JPS5852371B2 - surface acoustic wave filter - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to improvements in surface acoustic wave filters constructed using surface acoustic wave circuits.

This kind of surface acoustic wave filter is made of a piezoelectric substrate such as crystal or lithium niobate, or a piezoelectric material is placed on a non-piezoelectric substrate, as shown in FIG. first and second transducers Q1 and Q2 that convert an electrical signal formed on a substrate 1 having a magnetic layer into a surface acoustic wave or vice versa;
, each of the second order transducers Q1 and Q2 has first and second electrodes E1 and E2 disposed on the substrate 1, and each of the second order electrodes E1 and E2 is connected to the transducer Q.
It has a plurality of electrode elements e arranged sequentially in a first direction a connecting E1 and Q2 and extendable in a second direction substantially perpendicular to the first direction a, and the electrodes E1 and E2 are connected to the electrode E1. The - electrode elements of the electrode E2 are arranged between the adjacent electrode elements of the transducer. Input terminals T1 and T2 from electrodes E1 and El of Q1
However, it is usually constructed using a surface acoustic wave circuit U having a configuration in which output terminals 01 and 02 are led out from electrodes E1 and El of transducer Q2, respectively.

However, such a surface acoustic wave circuit U generally has electrodes E of transducers Q1 and Q2, as shown by curve 3 in FIG.
Frequencies fL and fH according to the intervals that take the maximum and minimum values in the intervals between successive adjacent electrode elements of 1 and El.
Although it has a band-pass characteristic in which the band in between is a pass band, the band-pass characteristic is generally obtained with a Fresnel ripple as shown by the symbol d in the pass band.

Furthermore, the surface acoustic wave circuit U described above has an admittance as seen from the input terminals T1 and T2 side to the transducer Q1 side, and an admittance seen from the output terminals 01 and 02 side to the transducer Q2 side.
When looking at the admittance from the side, generally, when G is the conductance component and jB is the susceptance component, generally Y=G+jB・・・・・・・・・・・・・・・・・・・・・
... exhibits the admittance Y given by (1), but since the conductance component B and susceptance component jB of the admittance Y take values according to the frequency, the Smith admittance diagram in Fig. 3 As is generally indicated by the line y1 above, there is a significant difference between the value at frequency fL and the value at frequency fH, and therefore the surface acoustic wave circuit U described above has The impedance of the transducer Q2 from the input terminals T1 and T2 and the transducer Q2 from the output terminals 01 and 02 in its passband, assuming that it has a bandpass characteristic.
The impedances seen from the side are generally not matched.

Therefore, if a surface acoustic wave filter is constructed using only the surface acoustic wave circuit U described above, and in which the input terminals T1 and T2 are used as an input terminal pair, and the output terminals 01 and 02 are used as an output terminal pair. , the surface acoustic wave filter generally exhibits bandpass characteristics as shown in Fig. 2, but the bandpass characteristics are obtained with Fresnel ripples in the passband, and the input terminal pair and output terminal The impedances seen from the side of the surface acoustic wave circuit U from the pair of husbands were not matched in the pass band.

For this reason, conventionally, the above-mentioned surface acoustic wave circuit U is used, and between its input terminals T1 and T2, and between its output terminals 01 and 0.
A surface acoustic wave filter has been proposed in which an inductance element for impedance matching is connected between the two.

However, in the case of such a surface acoustic wave filter, between the input terminals T1 and T2 of the surface acoustic wave circuit U, and the output terminal 01
By appropriately selecting the constant of the impedance matching inductance element connected between Although obtained as an improved match,
It was not possible to obtain a sufficiently improved matching, and there was hardly any improvement in the bandpass characteristics accompanied by Fresnel ripples in the passband. Met.

Conventionally, in the surface acoustic wave circuit U shown in FIG. A surface acoustic wave filter constructed using only surface acoustic wave circuits similar to the surface acoustic wave circuit U has also been proposed.

However, in the case of such a surface acoustic wave filter, since the plurality of electrode elements of the transducer electrode in the surface acoustic wave circuit are weighted in terms of length, by appropriately selecting the weights, Although the band-pass characteristics may improve the Fresnel ripple in the pass band to some extent, it is possible to improve the Fresnel ripple in the passband to some extent by using a surface acoustic wave circuit with length-related weights between multiple electrode elements of the transducer electrode. In addition, there was a manufacturing problem in that the length of the electrode elements of a transducer in a surface acoustic wave circuit was small. Since the surface acoustic wave is diffracted at the position of the electrode element, it has not been possible to obtain sufficiently satisfactory bandpass characteristics in the passband.

Further, there is little improvement in the problem that the impedances seen from the input terminal pair and the output terminal pair toward the surface acoustic wave circuit are not matched in the pass band.

