JP3226645B2 - Surface acoustic wave filter - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a transversal type surface acoustic wave filter.

[0002]

2. Description of the Related Art FIG. 4 shows a general example of an electrode structure in a transversal type surface acoustic wave filter. In the example shown in the figure, a pair of comb-shaped electrodes 1 and 2 weighted with apodization are provided on the input side, and a pair of comb-shaped electrodes 3 and 4 not weighted with apodization are provided on the output side. To operate this as a filter, as shown in FIG. 5, a signal source 5 is connected to one of the comb electrodes 1 on the input side, the other comb electrode 2 is grounded, and An amplifier 6 is connected to one comb-shaped electrode 3 and the other comb-shaped electrode 4 is grounded.

However, in such a circuit configuration, the effective voltage Vin of the signal input to the comb electrode 1 on the input side and the potential (0) of the comb electrode 2 are output from the comb electrode 3 on the output side. The effective voltage Vout of the signal and the potential (0) of the comb-shaped electrode 4 are unbalanced. In some cases, amplitude ripples and group delay ripples occur, and desired filter characteristics cannot be obtained.

Therefore, as shown in FIG. 6, a signal source 5 is connected to each of the comb electrodes 1, 2 on the input side via a balun transformer 7.
(Hereinafter, the comb-shaped electrode 1 is hot1, and the comb-shaped electrode 2 is co.
Call it ld2. ), And each of the comb-shaped electrodes 3 and 4 on the output side (hereinafter, the comb-shaped electrode 3 is referred to as “hot3” and the comb-shaped electrode 4 is referred to as “cold4”). That is being done. With such a circuit configuration, when noise is present on the input side, in-phase noise is superimposed on hot1 and cold2 on the input side, and each noise signal passing through the filter is multiplied by a predetermined transfer function. Output hot3 and col
Although output from d4, the output noise signal is canceled by the differential amplifier 8 because it is in phase and the same magnitude. Further, noise newly loaded on the output side is similarly canceled by the differential amplifier 8. In addition, the induction between the input and output is performed by the comb electrodes 1 and 2 and the comb electrodes 3 on the piezoelectric substrate.
4 can be suppressed effectively by providing a shield electrode between them.

Further, the induction between the input and output of the external terminal is canceled by the differential amplifier 8 assuming that the surface acoustic waves reach the hot 3 and the cold 4 on the output side at the same level and in the same phase.

In addition, by using a balanced circuit configuration, induction can be electrically removed. Here, FIG. 7 shows the relationship between the voltage generated at the input side electrode and the signal source voltage in the case of the balanced type circuit configuration, and FIG. 8 shows the voltage generated at the input side electrode in the case of the unbalanced type circuit configuration. And the relationship between the signal and the signal source voltage. As shown in FIG. 7, in the case of a balanced circuit configuration, hot1 is 1/2 V, cold2
Has a potential of -1 / 2V. As shown in FIG. 8, in the case of an unbalanced circuit configuration, hot1 is V, cold2
Becomes 0 potential. Therefore, in the case of a balanced circuit configuration, the effective voltage of the electrode is 0, and in the case of the unbalanced circuit configuration, the effective voltage of the electrode is 1 / 2V. That is, if the effective voltage is 0, there is no electromagnetic radiation. Therefore, if a circuit configuration of a balance type is used, induction by electromagnetic radiation can be eliminated. This also applies to the output side electrode.

By the way, in a balanced circuit configuration, in order to make the effective voltage between hot and cold actually zero, it is assumed that hot and cold are geometrically symmetric. In other words, cold from the viewpoint of hot,
It is required that the hots seen from the cold are congruent.

However, in the conventional electrode structure shown in FIG. 4, since the hot electrode finger and the cold electrode finger cross each other, the hot and cold are not completely geometrically symmetric. . For this reason, hot, col
The effective voltage between d and d does not actually become 0, and
An electromagnetic wave is generated by generating a certain potential between t and cold, and this electromagnetic wave jumps into the output-side electrode as a direct wave, causing characteristic deterioration due to induction.

[0009]

As described above, even if the surface acoustic wave filter is configured as a balanced circuit, in the electrode structure of the conventional surface acoustic wave filter, hot and cold are not completely geometrically symmetric. Because of hot, cold
The effective voltage between them did not become 0, causing characteristic deterioration due to induction.

