass band is 1.5 dB or less, the specific bandwidth is 1.27% or more and less than 2.36%. A ladder filter according to any one of claims 2 to 5.
According to a seventh aspect of the invention, the ladder type filter according to any one of the first to sixth aspects is mounted on a GPS receiving circuit to constitute a device.

  Since the ladder-type SAW filter of the present invention is configured without using an inductor, the filter can be miniaturized, the insertion loss at the center frequency is reduced, and the guaranteed attenuation near the passband is improved. Since the ladder-type SAW filter can be realized, there is an advantage that the performance of the RF circuit and the communication device is improved.

FIG. 1 is a diagram showing a first embodiment of a ladder-type SAW filter according to the present invention, where FIG. 1 (a) is a circuit configuration and FIG. 1 (b) is a schematic sectional view showing the structure. As shown in FIG. 1A, the circuit configuration of the ladder-type SAW filter of the first embodiment is the same as that shown in FIG. A parallel arm having a structure in which two SAW resonators Xp are connected in series is arranged inside each of them, and a series arm resonator having a structure in which two SAW resonators Xs are connected in series between these two parallel arms. This is a ladder-type SAW filter that is arranged in a ladder shape.
The ladder type SAW filter has a chip size package (CSP) structure similar to that shown in FIG. 18 as shown in FIG. The common parts are denoted by the same reference numerals and description thereof is omitted.

  2 (a), (b), (c) and FIG. 3 show frs as the resonance frequency of the series arm SAW resonator Xs of the filter circuit shown in FIG. 1 (a) and anti-resonance frequency of the parallel arm SAW resonator Xp. Is a diagram showing the relationship between Δf / fo and the maximum insertion loss in the GPS band of the ladder-type SAW filter, where Δf is fap, and Δf = frs−fap and fo = ((frs + fap) / 2). Cin and Cout shown in FIG. 1A represent the ground capacitance of the input wiring and the output wiring. The parameters of the SAW resonator used in the ladder filter shown in FIG. 1A are the same as those used in FIG. 21 except for the wavelength λ, and the wavelength λ of the SAW resonator is set according to Δf / fo. Yes. 2 (a), (b), (c) and FIG. 3, using the ground capacitance values Cin and Cout as parameters, the measurement of the SAW resonator is performed in order to obtain the relationship between Δf / fo and the maximum insertion loss in the GPS band. Circuit analysis was performed using S parameters (hereinafter, such a simulation method is referred to as lumped constant simulation). As shown in FIGS. 2 (a), (b), (c) and FIG. 3, in consideration of ground capacitances Cin and Cout in the input wiring and output wiring, the maximum insertion in the GPS band is made in the region where Δf / fo is negative. It has been found that there exists Δf / fo that minimizes the loss value. This is because by setting Δf / fo to be negative, the induction regions of the parallel arm resonator Xp and the series arm resonator Xs overlap each other in the vicinity of the center of the pass band (GPS band), and the overlapped induction regions are input wiring and This is considered to work to cancel the ground capacitance generated in the output wiring.

  FIG. 4 is a diagram showing the relationship between Δf / fo of the ladder filter of FIG. 1A and the imaginary part of the input impedance at 1575.42 MHz. When ground capacitances Cin and Cout do not exist (Cin = Cout = 0 pF), the imaginary part of the impedance becomes 0Ω when Δf / fo is 0, but when Δf / fo exists, Δf / fo is negative Therefore, the imaginary part of the impedance becomes 0Ω, and the loss due to impedance mismatch is minimized. Furthermore, it was found that when Δf / fo is increased to the negative side and the absolute value is increased, the impedance becomes inductive and loss due to impedance mismatch increases. Δf / fo at which the imaginary part of the impedance becomes 0Ω differs depending on the values of the ground capacitances Cin and Cout. As the value of the ground capacitance increases, Δf / fo at which the imaginary part of the impedance becomes 0Ω increases on the negative side. Turned out to be.