Therefore, the present invention provides the surface acoustic wave circuit U described above in FIG.
Even if a surface acoustic wave circuit such as the one shown in FIG. The purpose of the present invention is to propose a new surface acoustic wave filter whose impedance is matched in the pass band, and this will become clear from the detailed description below.

FIG. 4 shows an example of a surface acoustic wave filter according to the present invention, in which the surface acoustic wave circuit U and the input terminal T shown in FIG.
3 and T4 and the input terminals T1 and T2 of the surface acoustic wave circuit U
a matching circuit M1 having output terminals 03 and 04 respectively connected to the output terminals 01 and 02 of the surface acoustic wave circuit U;
Input terminals T5 and T6, respectively connected to output terminal 05
and 06.

In this case, the matching circuit M1 includes a capacitance element C1 connected between input terminals T3 and T4, an inductance L1 connected between input terminal T3 and output terminal 03, and an inductance element connected between output terminals 03 and 04. It is composed of L2.

Furthermore, the matching circuit M2 includes an inductance element L3 connected between input terminals T5 and T6, an inductance element L4 connected between input terminal T5 and output terminal 05, and a capacitance element connected between output terminals 05 and 06. C
It is composed of 2.

Furthermore, input terminals 1 and 2 as surface acoustic wave filters are connected to input terminals T3 and T4 of the matching circuit M1, and output terminals H1 and H2 as surface acoustic wave filters are connected to output terminals 05 and 06 of the matching circuit M2 as surface acoustic wave filters. are derived respectively.

The above is an example of the configuration of a surface acoustic wave filter according to the present invention. According to this configuration, the surface acoustic wave circuit U described above in FIG. 1 is used, and the input side of the surface acoustic wave circuit U is Although matching circuits M1 and M2 are provided on the output side, as will be explained later, if the matching circuits M1 and M2 do not have the second matching circuits M1 and M2 and are elastic. Assuming that a surface acoustic wave filter is constructed with only the surface wave circuit U, in this case, the surface acoustic wave circuit U side is seen from between the input terminals 1 and 2, and between the output terminals H1 and H2. In order to compensate for the fact that the impedance cannot be matched in the passband as described above with reference to FIG.
If the surface acoustic wave filter is constituted by only the surface acoustic wave circuit U without the secondary matching circuits M1 and M2, the bandpass characteristic in this case is as shown in FIG. 1 above. A surface acoustic wave filter constructed only of surface acoustic wave circuits U is provided in order to avoid the occurrence of Fresnel ripples in the passband as shown in FIG. Electrodes E1 and E of transducers Q1 and Q2 of surface acoustic wave circuit U as well as the bandpass characteristics of
It exhibits a band-pass characteristic in which the frequency bands fL and fH correspond to the maximum and minimum intervals between two adjacent electrode elements, respectively.

However, in the case of the surface acoustic wave filter according to the present invention described above, matching circuits M1 and M2 are provided on the input side and output side of the surface acoustic wave circuit U, respectively, and the input terminal is connected from the input side of the matching circuit M1 to the input terminal. Since 1 and 2 have a configuration in which the output terminals H1 and H2 are derived from the output side of the matching circuit M2, the input terminal has an impedance as seen from the surface acoustic wave circuit U side from 1 and 2, and the output terminal. H1 and H
2, the impedances seen on the U side of the surface acoustic wave circuit are obtained as being matched in the passband, and the bandpass characteristics are obtained as being substantially free of Fresnel ripples in the passband. .

The reason for this will be described below.If the matching circuits M1 and M2
If the surface acoustic wave filter is configured with only the surface acoustic wave circuit U without the The observed admittance is the same as the admittance Y described above when looking at the transducer Q1 and Q2 sides from the input terminals T1 and T2 and the output terminals 01 and 02, respectively, of the surface acoustic wave circuit U, and therefore, it is the same as the admittance Y seen on the Smith admittance diagram. This was obtained as shown by y1 in Fig. 3.

However, it is assumed that the matching circuits M1 and M2 are provided, provided that the matching circuit M1 has a capacitance element C1 and an inductance element L2 having an inductance element L1.
If the matching circuit M2 has only an inductance element L3 having a capacitance element C2 and an inductance element L4, the values of the inductance elements L2 and L3 are set to the same inductance La.
And when the frequency is f, 1 and 2 are connected to the input terminal.
, and the admittance seen from the output terminals H1 and H2 toward the surface acoustic wave circuit U side (this is designated as Y2), the admittance Y (this is designated as Yl) described above is obtained in proportion to 1/2πfLa. Therefore, if the inductance La is appropriately selected, each point on the line y1 passes through a circle with constant conductance on the Smith admittance diagram as shown in Figure 5A, and the lower the frequency f, the greater the amount. This is obtained as shown by the line y2, which is equivalent to that obtained by rotating counterclockwise.

