JPH0715276A - Compound filter - Google Patents

Abstract

PURPOSE:To reduce attenuation at a passed band and to enlarge attenuation at a stopped band by providing a circuit equipped with a band stop characteristic at serial arms and a circuit to be serially resonated at a prescribed frequency at parallel arms and providing stopped band attanuation and shield with those circuits. CONSTITUTION:The compound filter is provided with a dielectric resonantor DR and a serial capacitor Cs at the parallel arms and equipped with SAW (surface acoustic wave) blocks 10 composed of four SAW resonators at the serial arms. Capacitors C11 and C12 and inducers L11 and L12 are also provided with functions as a matching circuit and a low-pass filter. For example, the amount of attenuation at the stopped band is a characteristic at the degree of about 50dB and the loss within the passed band is a characteristic at the degree of about 0.5dB-1.0dB. In this case, the respective merit of the dielectric resonator and the SAW resonator is effectively presented, and the stopped-band attenuation amount is secured. On the other hand, a cut-off characteristic can be secured differentially from the configuration of only the dielectric resonator DR.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a filter mounted on a mobile communication device, for example.

[0002]

2. Description of the Related Art Various filters are mounted on a communication device. When the types of mounted filters are classified according to their characteristics, they are classified into bandpass filters, lowpass filters, highpass filters, bandstop filters, and the like. Also,
When classified according to the configuration method, it can be classified into an LC filter, a dielectric filter, a surface acoustic wave filter, and the like.

Of these filters, the dielectric filter is usually a filter constructed by using a parallel resonator utilizing quarter wavelength resonance. The equivalent circuit of this resonator, that is, the dielectric resonator is a parallel resonance circuit as indicated by DR in FIG. In this figure, a resistance is drawn in parallel with the equivalent capacitance and the inductance of the dielectric resonator DR. This resistance represents the Q of the dielectric resonator DR.

[0004] For example a capacitor C _s in series with the dielectric resonator DR connected in series, placing it in parallel arms, the capacity of the equivalent inductance and a capacitance element C _s of the dielectric resonator DR is series resonance, A band stop filter is obtained. The characteristic of this band elimination filter depends on the capacitance value of the capacitive element C _s as shown in FIG. The characteristics of this figure are
As shown in, the calculation is performed through a matching circuit including inductive elements L _m1 and L _m2 and capacitive elements C _m1 and C _{m 2} . For comparison, the capacitance value of the capacitive element C _s is adjusted to (a) while adjusting the resonance frequency to the same value.
Calculations are performed for two types of configurations, 1.5 pF and (b) 5.48 pF. As you can see from this figure,
The smaller the capacitance value of the capacitive element C _s, the steeper the cutoff is obtained, and the larger the capacitance value, the larger the stopband attenuation amount.

As described above, as the advantage of the dielectric filter, since the resonator is arranged in the parallel arm, the insertion loss in the pass band can be lowered to about 1 dB, and the attenuation amount in the stop band can be increased. is there. On the other hand, as a drawback, it is difficult to realize a steep characteristic because Q is as low as about 200 to 300, and therefore it is necessary to design the filter while considering the relationship with the series capacitance.

Among the filters that can be used in the communication device, the surface acoustic wave filter is a filter that utilizes surface acoustic wave resonance on the surface of the piezoelectric body. That is,
The surface acoustic waves are excited by the electrodes formed on the surface of the piezoelectric body to generate resonance. The equivalent circuit of the surface acoustic wave resonator has a circuit configuration as shown in FIG. Therefore,
By providing this surface acoustic wave resonator on the series arm, band stop characteristics can be obtained. An advantage of the surface acoustic wave filter is that steep characteristics can be obtained. The disadvantage is the low bandwidth.

FIG. 7 shows an example of characteristics required for the filter. For example, in a demultiplexing circuit of a mobile communication device, that is, a circuit that separates a transmission frequency and a reception frequency, as shown in this figure, the attenuation at the transmission frequency is small, the attenuation at the reception frequency is large, and the cutoff It is necessary to have a filter on the transmission side that is steep. The steep characteristics are required because the transmission frequency and the reception frequency are close to each other.

In order to realize such required specifications with a dielectric filter, it is necessary to have a circuit configuration as shown in FIGS. 8 and 9, for example. The circuit shown in this figure includes three dielectric resonators DR arranged in parallel arms.
_{1 to} DR ₃ , and the capacitive elements CS _{1 to} C serially connected thereto
S _3, and a capacitive element _C 1 -C ₂ arranged in the inductive element _L 1 ~L ₂ and the parallel arm are arranged in the series arm.

When adopting such a configuration, the capacitances of the capacitive elements CS _{1 to} CS ₃ are increased. That is, in terms of focusing on that attenuation of the stop band higher the series capacitance value is large, to increase the capacitance of the capacitor CS ₁ to CS _3,
The lack of cut-off characteristics is compensated by using multiple resonators.

In order to realize the required specifications as shown in FIG. 7 with a surface acoustic wave filter, for example, FIG.
It is necessary to have a circuit configuration as shown in. In this figure, SAW _{1 to} SAW ₄ are surface acoustic wave resonators, and these four surface acoustic wave resonators SAW _{1 to} SAW _{4 are shown.}
Are cascaded in series arms. Also, C _j11
-C _J42, each surface acoustic wave resonator _SAW 1 _{~SAW 4}
Is the electrode stray capacitance of. Further, C _p1 and C _p2 are the pad of the package (not shown) and the surface acoustic wave resonator SAW.
₁ , stray capacitance between SAW ₄ and L _p1 and L _p2 are stray inductances between the pad and the surface acoustic wave resonators SAW ₁ and SAW ₄ .

In the case of this configuration, each surface acoustic wave resonator SA
The resonance frequencies of W _{1 to} SAW ₄ are set to frequencies slightly different from each other. This compensates for the narrow stopband. However, as is clear from FIG. 11 in which the characteristics of the circuit of FIG. 10 are measured through the matching circuit, four SAW resonators cannot secure a sufficient bandwidth.

[0012]

As described above, when a filter having a small attenuation in the pass band, a large attenuation in the stop band, and a sharp cutoff is required, it is realized as a dielectric filter. In order to obtain a steep characteristic, many dielectric resonators must be used. Further, in order to realize it as a surface acoustic wave filter, it is necessary to use a large number of surface acoustic wave resonators in order to secure a stop band width. Also, the use of a large number of resonators in the series arm leads to an increase in insertion loss and a deterioration in power resistance.

The present invention has been made to solve the above-mentioned problems, and has a small attenuation in the pass band without blocking the use of a large number of resonators, and hence the enlargement of the structure, and prevents it. The purpose of the present invention is to realize a filter with a large attenuation in the band and a sharp cutoff.

[0014]

In order to achieve such an object, a composite filter of the present invention comprises a surface acoustic wave resonance circuit arranged in a series arm and having a band stop characteristic, and a predetermined frequency arranged in a parallel arm. A dielectric resonance circuit that resonates in series with
And the stop band attenuation is realized by the dielectric resonance circuit, and the cutoff is realized by the surface acoustic wave resonance circuit.

The composite filter of the present invention is characterized in that the surface acoustic wave resonance circuit includes a plurality of surface acoustic wave resonators having different resonance frequencies and connected in cascade.

The composite filter of the present invention is characterized in that a plurality of surface acoustic wave resonators are housed in one package.

The composite filter of the present invention is characterized in that the dielectric resonance circuit includes a dielectric element that functions as an inductive element at a predetermined frequency, and a capacitive element connected in series with this dielectric element.

The composite filter of the present invention is characterized in that the mobile communication transmission signal is filtered and supplied to the antenna side.

A communication apparatus of the present invention is characterized by including the composite filter of the present invention, and filtering a signal by the composite filter.

In the method of designing the composite filter of the present invention, the dielectric resonance circuit is arranged in the parallel arm and the surface acoustic wave resonance circuit is arranged in the series arm, and the series capacitance of the dielectric resonance circuit is relatively large. It is characterized by increasing the stopband attenuation amount by setting the number of surface acoustic wave resonators constituting the surface acoustic wave resonance circuit and decreasing the insertion loss.

[0021]

In the composite filter of the present invention, the surface acoustic wave resonance circuit is arranged in the series arm and the dielectric resonance circuit is arranged in the parallel arm. The dielectric resonance circuit has a stopband attenuation characteristic, and the surface acoustic wave resonance circuit has a cutoff characteristic.
Therefore, a filter having a small attenuation in the pass band, a large attenuation in the stop band, and a sharp cutoff is realized. In that case, since it is not necessary to use a large number of resonators, the shape and size are also small.

Further, the surface acoustic wave resonance circuit can be composed of, for example, a plurality of surface acoustic wave resonators having different resonance frequencies and connected in cascade. In this way, the bandwidth becomes wider. At that time, the number of resonators is small. Further, if a plurality of surface acoustic wave resonators are housed in one package, the size can be further reduced. In addition, the dielectric resonance circuit
It can be composed of a dielectric element and a capacitive element connected in series to it.

The composite filter of the present invention can be used for filtering signals in various communication devices. For example, it can be used as a filter for filtering a transmission signal for mobile communication and supplying it to the antenna side. In other words, this type of filter can be made compact while sufficiently fulfilling the required use.

In the composite filter designing method of the present invention, the dielectric resonance circuit is arranged in the parallel arm and the surface acoustic wave resonance circuit is arranged in the series arm. With this,
The series capacitance of the dielectric resonant circuit is set to be relatively large, and thus the stopband attenuation amount is set to be large. Also,
The number of surface acoustic wave resonators forming the surface acoustic wave resonance circuit is set to a small number, thereby reducing the insertion loss.

[0025]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT A preferred embodiment of the present invention will be described below with reference to the drawings. The same components as those of the conventional example shown in FIGS. 4 to 11 are designated by the same reference numerals and the description thereof will be omitted.

FIG. 1 shows the structure of a composite filter according to the first embodiment of the present invention. Composite filter shown in this figure, on the parallel arm, and a dielectric resonator DR and the series capacitor element C _s. In addition, the surface acoustic wave block 10 composed of four surface acoustic wave resonators is provided in the series arm.
Is equipped with. The dielectric resonator DR has the same configuration as that of FIG. 4 described above, and the surface acoustic wave block 1
0 has the same configuration as in FIG. Capacitance element C ₁₁
C ₁₂ and the inductive elements L ₁₁ and L ₁₂ have a function as a matching circuit and a function as a low-pass filter.

FIG. 2 shows the insertion loss characteristic of this embodiment. As shown in this figure, the attenuation amount in the stop band is about 50 dB, and the loss in the pass band is 0.5 d.
Good characteristics of about B to 1.0 dB are realized. As described above, in this embodiment, a good cutoff characteristic and a large amount of attenuation can be obtained.

The reason why such characteristics are obtained is that the advantages of the dielectric resonator and the surface acoustic wave resonator are effectively exhibited. That is, unlike the case where the filter is configured by using only the surface acoustic wave resonator, the stop band attenuation amount can be secured by using the dielectric resonator DR, and thus the number of cascade connection stages of the surface acoustic wave resonator can be reduced. it can. Further, unlike the configuration using only the dielectric resonator DR,
Since the cut-off characteristic can be secured by the surface acoustic wave block 10, the capacitance value of the series capacitive element C _s can be increased, and as a result, an effective stopband attenuation amount can be secured. Further, when the filter according to the present embodiment is used as a transmission filter of a mobile communication device, the surface acoustic wave block 10 operates as a simple capacitor in the pass band where a large amount of power is transmitted, and energy conversion into elastic waves. Therefore, a filter having excellent power resistance can be obtained.

FIG. 3 shows the structure of a composite filter according to the second embodiment of the present invention. As shown in this figure, two surface acoustic wave blocks 10-1 and 10-2 may be arranged in the series arm. The dielectric block 12 is
This is a configuration including the above-mentioned dielectric resonator DR and series capacitance element C _s . Also, the surface acoustic wave blocks 10-1 and 10-
2 are housed in a single package 14.

With such a structure, that is, a plurality of surface acoustic wave blocks 10-1 and 10-1 in a single package.
With the configuration for storing 0-2, the device configuration can be further downsized. In addition, each surface acoustic wave block 10-
If a plurality of surface acoustic wave resonators are enclosed in 1 and 10-2, the stop band width can be appropriately widened.

In the above description, the case where the pass band is higher than the stop band has been described.
Even in the case where the pass band is lower than the stop band, the composite filter can be designed based on the same idea and the same effect can be obtained.

[0032]

As described above, according to the present invention,
Since the surface acoustic wave resonance circuit is arranged in the series arm and the dielectric resonance circuit is arranged in the parallel arm, the stop band attenuation characteristic is realized by the dielectric resonance circuit and the cutoff characteristic is realized by the surface acoustic wave resonance circuit. A filter having a small attenuation in the pass band, a large attenuation in the stop band, and a sharp cutoff can be realized in a small size. Therefore, it is possible to obtain a filter suitable for various communication devices, particularly for filtering the transmission signal for mobile communication. In other words, this type of filter can be made compact while sufficiently fulfilling the required use.

Further, since the surface acoustic wave resonance circuit has a structure in which a plurality of surface acoustic wave resonators having different resonance frequencies are connected in series, the number of resonators can be reduced while widening the bandwidth. Further, since a plurality of surface acoustic wave resonators are housed in one package, the size becomes smaller. Further, the dielectric resonance circuit can be composed of a dielectric element and a capacitive element connected in series with the dielectric element.

According to the composite filter designing method of the present invention, the dielectric resonance circuit is arranged in the parallel arm and the surface acoustic wave resonance circuit is arranged in the series arm, and the series capacitance of the dielectric resonance circuit is made relatively small. Since it is set to a large value, the stopband attenuation amount can be increased. Further, since the number of surface acoustic wave resonators forming the surface acoustic wave resonance circuit is set to be small, insertion loss can be reduced.

[Brief description of drawings]

FIG. 1 is a block diagram showing a configuration of a composite filter according to a first embodiment of the present invention.

FIG. 2 is a characteristic diagram showing an insertion loss characteristic of the first embodiment.

FIG. 3 is a block diagram showing a configuration of a composite filter according to a second embodiment of the present invention.

FIG. 4 is an equivalent circuit diagram showing an equivalent circuit configuration of a dielectric resonator.

FIG. 5 is a characteristic diagram showing insertion loss characteristics of a dielectric resonator.

FIG. 6 is an equivalent circuit diagram showing an equivalent circuit configuration of a surface acoustic wave resonator.

FIG. 7 is a diagram showing an example of required specifications.

FIG. 8 is a circuit diagram showing a configuration of a filter according to a first conventional example.

FIG. 9 is an equivalent circuit diagram showing an equivalent circuit configuration of a first conventional example.

FIG. 10 is a circuit diagram showing a configuration of a filter according to a second conventional example.

FIG. 11 is a characteristic diagram showing an insertion loss characteristic of a second conventional example.

[Explanation of symbols]

DR dielectric resonator C _s series capacitor element 10,10-1,10-2 SAW block 12 the dielectric block 14 Package

Claims (7)

[Claims] 1. A surface acoustic wave resonance circuit arranged in a series arm and having a band stop characteristic, and a dielectric resonance circuit arranged in a parallel arm that resonates in series at a predetermined frequency. A stop band attenuation is performed by the dielectric resonance circuit. Is realized by a surface acoustic wave resonance circuit. 2. The composite filter according to claim 1, wherein the surface acoustic wave resonance circuit includes a plurality of surface acoustic wave resonators having different resonance frequencies and connected in cascade. 3. The composite filter according to claim 2, wherein a plurality of surface acoustic wave resonators are housed in one package. 4. The composite filter according to claim 1, wherein the dielectric resonant circuit includes a dielectric element that functions as an inductive element at a predetermined frequency, and a capacitive element connected in series with the dielectric element. A featured composite filter. 5. The composite filter according to claim 1, wherein the transmission signal for mobile communication is filtered and supplied to the antenna side. 6. A communication device comprising the composite filter according to claim 1, wherein a signal is filtered by the composite filter. 7. A dielectric resonance circuit is arranged in the parallel arm and a surface acoustic wave resonance circuit is arranged in the series arm, and the series capacitance of the dielectric resonance circuit is set to be relatively large to increase the stopband attenuation. , A method of designing a composite filter, characterized in that the number of surface acoustic wave resonators constituting a surface acoustic wave resonance circuit is set to be small to reduce insertion loss.