As a result of the above simulation, when there is a ground capacity in the input wiring and output wiring, Δf / fo at which the maximum insertion loss value in the GPS band is minimum exists in a region where Δf / fo is negative, It was found that the absolute value of the value increases on the negative side as the value of the ground capacity increases. However, if Δf / fo is set to the negative side and its absolute value is excessively increased, the specific bandwidth of the filter may be significantly narrower than the required standard. Further, comparing FIGS. 2A, 2B, 2C and FIG. 3, the in-band maximum insertion loss value at which the in-band maximum insertion loss is minimized tends to increase as the ground capacities Cin and Cout increase. I found out that That is, there is an upper limit and a lower limit for Δf / fo and ground capacity, respectively.

The GPS band insertion loss standard is 1.5 dB or less, and the 1.5 dB bandwidth is due to frequency fluctuation due to temperature change applied to the SAW filter, frequency fluctuation due to reflow process when mounted on a circuit board, thermal shock, and mechanical shock. In consideration of various frequency fluctuations such as frequency fluctuations, in addition, it is necessary to set the frequency to 20 MHz (specific band 1.27%) or more in consideration of the frequency variation at the time of manufacture.
FIG. 5 shows filter characteristics obtained by simulating the ladder filter shown in FIG. 1A using a lumped constant. The solid lines indicate the ground capacitances Cin and Cout of 1.8 pF, and Δf / fo is −0.00635. Is the filter characteristic. On the other hand, the broken lines are filter characteristics when the ground capacitances Cin and Cout are both 2.0 pF and Δf / fo is −0.00762. Other parameters are the same as those in FIG. The bandwidth of the solid line was 20.7 MHz, and the bandwidth of the broken line was 18.5 MHz. In other words, when the ground capacity is larger than 1.8 pF, the optimum design is selected so that the absolute value of Δf / fo is increased to the negative side of −0.00635 and the insertion loss in the GPS band is 1.5 dB or less. However, the bandwidth of 20 MHz or more could not be realized. From this, it became clear that the ground capacity needs to be 1.8 pF or less and Δf / fo needs to be −0.00635 or more.

As described above, it is inevitable that the input wiring and the output wiring each generate a ground capacitance of about 0.2 pF. As is apparent from the diagram of FIG. 2A, both Cin and Cout are 0.2 pF. In this case, the insertion loss in the GPS band is minimized by setting Δf / fo to −0.0013 to −0.00025. In other words, if the ground capacitance generated in the input wiring and output wiring is in the range of 0.2 pF to 1.8 pF, by appropriately setting Δf / fo, the insertion loss in the GPS band is 1.5 dB or less, and the bandwidth is reduced. It was found that a ladder-type SAW filter of 20 MHz or higher (specific bandwidth of 1.27% or higher) can be realized.
Further, as described above, the lower limit value of Δf / fo is −0.00635, but the upper limit value of Δf / fo may be less than 0 from FIG.
From the above results, a chip inductor or the like is used if −0.00635 ≦ Δf / fo <0 is satisfied and the ground capacitance generated in the input wiring and the output wiring is within the range of 0.2 to 1.8 pF. Thus, it was found that a loss due to impedance mismatch within the GPS band can be reduced, and a low-loss ladder type SAW filter having a bandwidth of 20 MHz or more can be realized.

FIG. 6 shows the electromagnetic wave when the resonance frequency frs of the series arm SAW resonator Xs is 1572.423 MHz, the antiresonance frequency fap of the parallel arm SAW resonator Xp is 1578.416 MHz, and Δf / fo is set to −0.00380. In the transmission characteristics obtained by the simulation by the field analysis, FIG. 9A shows the passband characteristics and FIG. 10B shows the attenuation characteristics. The parameters were the same as those used in FIG. 21 except that the wavelength λ of the SAW resonator of each arm was changed so that Δf / fo was −0.00380. The ground capacitances generated in the input wiring and the output wiring were 0.499 pF and 0.516 pF, respectively. As is clear from FIG. 6, the bandwidth was 32.5 MHz (2.06% in specific bandwidth), and the attenuation characteristics were satisfactory with sufficient standards.
FIGS. 7A and 7B are Smith charts of impedance characteristics as seen from the input and output sides, respectively, and the imaginary part of the input impedance and output impedance in the GPS band is smaller than in FIG. 22 which is a conventional example. I understand.