It also has matching circuits M1 and M2, provided that the matching circuit M1 has inductance elements L1 and L2 having a capacitance element C1, and the matching circuit M2 has inductance elements L3 and L4 having a capacitance element C2. If the inductance elements L1 and L4 have the same inductance Lb, the input terminals have 1 and 2, and the admittance seen from the output terminals H1 and H2 toward the surface acoustic wave circuit U side (this is called Y3). ), the admittance Y2 mentioned above is 2π,
Since it is obtained in proportion to fLb, if the inductance Lb is appropriately selected, each point on the line y2 on the Smith admittance diagram will be centered on the center of the Smith admittance diagram, as shown in Figure 5B. The result obtained by passing on a concentric circle with , and rotating clockwise by an amount that increases the higher the frequency f, as shown by the line y3, which intersects the circle of G = 1. It is.

Furthermore, if it has matching circuits M1 and M2, but if the matching circuit M1 has a capacitance CL and inductances L1 and L2, and the matching circuit M2 has a capacitance C2 and inductances L3 and L4, then the capacitance When elements C1 and C2 have the same capacitance Ca, 1 is applied to the input terminal.
and 2, and the admittance seen from the output terminals H1 and H2 toward the surface acoustic wave circuit U side (this is referred to as Y4), since the admittance Y3 mentioned above is obtained in proportion to 2πfCa, set the capacitance Ca appropriately. If we select , as shown in Figure 5C on the Smith admittance diagram, each point on line y3 is obtained by rotating clockwise on a circle with constant conductance by an amount that increases as the frequency increases. This is obtained as shown by the line y4, which is equivalent to the line y4 and roughly follows a small circle centered on the center of the Smith admittance diagram.

Therefore, the admittance Y4 obtained on the Smith admittance line as shown by the line y4 is obtained by inputting terminals 1 and 2 at the input terminals and output terminal H1 of the surface acoustic wave filter shown in FIG. 4 according to the present invention. and H2, which is the admittance seen from the surface acoustic wave circuit U side.

Therefore, according to the surface acoustic wave filter according to the present invention shown in FIG. It is something that can be obtained if you are able to get it.

As described above, according to the surface acoustic wave filter according to the present invention, impedances 1 and 2 at the input terminals and impedances viewed from the output terminals H1 and H2 toward the surface acoustic wave circuit U side are matched in the pass band. Next, we will discuss the reason why the bandpass characteristic can be obtained with substantially no Fresnel ripple in the passband. When a surface acoustic wave filter is constructed of only the surface acoustic wave circuit U, the surface acoustic wave filter exhibits an impulse response function expressed by eJllt when time is t and f=2 μt. Therefore, the bandpass characteristic of the surface acoustic wave filter exhibits a Fresnel ripple in the passband as shown by curve d of curve 2 in FIG.

Since each of the matching circuits M1 and M2 is composed of an inductance element and a capacitance element, it exhibits a frequency function (this will be referred to as β), and its frequency function β(f) The frequency f is f=2μt
Therefore, each of the matching circuits M1 and M2 exhibits a time function β (2 μt).

Therefore, the surface acoustic wave filter according to the present invention having matching circuits Ml and Ml on the input side and output side of the surface acoustic wave circuit U, respectively, exhibits an impulse response function expressed by β(2μt)·ejμt2.

In the case where the surface acoustic wave filter is constituted by only the surface acoustic wave circuit U without the matching circuits M1 and Ml,
The surface acoustic wave filter exhibits an impulse response function expressed by <eJμt as described above, and the electrodes E1 and E2 of the transducers Q1 and Q2 of the surface acoustic wave circuit U are connected between successive adjacent electrode elements. Since the values increase or decrease sequentially at intervals, the instantaneous frequency of the impulse response function eJμt corresponds to 2μt and changes with time t.

Therefore, according to the surface acoustic wave filter according to the present invention, which has the matching circuits M1 and Ml on the input side and the output side of the surface acoustic wave circuit U, respectively, as described above (as described above),
, and the impedances seen from the output terminals H1 and H2 toward the surface acoustic wave circuit U are matched in the passband, that is, 1 and 2 at the input terminals, and 2 at the output terminals H1 and From the state in which the admittance seen from H2 to the surface acoustic wave circuit U side is obtained as shown by line y4 in Figure 5C on the Smith admittance diagram, it is 6th on the Smith admittance diagram. An admittance as shown by line y5, which is approximately along a small circle or ellipse centered on a point slightly eccentric from the center of the Smith admittance diagram as shown in the figure, is connected to input terminals 1 and 2, and output terminal H1. As can be seen from the surface acoustic wave circuit U side from H2 and H2, matching circuits M1 and Ml
By appropriately changing the inductance of the inductance element and the capacitance of the capacitance element constituting the circuit, a band-pass characteristic can be obtained in which substantially no Fresnel ripple occurs in the pass band, as shown by the curve ■ in Fig. 2. It is something.