The present invention has been made in view of the above circumstances, and has a balanced type circuit configuration to reduce the effective voltage between opposing comb-shaped electrodes to zero, and to provide a surface acoustic wave filter having excellent induction resistance. It is intended to provide.

[0011]

Means for Solving the Problems In order to solve the problem, the present invention provides a pair of comb-shaped electrodes for exciting surface acoustic waves and a pair of comb-shaped electrodes for receiving surface acoustic waves on a piezoelectric substrate. , together arranged to rest on the same straight line with one another bus bars that is grounded between before Symbol interdigital electrodes of each pair, each of the comb electrodes of each pair interdigital electrodes of each pair across the bus bars The electrode fingers are arranged so as to face each other and are mirror-symmetrical, and are arranged so as to be mirror-symmetrical with the electrode fingers of the comb-shaped electrode intersecting and connected to the bus bar between the bus bars , And either one
The method is characterized in that the apodized weight is applied to the pair of electrode pairs .

In the above-mentioned surface acoustic wave filter, the bus bar sandwiched between each pair of comb-shaped electrodes may be constituted by one bus bar. Further, a shield electrode may be arranged between bus bars sandwiched between each pair of comb electrodes.

[0013]

In the present invention, since the comb-shaped electrode pairs are mirror-symmetrical with respect to each other with the bus bar interposed therebetween, the effective voltage between the comb-shaped electrode pairs is completely zero by employing a balanced circuit configuration. Therefore, no electromagnetic wave is radiated from each comb-shaped electrode pair, and no characteristic deterioration due to induction is caused.

[0014]

Embodiments of the present invention will be described below with reference to the drawings.

FIG. 1 is a plan view of a surface acoustic wave filter according to one embodiment of the present invention.

In FIG. 1, reference numeral 10 denotes a piezoelectric substrate. On the piezoelectric substrate 10, a pair of comb electrodes 11, 12 on the input side for exciting surface acoustic waves and a pair of comb electrodes 13, 14 on the output side for receiving surface acoustic waves are provided.
The comb-shaped electrodes 11 and 12 on the input side are weighted with apodization.

Between the comb electrodes 11 and 12, and between the comb electrodes 13 and 14, bus bars 15, 1
6 are arranged. The bus bar 15 and the bus bar 16 are arranged on the same straight line.

The electrode fingers 11a of the comb electrodes 11 and 12 sandwich the bus bar 15 therebetween.
12a are arranged so as to face each other and have mirror symmetry. Similarly, each of the comb electrodes 13 and 14 has the electrode fingers 13 a and 14 a of the comb electrodes 13 and 14 with the bus bar 16 interposed therebetween.
Are arranged so as to face each other and have mirror symmetry. Further, the bus bar 15 is provided with an electrode finger 15a intersecting with the electrode fingers 11a and 12a of the comb-shaped electrodes 11 and 12.
5 are connected so as to be mirror-symmetrical. Similarly, the bus bar 16 is also provided with an electrode finger 16 a crossing the electrode fingers 13 a and 14 a of the comb electrodes 13 and 14.
Are connected so as to be mirror-symmetrical with respect to.

Using the surface acoustic wave filter thus configured, a balanced circuit configuration as shown in FIG. 6 is obtained.

Here, when the comb-shaped poles 11 and 12 on the input side are driven by a balance signal, these comb-shaped poles 11 and 12 are driven.
Are arranged so that the electrode fingers 11a and 12a of the comb-shaped electrodes 11 and 12 face each other with the bus bar 15 therebetween and are mirror-symmetrical, so that the effective voltage of the entire comb-shaped poles 11 and 12 on the input side is zero. Becomes Thereby, the comb-shaped pole 1 on the input side
Electromagnetic waves are not radiated from the electrodes 1 and 12 to the comb electrodes 13 and 14 on the output side.

The surface waves excited by the comb-shaped poles 11 and 12 and the bus bar 15 on the input side are received by the comb-shaped electrodes 13 and 14 and the bus bar 16 on the output side and are converted into electric signals. When the output signal is taken out as a balance signal and input to the differential amplifier 8 as shown in (1), the comb electrodes 13 and 14 on the output side are connected to the electrode fingers of the comb electrodes 13 and 14 with the bus bar 16 interposed therebetween. 13a, 14a
Are arranged so as to face each other and have mirror symmetry, so that the entire effective voltage of the comb electrodes 13 and 14 on the output side becomes 0 as in the case of the comb electrodes 11 and 12 on the input side. As a result, radiation of electromagnetic waves from the comb electrodes 13 and 14 on the output side to the comb electrodes 11 and 12 on the input side is eliminated.