  8A and 8B are a comparison of transmission characteristics between a conventional ladder-type SAW filter and a ladder-type SAW filter according to the present invention. A solid line indicates a transmission characteristic of the ladder-type SAW filter according to the first embodiment, a broken line. These are the transmission characteristics of the conventional ladder-type SAW filter. Since the ladder-type SAW filter according to the present invention has a smaller insertion loss and a narrower band than the conventional ladder-type SAW filter, the manufacturing margin for guaranteeing the MCA band attenuation is higher than that of the conventional ladder-type SAW filter. It became possible to secure with a margin.

FIG. 9 is a diagram showing a circuit configuration of the ladder-type SAW filter according to the second embodiment of the present invention. The difference from FIG. 1A is the ground capacitance Ci generated in the wiring between the series arm resonators Xs. The circuit design is performed in consideration of (= C1, C2, C3). In FIG. 9, if the sum of the ground capacitances C1, C2, and C3 generated in the wiring between the series arm resonators Xs is 1.5 pF or less, and the method of the first embodiment of the present invention is used for this, the first implementation is performed. It has been found that a ladder-type filter having a wider band than that of the example ladder-type SAW filter can be realized.
FIG. 10 shows transmission characteristics obtained by lumped constant simulation of the ladder filter according to the second embodiment. FIG. 10A shows the passband characteristics and FIG. 10B shows the attenuation characteristics. The broken line is the transmission characteristic according to the first embodiment, and the solid line is the transmission characteristic according to the second embodiment. The ground capacitances Cin and Cout generated in the input wiring and output wiring of the ladder-type SAW filter are both 0.5 pF, Δf / fo is −0.00254, and parameters other than the wavelength λ are the same as those in FIG. The only difference between the solid line and the broken line in FIG. 10 is the ground capacitance generated in the wiring between the series arm SAW resonators Xs. In the first embodiment indicated by a broken line, the ground capacitance generated in the wiring between the series arm SAW resonators Xs is set to 0 pF. In the second embodiment indicated by a solid line, the ground generated in the wiring between the series arm SAW resonators Xs is set. The capacitance is C1 = 0.1 pF, C2 = 0.5 pF, and C3 = 0.1 pF, and the total ground capacitance (C1 + C2 + C3) generated in the wiring between the series arm SAW resonators is 0.7 pF. In the first embodiment indicated by the broken line, the bandwidth is 29.5 MHz, and in the second embodiment indicated by the solid line, the bandwidth is 36.7 MHz. In the second embodiment, a ladder-type filter having a wider bandwidth than the first embodiment is obtained. It turns out that it can be realized.
Further, as is apparent from FIG. 10B, it has also been found that the second embodiment has a larger attenuation in the MCA band and the Inmarsat band than the first embodiment.

  FIG. 11 is a diagram for explaining the upper limit value of the total sum of the ground capacitance Ci generated in the wiring between the series arm resonators Xs in the second embodiment. The broken lines are transmission characteristics obtained by simulation using a lumped constant with ground capacitances C1, C2, and C3 all set to 0.7 pF (sum of 2.1 pF). The solid line represents the transmission characteristics when C1, C2, and C3 are all 0.5 pF (total is 1.5 pF). Other parameters are the same as those in FIG. As is clear from FIG. 11, it was found that when the total ground capacitance Ci generated in the wiring between the series arm resonators Xs exceeds 1.5 pF, the insertion loss in the GPS band becomes 1.5 dB or more. The simulation was performed by changing the ground capacitance Ci in various ways. When the sum of the ground capacitance Ci exceeded 1.5 pF, the insertion loss became 1.5 dB or more. The total of the ground capacitance Ci needs to be 1.5 pF or less.

FIG. 12 is a diagram comparing the transmission characteristics of a conventional ladder type filter and a ladder type SAW filter using the second embodiment. The broken line represents the transmission characteristics of the conventional ladder-type SAW filter. Δf / fo = 0.00127, and the ground capacitance Ci generated in the wiring between the series arm resonators Xs is all 0 pF. The solid line shows the transmission characteristics of the ladder-type SAW filter according to the second embodiment. Δf / fo is −0.00254, the ground capacitance C1 is 0.1 pF, C2 is 0.5 pF, and C3 is 0.1 pF. The other parameters were the same as in FIG. FIG. 12A shows that the conventional ladder-type SAW filter (broken line) has a maximum GPS band insertion loss of 1.12 dB and a bandwidth of 36.0 MHz, whereas the second embodiment of the present invention uses GPS. The maximum in-band insertion loss is 0.91 dB, the bandwidth is 36.7 MHz (ratio band 2.33%), and the ladder-type SAW filter of the second embodiment has a wider bandwidth and lower loss than the conventional one. It turns out that. FIG. 12B shows the attenuation characteristics of the same thing as FIG. 12A. As is clear from this, the attenuation characteristics of the ladder-type SAW filter using the second embodiment are better. The characteristics are steeper than those of the conventional one, and the manufacturing margin such as frequency variation becomes larger in guaranteeing the MCA band attenuation amount and the Inmarsat band attenuation amount.
If at least one of the ground capacitances C1, C2, and C3 shown in FIG. 9 cannot be realized only by the wiring between the series arm resonators Xs, the capacitance may be configured by a comb capacitor formed on the piezoelectric substrate. good.

FIG. 13 shows a circuit configuration of a ladder-type SAW filter according to a third embodiment of the present invention, in which a series arm SAW resonator Xs, a parallel arm in which two SAW resonators Xp are connected in series, and two SAW resonances. A series arm in which the child Xs2 is connected in series, a parallel arm in which the two SAW resonators Xp are connected in series, and the series arm SAW resonator Xs are alternately connected to form a ladder shape. The parallel arm resonator Xp and the series arm resonator Xs arranged on the outermost side satisfy −0.00635 ≦ Δf / fo <0 so that the series arm resonator Xs2 arranged between the two parallel arms The resonance frequency is set higher than that of Xs.
FIG. 14 shows the transmission characteristics of the ladder-type SAW filter according to the third embodiment obtained by using a simulation by electromagnetic field analysis. The broken line indicates the transmission characteristics of the ladder-type SAW filter according to the first embodiment, and the solid line indicates the first. It is the transmission characteristic by a 3rd Example. In the broken line of the first embodiment, the resonance frequency frs of the series arm SAW resonator Xs is 1572.423 MHz, the anti-resonance frequency fap of the parallel arm SAW resonator Xp is 1578.416 MHz, and Δf / fo is −0.00380, The ground capacitances generated in the input wiring and the output wiring are 0.499 pF and 0.516 pF, respectively. In the solid line of the third embodiment, only the wavelength λ of Xs2 is reduced, and the resonance frequency frs2 of Xs2 is set to 1575.428 MHz. That is, the resonance frequency frs2 of Xs2 is set higher by 3.005 MHz than the resonance frequency frs of Xs. Other parameters were the same as those in FIG. The bandwidth of the ladder-type SAW filter according to the first embodiment is 32.5 MHz and the maximum insertion loss within the GPS band is 0.89 dB, whereas the bandwidth of the ladder-type SAW filter according to the third embodiment is 35. The maximum insertion loss in the GPS band at 1 MHz is 0.86 dB, and it has been found that wideband characteristics can be realized without degrading the insertion loss in the GPS band. The ladder type SAW filter according to the third embodiment can also be used in combination with the method of the second embodiment.

FIG. 15 is a diagram showing a circuit configuration of the ladder-type SAW filter according to the fourth embodiment of the present invention. The parallel arm of the SAW resonator Xp, the series arm of the SAW resonator Xs, and the SAW resonator Xp2 A parallel arm, a serial arm of the SAW resonator Xs, and a parallel arm of the SAW resonator Xp are alternately connected to form a ladder shape. Further, the ground capacitances Cin and Cout generated in the input wiring and the output wiring and the ground capacitance C1 generated in the wiring between the series arm SAW resonators Xs are taken into consideration.
In FIG. 15, the series arm resonator Xs and the parallel arm resonator Xp arranged on the outermost side are set so as to satisfy −0.00635 ≦ Δf / fo <0. In the fourth embodiment, two more The antiresonance frequency of the parallel arm resonator Xp2 arranged between the series arms is set lower than that of Xp. As a result of the simulation, it was found that by setting in this way, the same effect as the third embodiment, that is, a broadband characteristic can be realized without deteriorating the insertion loss of the GPS band.
The ladder type SAW filter according to the fourth embodiment can also be used in combination with the method of the second embodiment. When the circuit configuration of FIG. 15 is used in combination with the second embodiment, the ground capacitance C1 generated in the wiring between the series arm SAW resonators may be 1.5 pF or less.

When the number of ladder circuits is increased as compared with FIGS. 13 and 15, a plurality of or all of the second, third, and fourth embodiments of the present invention can be used together.
When the ladder-type SAW filter according to the present invention uses a rotating Y-cut X-propagating LiTaO ₃ for the piezoelectric substrate, low loss and steep transmission characteristics can be realized with a specific bandwidth of 1.27% or more and less than 2.36%. That is, as disclosed in Japanese Patent Application Laid-Open No. 10-126212, it is common knowledge that a ladder-type SAW filter can obtain only insufficient transmission characteristics with a narrowband filter having a relative bandwidth of less than 2.36%. However, according to the present invention, good transmission characteristics can be realized even in a narrow band.

  When the ladder-type SAW filter according to the present invention is used for an RF circuit or a communication device (for example, a GPS receiver circuit, a GPS receiver, or a mobile phone equipped with the same), input wiring and output wiring included in the ladder-type SAW filter alone. The ground capacitance of the circuit board to be mounted is further added to the ground capacitance. Even in that case, if the ground capacitance of each of the input wiring and the output wiring is within the range of 0.2 to 1.8 pF after the ladder-type SAW filter is mounted on the circuit board, −0.00635 ≦ Δf / By appropriately setting Δf / fo of the ladder-type SAW filter within the range of fo <0, a low-loss RF circuit and a communication device resistant to interference waves can be realized.

  In the above, the case where the SAW resonator is used as the resonator which is a constituent element of the ladder-type SAW filter has been described. However, the resonator is a piezoelectric thin film resonator, a bulk wave resonator using a ceramic, a coaxial dielectric resonator, or the like. Even in this case, the same effect can be obtained.

  In each of the above embodiments, a ladder-type SAW filter in which three series arm SAW resonators and two parallel arm SAW resonators are formed in a ladder shape, two series arm SAW resonators, and three parallel arms are provided. The ladder type SAW filter in which the arm SAW resonator is formed in a ladder shape has been described. If the parallel arm SAW resonator is a first SAW resonator and the series arm SAW resonator is a second SAW resonator, A ladder-type SAW filter can be generalized by describing a ladder-type SAW filter in which the first SAW resonator is arranged in parallel arms and the second SAW resonator is arranged in series arms.

Japanese Patent Laid-Open No. 2004-242281 describes a ladder-type SAW filter in which Δf / fo is negative, but there are fundamental differences from the present invention as described below.
One of the objects of the invention of Japanese Patent Application Laid-Open No. 2004-242281 is aimed at widening the pass band equivalent to or higher than that of Japanese Patent Application Laid-Open No. 10-126212. This is to realize a ladder-type SAW filter having a narrower band than that of the publication with good characteristics.
Japanese Patent Application Laid-Open No. 2004-242281 includes a first series arm resonator in which an inductor is connected in parallel to the series arm resonator, and a second series arm resonator in which no inductor is connected. However, when this is applied to the present invention, the above-described effects of the present invention were not obtained at all.

In the present invention, the ground capacitance of the input wiring and the output wiring is numerically limited, and the grounds for the limitation are also described. However, JP 2004-242281A does not describe ground capacitance. Even if it is a well-known fact that ground capacitance is inevitably generated in the input wiring and output wiring, the stray capacitance is generated between the induction regions of the parallel arm resonator and the series arm resonator. The idea that it is countered by overlapping can not be easily imagined.

BRIEF DESCRIPTION OF THE DRAWINGS It is a figure which shows the form of the 1st Example of the ladder type SAW filter based on this invention, (a) is a circuit diagram, (b) is the schematic sectional drawing which showed the structure. (A) ~ (c) is a diagram showing the relationship between Delta] f / f ₀ and bandwidth within the maximum insertion loss in the case of the earth capacitance of the input lines and output lines as parameters. Is a diagram showing the relationship between Delta] f / f ₀ and band maximum insertion loss when the earth capacitance of the input lines and output lines as a parameter. The earth capacitance of the input lines and the output lines is a diagram showing the relationship between Delta] f / f ₀ and the impedance imaginary part when the parameters. The solid line indicates the filter characteristics when the ground capacitance Cin = Cout = 1.8 pF and Δf / fo = −0.00635, and the broken line indicates the ground capacitance Cin = Cout = 2.0 pF and Δf / fo = −0.00762 It is a filter characteristic. Is Transmission characteristics of simulation by electromagnetic field analysis when the resonance frequency frs of the series arm SAW resonator Xs is 1572.423 MHz, the anti-resonance frequency of the parallel arm SAW resonator Xp is fap = 1578.416 MHz, and Δf / fo = −0.00380. FIG. 9A shows the passband characteristics, and FIG. 10B shows the attenuation characteristics. (A), (b) It is a Smith chart of the impedance seen from the input-output side. The solid line shows the transmission characteristics of the ladder-type SAW filter according to the first embodiment, the broken line shows the conventional transmission characteristics, FIG. 9A shows the passband characteristics, and FIG. is there It is a figure which shows the circuit structure of the ladder type SAW filter of a 2nd Example. The broken line is the transmission characteristic according to the first embodiment, the solid line is the transmission characteristic according to the second embodiment, the figure (a) is the passband characteristic, and the figure (b) is the attenuation characteristic. The broken line is the transmission characteristic when the ground capacitances C1, C2, and C3 of the second embodiment are all 0.7 pF, and the solid line is the transmission characteristic when C1, C2, and C3 are both 0.5 pF. The solid line represents the transmission characteristic when Δf / fo = −0.00254, the ground capacitance C1 = 0.1 pF, C2 = 0.5 pF, and C3 = 0.1 pF in the second embodiment, and the broken line represents the conventional ladder-type SAW filter. The transmission characteristics when Δf / fo = 0.00127 and the ground capacitance Ci = 0 pF are shown, (a) shows the passband characteristics, and (b) shows the attenuation characteristics. This is a circuit configuration of the ladder-type SAW filter of the third embodiment. In the simulation by electromagnetic field analysis, the solid line represents the transmission characteristic according to the third embodiment, and the broken line represents the transmission characteristic according to the first embodiment. It is a circuit structure of the ladder type SAW filter of a 4th Example. (A) is a figure which shows the basic area of the conventional ladder type SAW filter, (b) is a figure which shows a reactance curve and a filter characteristic. It is a figure which shows the circuit structure of the conventional ladder type SAW filter. It is a schematic sectional drawing which shows the structure of the conventional ladder type SAW filter. It is a figure which shows the electrode pattern of a serial arm SAW resonator. It is a figure which shows the electrode pattern of a parallel arm SAW resonator. It is a figure which shows the transmission characteristic of the conventional ladder type SAW filter, The figure (a) is a passband characteristic, The figure (b) is an attenuation characteristic. The impedance characteristics of a conventional ladder-type SAW filter, (a) is a Smith chart viewed from the input side, and (b) is a Smith chart viewed from the output side. It is a figure which shows the circuit structure of the conventional ladder type SAW filter.