Therefore, according to the surface acoustic wave filter according to the present invention shown in FIG. 4, a bandpass characteristic substantially free of Fresnel ripples in the passband can be obtained.

As described above, according to the surface acoustic wave filter according to the present invention, a bandpass characteristic substantially free of Fresnel ripples in the passband can be obtained, and surface acoustic wave It has the characteristic that the impedance seen on the circuit side is matched in the pass band.

Further, according to the surface acoustic wave filter according to the present invention, since there is no length weighting between the plurality of electrode elements of the transducer electrode of the surface acoustic wave circuit used therein, the transducer electrode The manufacturing problem is that length weighting must be applied between the plurality of electrode elements of the transducer electrode. It also has the characteristic that it does not have the problem of not being able to obtain sufficiently satisfactory bandpass characteristics in the passband due to diffraction of surface acoustic waves at the position of the electrode element where the weight is small. It is something that you have.

In the above description, it is assumed that the admittance or impedance viewed from the input terminal side of the surface acoustic wave circuit U to the transducer Q1 side is equal to the admittance or impedance viewed from the output side to the transducer Q2 side, and that the matching circuit M1 The inductance of inductance element L1 and that of inductance element L4 of matching circuit M2, the inductance of inductance element L2 and that of inductance element L3, and the capacitance of capacitance element C1 and that of capacitance element C2,
Although the explanation has been made assuming that the two are equivalent to each other, it is clear that the above-mentioned characteristics can be obtained even if they are not equal in this way.

Furthermore, it is clear that the above-mentioned characteristics can be obtained whether the inductance elements and capacitance elements constituting the above-mentioned matching circuits M1 and Ml are composed of lumped constant elements or distributed constant elements. Dew.

[Brief explanation of drawings]

Fig. 1 is a route diagram showing an example of a surface acoustic wave circuit applicable to the surface acoustic wave filter according to the present invention, Fig. 2 is a diagram showing its admittance on a Smith admittance diagram, and Fig. 3 is a diagram showing a conventional surface acoustic wave circuit. A diagram showing bandpass characteristics of a surface acoustic wave filter and a surface acoustic wave filter according to the present invention, FIG. 4 is a route map showing an example of a surface acoustic wave filter according to the present invention, and FIG. 5A
-C and FIG. 6 are diagrams showing admittance on a Smith admittance diagram for explaining the present invention. In the figure, U is a surface acoustic wave circuit, Ml and M2 are matching circuits,
1 is a substrate, Ql and Q2 are transducers, El and E2 are electrodes, e is an electrode element, T1 to T6, 1 and 2 are input terminals, 01 to 06°Hl and H2 are output terminals,
L1 to L4 represent inductance elements, and C1 and C2 represent capacitance elements, respectively.

Claims (1)

[Claims] 1. A surface acoustic wave circuit having first and second input terminals and first and second output terminals, a third and fourth input terminal, and a first surface acoustic wave circuit of the surface acoustic wave circuit. and third and fourth output terminals respectively connected to the second input terminal; and fifth and fourth matching circuits respectively connected to the first and second output terminals of the surface acoustic wave circuit. a second matching circuit having a sixth input terminal and fifth and sixth output terminals;
The surface acoustic wave circuit has first and second transducers that convert an electrical signal formed on a substrate that propagates a surface acoustic wave into a surface acoustic wave or vice versa, and the first and second transducers each has first and second electrodes disposed on the substrate, each of the first and second electrodes being sequentially disposed in a first direction connecting the first and second transducers. The invention further includes a plurality of electrode elements extending in a second direction substantially orthogonal to the first direction, and the first and second electrodes are arranged between adjacent electrode elements of the first electrode. The negative electrode elements of the second electrode are arranged in a relationship such that the spacing between successively adjacent electrode elements of the first and second electrodes is made larger or smaller in order, and 1
first and second electrodes of the transducer are connected to the first and second input terminals, respectively, and first and second electrodes of the second transducer are connected to the first and second output terminals, respectively. and the first matching circuit includes a first capacitance element connected between the third and fourth input terminals, and a first capacitance element connected between the third input terminal and the third output terminal. and a second inductance element connected between the third and fourth output terminals, and the second matching circuit is connected between the fifth and sixth input terminals. a fourth inductance element connected between the fifth input terminal and the fifth output terminal, and a second inductance element connected between the fifth and sixth output terminals. a capacitance element, the third and fourth input terminals of the first matching circuit are used as an input terminal pair, and the fifth and sixth output terminals of the second matching circuit are used as an output terminal pair. A surface acoustic wave filter characterized by: