JPH04253414A - Branch filter using surface acoustic wave resonator composite filter - Google Patents

Abstract

PURPOSE:To reduce energy consumption by deciding the arrangement and connecting method of one-opening surface acoustic wave resonators so that an impedance characteristic on the side or a common terminal can have reactivi ty. CONSTITUTION:With a single antenna 1 as the common terminal, a filter 2 for reception and a filter 3 for transmission are parallelly connected. In this case, the four one-opening surface acoustic wave resonators composed of electrodes 4-1-4-6 or the like and gap capacitors composed of electrode patterns 5-1 and 5-2 are provided and formed on a piezoelectric substrate 7, which can propagate surface acoustic waves, so as to be used as the filter 3 for transmission. Further, acoustic absorbing materials 6-1-6-6 for absorbing the surface acoustic waves and inductances 8-1-8-4 for matching an outside circuit are provided. Moreover, each one-opening surface acoustic wave resonator is composed of an equivalent inductance 11-1, equivalent capacitor 12-1 and electrostatic capacitor 13-1.

Description

[Detailed description of the invention]

0001

[Industrial Application Field] The present invention relates to a duplexer, and more specifically, a bandpass device which is a combination of a plurality of single-aperture surface acoustic wave resonators each having interdigital electrode fingers arranged on a piezoelectric substrate. Or, it relates to a duplexer configured with a surface acoustic wave filter for band rejection, and in particular to a duplexer suitable for configuring a plurality of filters connected in parallel, such as an antenna duplexer. It is.

0002

2. Description of the Related Art A duplexer is a device that separates signals propagating through a common signal propagation medium based on the wave characteristics of the signals.
On Ultrasonics, Ferroelectrics and Frequency Control (IEEE
Transaction on Ultrasonic
s, Ferroelectrics, and Freq.
control), Vol. 36, No.
．． 5, pp. In the antenna duplexer described in No. 531-539, 1989, the antenna is used as a common signal propagation medium, and a path for obtaining a received signal through the antenna and a path for sending the signal to the antenna are separated. In the conventional technology described above, in order to achieve the demultiplexing function, a demultiplexer was constructed by connecting transmitting and receiving surface acoustic wave filters with different passbands in parallel from the antenna terminal, which is the branching point. . In addition, the surface acoustic wave resonator composite filter used as a band rejection type filter in the transmission system in the above-mentioned duplexer has a plurality of surface acoustic wave resonators formed on a piezoelectric substrate, as described in Japanese Patent Application Laid-Open No. 1-260911. It was constructed by a combination of single-aperture surface acoustic wave resonators.

0003

[Problems to be Solved by the Invention] The conventional duplexer described above suffers from absorption loss due to parallel connection and diffraction phenomenon inherent to a surface acoustic wave resonator composite filter when two different passbands are close to each other. No consideration was given to the impact. For this reason, there have been problems with increased loss in the duplexer and generation of ripples within the passband.

[0004] Below, problems with a conventional duplexer constructed using a single antenna will be explained in detail. As shown in FIG. 2, the duplexer is constructed by connecting a receiving filter 2 and a transmitting filter 3 in parallel using a single antenna 1 as a common terminal. Passband fr of filters 2 and 3
By setting ft and ft to different bands as shown in FIG. At this time, by connecting two transmitting and receiving filters in parallel, a parallel connection loss occurs. This loss is caused by the fact that when a plurality of filters with different pass frequency bands are connected in parallel, a signal in the pass band of one filter is absorbed and dissipated outside the pass band of the other filter.

[0005] In order to suppress parallel connection loss, it is necessary that the input impedance of each filter be reactive outside the passband. Therefore, when a surface acoustic wave resonator composite filter is used in such a duplexer, its impedance characteristics are important. For example, in the duplexer of this example, the surface acoustic wave resonator composite filter is used as a transmission filter through which a large amount of power passes because it has excellent low loss properties and power durability. At this time, the impedance characteristic is required to be reactive in the passband fr of the reception filter that is the other side of the parallel connection.

FIG. 4 shows an example of the structure of a surface acoustic wave resonator composite filter. Each one-aperture surface acoustic wave resonator used in the filter is composed of an interdigital transducer consisting of multiple pairs of interdigitated electrode fingers. These transducers become single-aperture resonators because vibrational energy is trapped within the transducer by internal reflection from the transducer's electrode fingers themselves, even if there are no reflectors on either side. In the figure, the resonance frequencies of the one-aperture surface acoustic wave resonators 16-1 to 16-n are set in the following relationship.

[0007]

[Math 1]

At this time, the input impedance characteristics when looking at the element from the terminal 14 side and the terminal 15 side are shown in FIG.
Shown in a) and (b). This figure is a Smith diagram, ft1~f
The area between t2 is the transmission band ft, and the area between ft1 and ft2 is the reception band fr. Since the transmitting filter needs to achieve high attenuation characteristics in the receiving band, the region between fr1 and fr2 as shown in (a) above is the right end on the real axis of the Smith diagram, that is, the position where the impedance is extremely large. It is desirable that the Generally, the impedance of the surface acoustic wave resonator composite filter in the region between fr1 and fr2 is not necessarily high impedance when the filter is alone.

However, if we could achieve a state closer to the periphery of the Smith diagram, that is, a more reactive state,
By passing through a suitable line, it can always be located at the right end on the real axis as shown in the above figure (a), resulting in a very high impedance. Comparing both of the above figures (a) and (b), a large difference can be seen in the region between fr1 and fr2. The impedance characteristic on the terminal 15 side shown in (b) goes inside the Smith diagram, and the reactivity is degraded. This difference in impedance characteristics appears due to the positional relationship between the input terminal and the resonator, that is, the difference in the signal reflection point at each frequency. In conventional duplexers using surface acoustic wave resonator composite filters, no consideration was given to the arrangement or connection direction of the surface acoustic wave resonators, resulting in increased parallel connection loss due to deterioration of reactivity.

On the other hand, in the duplexer shown in FIG. 3 where the gap between the transmission band ft and the reception band fr is narrow, the influence of the diffraction phenomenon peculiar to the surface acoustic wave resonator composite filter cannot be ignored. . The diffraction phenomenon of the single-aperture surface acoustic wave resonator 16 as shown in FIG. The smaller the diffraction phenomenon is, the more pronounced the diffraction phenomenon is. In the following, the aperture length will be expressed and handled as a multiple of the wavelength λ.

FIG. 7 shows an example of the frequency characteristic of the impedance Im(Z) of a one-aperture surface acoustic wave resonator affected by the diffraction phenomenon, and a ripple was generated between the resonant frequency fs and the anti-resonant frequency fa. FIG. 8 shows an example of the amplitude frequency characteristics of a surface acoustic wave resonator composite filter constructed using a plurality of the above-mentioned one-aperture surface acoustic wave resonators, and a ripple Vp is generated in the passband near the falling part. did. The magnitude of the ripple is mainly determined by the impedance characteristics of a single-opening surface acoustic wave resonator with a low resonance frequency that forms falling characteristics.

In conventional surface acoustic wave resonator composite filters, the aperture length was set to be relatively small in order to reduce the chip size or increase impedance for the purpose of improving attenuation characteristics outside the passband. . For this reason, for example, in a duplexer with a narrow gap between transmission and reception bands, loss increases due to ripples generated in the passband near the falling edge.

An object of the present invention is to provide a low-loss duplexer by solving the problems of absorption loss caused by parallel connection and in-band ripple caused by the diffraction phenomenon of surface acoustic wave resonators.

[0014]

[Means for Solving the Problems] The first object is to provide a one-aperture elastic surface so that the impedance characteristic on the common terminal side of a surface acoustic wave resonator composite filter used in a duplexer has reactive properties. This is achieved by determining the arrangement and connection method of the wave resonators. Reactivity can be achieved by arranging a resonator with a lower resonant frequency on the common terminal side in each of the series circuit of the surface acoustic wave resonator composite filter and the circuit between the series circuit and the common ground.

The second object can be achieved by optimizing the aperture length of the one-aperture surface acoustic wave resonator constituting the surface acoustic wave resonator composite filter and suppressing the diffraction phenomenon. Specifically, by setting the aperture length of the single-aperture surface acoustic wave resonator with a low resonant frequency connected to the common terminal side to be at least 10 times the wavelength, the in-band ripple value can be reduced to a level that is practical for use as a demultiplexer filter. It can be made as small as possible.

[0016]

[Operation] Since the impedance characteristic on the common terminal side of the surface acoustic wave resonator composite filter used in the duplexer becomes reactive, the problem of absorption loss caused by parallel connection can be solved. Furthermore, since it is possible to suppress in-band ripples that occur due to diffraction phenomena, it is possible to solve the problem of increased loss at the trailing edge when two passbands are close to each other. As a result of the above action, a low-loss duplexer can be realized.

[0017]

[Examples] The contents of the present invention will be explained below using specific examples. FIG. 1(a) is a configuration diagram of an embodiment of a duplexer used in a mobile communication device or the like according to the present invention. Similar to FIG. 2, the receiving filter 2 and the transmitting filter 3 are connected in parallel using a single antenna 1 as a common terminal. In this example, the surface acoustic wave resonator composite filter is
On the piezoelectric substrate 7, which is used as a transmission filter and is capable of propagating surface acoustic waves, there are four one-aperture surface acoustic wave resonators including electrodes 4-1 to 4-6, and an electrode pattern 5-. A resonator composite filter is formed by providing a gap capacitance of 1 and 5-2. Also, 6-1 to 6-6
are sound-absorbing materials for absorbing surface acoustic waves, 8-1 to 8-
4 is an inductance for matching with an external circuit.

[0018] The surface acoustic wave resonator composite filter has the following features:
The electrical equivalent circuit is shown in FIG. 1(b). As shown in the figure, each one-aperture surface acoustic wave resonator consists of an equivalent inductance, an equivalent capacitance, and an electrostatic capacitance. Two single-aperture surface acoustic wave resonators are connected to the series circuit between the antenna 1 and the input terminal 10. The resonator on the antenna 1 side of this circuit has an equivalent inductance of 11-1 and an equivalent capacitance of 12-
1. Resonant frequency fr(1) determined by capacitance 13-1,
It has an anti-resonant frequency fa(1). In addition, input terminal 10
The side resonator has a resonant frequency fr(
4), has an anti-resonant frequency fa(4). On the other hand, two single-aperture surface acoustic wave resonators are also connected in the circuit between the electrodes of the series circuit and the common ground. The resonator on the antenna 1 side of this circuit has a resonant frequency f determined by the equivalent inductance 11-2, equivalent capacitance 12-2, and capacitance 13-2.
r(2) and anti-resonant frequency fa(2). In addition, the resonator on the input terminal 10 side has an equivalent inductance of 11-3
, equivalent capacitance 12-3, and electrostatic capacitance 13-3.

FIG. 9 shows the results of a computer simulation of the frequency characteristics of the filter of this configuration using the electrical equivalent circuit described above. In the figure, the lower stopband of the passband is formed by a resonator between the electrodes of the series circuit and the common ground, and the resonant frequencies fr(2) and fr(3) coincide with their attenuation poles. In addition, the high-frequency side stopband is formed by a series circuit resonator, and anti-resonant frequencies fa(1), fa(
4) coincides with their attenuation poles. In the above embodiment, the relationship between the anti-resonant frequencies of the resonators connected in the series circuit and the relationship between the resonant frequencies of the resonators connected between the electrodes of the series circuit and the common ground are as follows.

[0020]

[Math 2]

[0021]

[Math 3]

It is set as follows. At this time,
As for the impedance characteristic seen from the common terminal side (antenna 1 side), reactivity is achieved outside the passband of the transmission filter, as in the case of FIG. The reactivity in this case is also determined by the positional relationship between the input terminal and each resonator.

Further, an example of a surface acoustic wave resonator composite filter having another configuration for realizing the filter characteristics shown in FIG. 9 is shown in an electrical equivalent circuit in FIG. Although the arrangement of the single-aperture surface acoustic wave resonator is different from that in Figure 1, the relationship between the anti-resonant frequencies of the series circuit resonators and the relationship between the resonator frequencies between the series circuit electrodes and the common ground are as described above (
This is the same as 2) and (3). Therefore, the common terminal side (
The impedance characteristics seen from the antenna 1 side) are reactive.

There are various configuration methods for the surface acoustic wave resonator composite filter, depending on the number of one-aperture surface acoustic wave resonators, the series circuit, and the combination of the resonators between the electrodes of the series circuit and the common ground. In either case, by arranging a resonator with a low resonant frequency on the common terminal side in each circuit, reactivity on the common terminal side can be realized.

In addition, when a filter is constructed from a large number of single-aperture surface acoustic wave resonators, even if the resonant frequency arrangement of the resonators is irregular in some parts, the average value of the resonant frequencies of all the resonators is Reactivity can be achieved by connecting a resonator with a lower resonant frequency to the common terminal side. The duplexer of the above embodiment using the surface acoustic wave resonator composite filter with excellent reactivity has low parallel connection loss and achieves low loss characteristics.

On the other hand, in order to suppress in-band ripple,
It is necessary to clarify the relationship between the diffraction phenomenon and the aperture length of a one-aperture surface acoustic wave resonator. FIG. 11 is an example of measurement of the relationship between the in-band ripple Vp and the aperture length W of a surface acoustic wave resonator composite filter. The aperture length in the figure is expressed as a multiple of the wavelength λ. As is clear from the figure, in order to suppress in-band ripples in the filter, it is necessary to configure the filter with a single-aperture surface acoustic wave resonator having a large aperture length. Considering the practical level of the duplexer, the permissible limit value of in-band ripple is estimated to be 0.3 dB. Therefore, the aperture length of a single-aperture surface acoustic wave resonator with a low resonant frequency used on the common terminal side is set to 1 of the wavelength λ.
By setting it to 0 times or more, the characteristics of a surface acoustic wave resonator composite filter for a duplexer can be realized. Furthermore, a more desirable aperture length is at least 20 times the wavelength λ at which the in-band ripple is about 0.1 dB or less. The surface acoustic wave resonator composite filter of the present invention described above has superior power durability characteristics compared to transversal filters. The microchip, which is made up of a plurality of substrates and has a total area of about 10 (mm2), can withstand output power on the order of watts, and is particularly advantageous when an output of 0.1 W or more is required. Therefore, the duplexer using the surface acoustic wave resonator composite filter of the present invention is suitable as an antenna duplexer for mobile radio terminals such as portable telephones, which require output power on the order of watts and be compact and lightweight. FIG. 12 shows a configuration using the duplexer 122 of the present invention as shown in FIG.
1 is a block diagram of a portable telephone 120. FIG. The speaker's voice is converted into an electrical signal by the microphone 126 and input to the transmitter 125. Here, the input signal is modulated and amplified into a transmission signal. The output is passed through a duplexer 122,
It is transmitted to the outside from the antenna 121. Further, the signal received by the antenna 121 is leaked by the duplexer 122 and then input to the receiving section 123, where it is amplified and demodulated to be reproduced as sound by the speaker 124. The logic part 127 follows the signal from the cellular radio base station.
Configure channels within the cell. As in the above embodiment, by using the duplexer of the present invention in a mobile radio terminal,
This allows devices to be smaller, lighter, and consume less power.

By using the surface acoustic wave resonator composite filter of the present invention in a duplexer as shown in the embodiment of FIG. 1, it is possible to reduce loss in the falling portion of the passband.

[0028]

[Effects of the Invention] By using the duplexer of the present invention, a high power transmission signal from a transmission filter via an antenna,
It is possible to propagate weak power reception signals from the antenna through the reception filter without attenuating each signal. As a result, it is possible to improve the quality of the transceiver and reduce the power consumption of the device.

The above-mentioned duplexer is applicable not only when a single antenna is shared for transmission and reception, but also to a duplexer etc. in which another local oscillation filter and a reception filter are connected in parallel to form a mixer. A similar effect can also be obtained by

[Brief explanation of the drawing]

FIG. 1 is a diagram of an embodiment of a duplexer using a surface acoustic wave resonator composite filter according to the present invention and an electrical equivalent circuit of the filter.

FIG. 2 is a diagram showing a configuration example of a duplexer.

FIG. 3 is a diagram showing an example of frequency characteristics of each filter of a duplexer.

FIG. 4 is a diagram illustrating a configuration example of a conventional surface acoustic wave resonator composite filter.

FIG. 5 is a diagram of an example of impedance characteristics seen from two terminals of the filter shown in FIG. 4;

FIG. 6 is a diagram showing the structure of a one-aperture surface acoustic wave resonator.

FIG. 7 is a diagram showing an example of impedance characteristics of a conventional one-aperture surface acoustic wave resonator.

FIG. 8 is a diagram showing an example of characteristics of a conventional surface acoustic wave resonator composite filter.

9 is a diagram showing an example of the characteristics of the surface acoustic wave resonator composite filter used in the embodiment of FIG. 1; FIG.

FIG. 10 is a diagram showing an electrical equivalent circuit of another embodiment according to the present invention.

FIG. 11 is a diagram showing the relationship between the aperture length and in-band ripple of a single-aperture surface acoustic wave resonator constituting a surface acoustic wave resonator composite filter.

FIG. 12 is a diagram showing an example of the configuration of a mobile radio terminal using the duplexer of the present invention.

[Explanation of symbols]

1... Antenna, 2... Reception filter, 3... Transmission filter,
4-1 to 4-6... Resonator electrode, 5-1, 5-2... Gap capacitance electrode, 7... Piezoelectric substrate, 9... Reception filter terminal, 1
0...Transmission filter terminal, 16...1 aperture surface acoustic wave resonator.

Claims (3)

[Claims] Claims: 1. A branching circuit comprising a plurality of filters having different passbands connected in parallel from a branch point, in which at least one of the two filters connected to the branch point includes a capacitive element on a piezoelectric substrate; An elastic surface formed by forming a series circuit in which a plurality of circuit elements including surface acoustic wave resonators are connected in series, and a second capacitive element and a surface acoustic wave resonator between the electrodes of the circuit elements and a common ground. In a duplexer that is a wave resonator composite filter, in each of the series circuit element or the second circuit element of the surface acoustic wave filter, the average of the resonant frequencies of all the resonators forming the filter on the branch point side. A duplexer characterized in that it is configured by connecting surface acoustic wave resonators having a resonant frequency lower than the value of the resonant frequency. 2. The duplexer according to claim 1, wherein each of the series circuit elements of the surface acoustic wave filter or the second circuit element generates surface acoustic wave resonance of a lower resonant frequency in order from the branching point side. A duplexer is characterized in that it is configured by connecting two devices. 3. The duplexer according to claim 1 or 2, wherein the aperture length of the low resonant frequency surface acoustic wave resonator connected to the branching point side is 10 times or more the arrangement period of the interdigital electrodes. A duplexer characterized in that it is constructed using a surface wave resonator composite filter.