Explanation of symbols

Xs, Xs2 Series arm SAW resonator Xp, Xp2 Parallel arm SAW resonator Cin Ground capacitance Cout of input wiring Ground capacitance C1, C2, C3 of output wiring Ground capacitance of wiring between series arm resonators Δf Δf = frs−fap
f ₀ f ₀ = (frs + fap) / 2
1 SAW chip 2 IDT electrode 3 pad electrode 4 alumina ceramic substrate 5 electrode 6 metal bump 7 sealing resin 8 space 9 outer electrode 10 internal wiring

Claims (7)

A ladder type filter in which a first resonator having a resonance frequency of frp and an antiresonance frequency of fap is arranged on a parallel arm, and a second resonator having a resonance frequency frs and an antiresonance frequency of fas is arranged on a series arm. In
When defined as Δf = (frs−fap), fo = ((frs + fap) / 2), the relationship of −0.00635 ≦ Δf / fo <0 is satisfied, and between the outermost resonator and the input / output terminal A ladder filter characterized in that a ground capacitance generated in each of an input wiring and an output wiring is in a range of 0.2 to 1.8 pF. Insulating substrate having external input electrode, external output electrode and external ground electrode for surface mounting on the bottom, and mounting substrate having upper input electrode, upper output electrode and upper ground electrode arranged on the insulating substrate An input pad electrode, an output pad electrode, a ground pad electrode, and first and second SAW resonators on one main surface of the piezoelectric substrate, a SAW chip, the external input electrode, the upper input electrode, and the input The first wiring having a resonance frequency (frp) and an anti-resonance frequency (fap), comprising: an input wiring connecting a pad electrode; and an output wiring connecting the external output electrode, the upper output electrode, and the output pad electrode. In the ladder-type SAW filter, the SAW resonators are arranged in parallel arms, and the second SAW resonator having a resonance frequency (frs) and an anti-resonance frequency (fas) is arranged in a series arm.
When defined as Δf = (frs−fap), fo = ((frs + fap) / 2),
Δf / fo satisfies a relationship of −0.00635 ≦ Δf / fo <0, and a ground capacitance generated in each of the input wiring and the output wiring is in a range of 0.2 to 1.8 pF. Ladder type filter. The ladder filter according to claim 1 or 2, wherein the total ground capacitance of the connection pattern connecting the resonators is 1.5 pF or less. The ladder type filter according to claim 1, wherein the ladder type filter has at least three series arms and at least two parallel arms.
A ladder filter, wherein a resonance frequency of a resonator of a series arm disposed between two parallel arms is set higher than a resonance frequency of a resonator disposed on the outermost series arm. The ladder type filter according to any one of claims 1 to 3, wherein the ladder type filter has at least two series arms and at least three parallel arms.
A ladder type characterized in that the anti-resonance frequency of the resonator of the parallel arm disposed between the two series arms is set lower than the anti-resonance frequency of the resonator disposed on the outermost parallel arm. filter. The piezoelectric substrate is LiTaO ₃ with rotational Y-cut X propagation, and the specific bandwidth is 1.27% or more and less than 2.36% when the insertion loss in the passband is 1.5 dB or less. The ladder type filter according to any one of claims 2 to 5. An apparatus comprising the ladder filter according to claim 1 mounted on a GPS receiving circuit.

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