As described above, when the surface acoustic wave filter of the present invention is used and the circuit configuration is of a balanced type as shown in FIG. 6, the comb-shaped poles 11 and 12 on the input side and the comb-shaped electrodes 13 on the output side are Since the radiation of the electromagnetic wave between the fourteen is eliminated, a filter having excellent induction resistance can be realized.

In the surface acoustic wave filter of the present invention,
The signal excited by the input-side bus bar 15 and the comb-shaped pole 11 is output by the output-side bus bar 16 and the comb-shaped electrode 13.
The signals excited by the busbar 15 on the input side and the comb-shaped pole 12 must be received by the busbar 16 and the comb-shaped electrode 14 on the output side.
6 are arranged on the same straight line.

Further, since the surface acoustic wave filter of the present invention can be realized by changing only the electrode structure, it can be manufactured by the conventional manufacturing process and equipment only by changing the exposure mask.

The present invention is not limited to the embodiment described above.

For example, as shown in FIG. 2, the bus bar sandwiched between each pair of comb electrodes may be constituted by one bus bar 17.

As shown in FIG. 3, a shield electrode 1 is provided between bus bars 15, 16 sandwiched between each pair of comb electrodes.
8 may be arranged.

[0028]

As described above, according to the surface acoustic wave filter of the present invention, the effective voltage between the opposing interdigital electrodes can be made zero by forming a balanced circuit configuration. No longer emits electromagnetic waves from
A filter having excellent induction resistance can be realized. Further, since only the electrode structure of the conventional surface acoustic wave filter needs to be changed, there is no problem in terms of manufacturing cost.

[Brief description of the drawings]

FIG. 1 is a plan view of a surface acoustic wave filter according to one embodiment of the present invention.

FIG. 2 is a plan view of a surface acoustic wave filter according to another embodiment of the present invention.

FIG. 3 is a plan view of a surface acoustic wave filter according to another embodiment of the present invention.

FIG. 4 is a plan view of an electrode structure in a conventional transversal surface acoustic wave filter.

FIG. 5 is a diagram showing an unbalanced circuit configuration using a surface acoustic wave filter.

FIG. 6 is a diagram showing a balanced circuit configuration using a surface acoustic wave filter.

FIG. 7 is a diagram illustrating a relationship between a voltage generated at an input electrode and a signal source voltage in the case of a balanced circuit configuration.

FIG. 8 is a diagram showing a relationship between a voltage generated at an input-side electrode and a signal source voltage in the case of an unbalanced circuit configuration.

[Explanation of symbols]

10: piezoelectric substrate, 11, 12: comb-shaped electrode on input side, 1
3, 14 ... comb electrodes on the output side, 15 ... busbars on the input side, 16 ... busbars on the output side, 11a, 12a ... electrode fingers of the comb electrodes on the input side, 13a, 14a ... comb electrodes on the output side Electrode finger, 15a ... electrode finger of busbar on input side,
16a: electrode fingers of the busbar on the output side.

Continuation of the front page (56) References JP-A-52-77646 (JP, A) (58) Fields investigated (Int. Cl. ⁷ , DB name) H03H 9/145 H03H 9/64

Claims (3)

(57) [Claims] 1. A provided a pair of comb electrodes for receiving a pair of comb electrodes and the surface acoustic wave for exciting surface acoustic waves on a piezoelectric substrate, which is grounded before SL between interdigital electrodes of each pair busbar Are arranged so as to be on the same straight line with each other, and the comb-shaped electrodes of each pair are arranged so that the electrode fingers of the comb-shaped electrodes of each pair face each other with the bus bar therebetween and are mirror-symmetrical, and a in said cross the electrode fingers of the interdigital electrodes and the electrode fingers connected to the bus bar is arranged so as to be mirror-symmetrical with respect to said bus bars, and one of the comb-shaped electrode pairs either
A surface acoustic wave filter characterized by being weighted with a potize . 2. The surface acoustic wave filter according to claim 1, wherein a bus bar sandwiched between each pair of comb electrodes is constituted by one bus bar. 3. The surface acoustic wave filter according to claim 1, wherein a shield electrode is arranged between bus bars sandwiched between each pair of comb electrodes.