JPH0238493Y2 - - Google Patents

Description

[Detailed description of the invention] The invention relates to an electromechanical transducer for surface acoustic waves.
In particular, the present invention relates to a weighting structure for a two-phase unidirectional surface acoustic wave electromechanical transducer.

Delay lines, filters, etc. using surface acoustic waves are currently being incorporated into communication equipment, television sets, etc., and a wide range of applications are being considered over frequencies from several tens of MHz to several GHz. However, the characteristics of the surface acoustic wave elements actually used are that they have a large insertion loss and a large in-band ripple.

In order to solve these problems, two-phase unidirectional electrode transducers have recently been developed. This two-phase unidirectional electrode converter is extremely effective in reducing insertion loss and in-band ripple.

This two-phase unidirectional surface acoustic wave electromechanical transducer has a first electrode to which a reference signal is applied, a second electrode to which a signal whose phase differs from the reference signal by 90° is applied, and a common ground electrode. The plurality of groups are connected so that the first and second electrodes form interdigital electrodes, and the surface waves generated from the first and second electrodes of each group are directed in the traveling direction. The width of the zigzag-shaped common ground electrode is determined and arranged so that the common ground electrodes are in phase in the same direction and out of phase in the opposite direction.

FIG. 1 is a front view of such a conventional two-phase unidirectional surface acoustic wave electromechanical transducer. In the figure, reference numeral 1 denotes a piezoelectric substrate made of piezoelectric crystal such as lithium niobate (LiNbO ₃ ), and a group of electrode fingers 2 are disposed on the piezoelectric substrate. These electrode finger group 2 have an electrode index N
a 0° side interdigital electrode 2a that provides a reference signal;
A 90° side interdigital electrode 2b with the same electrode index for giving signals with a phase difference of 90°, a 0° side interdigital electrode, and a 90° side interdigital electrode N
It has a common ground electrode 2c arranged in a zigzag pattern so as to cover each electrode finger of the book. The electrode finger group 2 is formed from a 0° side interdigital electrode 2a to a 90° side interdigital electrode 2b formed on both sides of the common ground electrode 2c.
The distance l between the centers of the two electrodes is defined as l=λ ₀ /4·{2(N+1)+1}, and each electrode placed between the centers of the common ground electrodes 2c on both sides is defined as one group, and these multiple groups It is constructed by connecting G ₁ , G ₂ , G ₃ . However, λ ₀ is the wavelength, and N is the number of interdigital electrodes on the 0° side and 90° side (in the example in Figure 1, N =
1). If the width of the common ground electrode 2c sandwiched between the 0° side and the interdigital electrode on the 90° side is λ ₀ /2, and the width of the common ground electrode 2c located on the other sides is λ ₀ . is illustrated. Then, the length of the intersecting fingers is changed for each group, and the beam width of the surface acoustic wave excited for each group is changed and weighted (referred to as apodized weighting).
In addition, the 0° side electrode fingers of each group G ₁ , G ₂ ...
The width of the common ground electrode 2c is taken into consideration so that the surface waves generated from the electrode fingers on the 90° side are in phase in the direction of propagation and out of phase in the opposite direction.

Now, in the conventional two-phase type unidirectional electrode transducer with such apodized weighting, the length of each group's 0° side electrode fingers and 90° side electrode fingers covered by the common ground electrode 2c is l ₁ i, l ₂ i (i=
1, 2...) are not equal, in other words, the acoustic power consumed by the 0° side electrode finger and the 90° side electrode finger of each group is not equal, so the directionality of the surface wave deteriorates and the loss increases. At the same time, ripples within the passband also increase. Therefore, although the two-phase unidirectional electrode transducer can significantly reduce insertion loss and ripple within the passband compared to a normal transducer, its effects have not been sufficient.

Therefore, in various types of weighting such as apodized weighting, the present invention equalizes the acoustic power consumed at the 0° side electrode finger and the 90° side electrode finger of each group, thereby reducing loss and ripple within the passband. The purpose is to

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

Fig. 2 is a front view of an electromechanical transducer for surface acoustic waves according to an embodiment of the present invention, which is an embodiment in which apodized weighting is applied. A detailed explanation thereof will be omitted.

The difference from the conventional example shown in Fig. 1 is that the common ground electrode 2
Protrusions 21a to 21e are provided on a part of c, and the length of the 0° side electrode finger l ₁ i (i = 1, 2...) and the length of the 90° side electrode finger of each group covered by the common ground electrode is determined. s l ₂ i
This is the point where (i=1, 2,...) are made equal to each other.
This makes the acoustic power consumed by the 0° side electrode finger and the acoustic power consumed by the 90° side electrode finger of each group equal to each other, eliminates deterioration of the directionality of surface waves, reduces loss, and improves transmission. Ripples within the band can be reduced.

FIG. 3 shows another embodiment of the present invention in which apodized weighting is applied, and the same parts as in FIG. 2 are given the same reference numerals. In the embodiment of FIG.
The only difference from the figure is that the weighting method of the apodization type is different, and the protrusions 21a to 21e are provided, and the length of the 0° side electrode finger of each group covered by the common ground electrode 2c is l ₁ i (i =1, 2...) and the length l ₂ i (i=1, 2,...) of the 90° side electrode fingers are the same.

Figures 4, 5, and 6 are center frequencies, respectively.
This is an insertion loss vs. frequency characteristic diagram when the two-phase unidirectional electrode converter according to the present invention shown in Fig. 2 and the conventional example shown in Fig. 1 are applied to filters of 197.6 MHz, 372.8 MHz, and 531 MHz, respectively, and the solid line is The one according to the present invention and the one-dot chain line are according to the conventional example.

As is clear from this, the insertion loss has been improved by approximately 1 dB, and the in-band ripple has become extremely small.

FIG. 7 is a reference diagram, and is an example in which capacitance weighting is applied to equalize the consumed radiation power.

In the figure, numeral 31 is a piezoelectric substrate made of piezoelectric crystal such as lithium niobate, and there are interdigital transducers 31a on which a reference signal is provided with an electrode number N on the substrate, and
They are arranged in a zigzag pattern so as to cover the 90° side interdigital electrode 31b, the 0° side interdigital electrode 31a, and the 90° side interdigital electrode 31b, which have the same electrode index for giving signals with 90° different phases. A common ground electrode 31c is formed. Interdigital electrode 3
The ends 31a' of each electrode finger forming 1a and 31b,
31b' is expanded to a predetermined size, dielectric films 32a and 32b (dotted chain lines) are deposited thereon, and an input signal electrode 33a for inputting a reference signal is further applied thereon.
(dotted line) and an input signal electrode 33b (dotted line) for inputting a signal having a phase different by 90 degrees from the reference signal are respectively deposited.

The equivalent electric circuit of this configuration is shown in Figure 7 A, B, C,
If we focus only on part D, we will see something like Figure 8a. In the figure, C ₂ is the capacitance due to the part D in Figure 7, C ₁ is the capacitance between AB and AC, and R is the radiation resistance, and the power consumed by this radiation resistance R is the surface wave energy, in other words. Then it becomes radiant power. Now, the voltage E appearing across the radiation resistance R,
That is, the potential of the 0° side electrode finger A in Fig. 7 is E=Y ₂ · Ein/(Y ₁ + Y ₂ ) (1) where, Y ₁ = 1/R + jwC ₁ , Y ₂ = jwC ₂ , and the 7th The potential of each electrode finger can be adjusted by changing the capacitance C ₂ of the part shown in figure D, that is, by adjusting the intersection area between the input signal electrode 33a and the part 31a' which is widened at the end of the electrode finger. weighting in this way is called capacity weighting.

On the other hand, if we focus only on the parts A', B', C', and D' in Fig. 7, the equivalent electric circuit will be as shown in Fig. 8b, and the potential of the electrode finger A' on the 90° side is E. =Y ₂ ′・Ein/(Y ₁ ′+Y ₂ ′) (2) However, Y ₁ ′=1/R+jwC ₁ , Y ₂ ′=jwC ₂ ′.

Therefore, from equations (1) and (2), if Y ₂ = Y ₂ ′ (C ₂ = C ₂ ′), the radiation power consumed by the 0° side electrode and the 90° side electrode of each group are The radiated power can be made equal to the radiated power.

Therefore, the capacitance between the reference signal input electrode 33a and the 0° side electrode finger of group Gi (i=1, 2...)
C ₂ , a signal input electrode 33b with a phase difference of 90° from the reference signal, and a group Gi (i=1, 2,...).
The capacitance C ₂ ' between the electrode fingers on the 90° side is made equal to each other. For this purpose, the signal input electrode 3
The intersection area between the electrode 3a and the electrode finger on the 0° side of group Gi is made to match the intersection area between the signal input electrode 33b and the electrode finger on the 90° side of group Gi.

Figure 9 is a reference diagram and shows each group Gi (i=
This is an example in which axial weighting is applied by changing the number of floating electrodes (1, 2, . . . ), and the same parts as in FIG. 9th
The difference between the figure and FIG. 2 is that the lengths of the electrode fingers are made the same and that floating electrodes are inserted in each group.

In the figure, 41a, 41b, 42a, 42a', 42
b and 42b' are floating electrodes having the same shape. Note that these floating electrodes are created as follows. That is, it is created by cutting off a pair (or several pairs) of the 0° side electrode fingers or the 90° side electrode fingers of each group Gi, and connecting the cut ends to each other. Reference numeral 43 denotes a common ground electrode arranged in a zigzag pattern, which includes one 0° side electrode finger of each group Gi and a predetermined number of floating electrodes, and one 90° side electrode finger of each group Gi. and a predetermined number of floating electrodes are arranged so as to alternately cover these sets.

Now, by changing the number of floating electrodes inserted into each group G ₁ , G _{2 .} . . , it is possible to change the excitation intensity in each group, and it is possible to perform weighting in the axial direction.

By the way, the power consumption of the 0° side electrode finger and the 90° side electrode finger of each group is equal to the power consumption of the 0° side electrode finger or the 90° side electrode finger of each group.
It depends on the number of floating electrodes paired with the 90° side electrode finger.

Therefore, the number of floating electrodes to be inserted into groups G ₁ , G ₂ , G ₃ . . . is 0, 2, 41a, 41b,
4, 42a, 42a', 42b, 42b'... and the number of floating electrodes paired with the 0° side electrode finger and the number of floating electrodes paired with the 90° side electrode finger of each group. are matched with each other.

As described above, according to the present invention, in a two-phase unidirectional electrode transducer, the acoustic power consumed by the 0° side electrode fingers of each group is made equal to the acoustic power consumed by the 90° side electrode fingers. It has become possible to reduce insertion loss and ripple within the band compared to conventional devices.

[Brief explanation of the drawing]

Figure 1 is a front view of a conventional two-phase unidirectional electromechanical converter, Figure 2 is a front view of a surface acoustic wave electromechanical converter according to the present invention, and Figure 3 is an apodized type weighted electromechanical converter. Another embodiment explanatory diagram of the present invention, No. 4
Figures 5 and 6 are insertion loss vs. frequency characteristics diagrams explaining the effects of the present invention, Figure 7 is a reference diagram applied to capacitance weighting, Figures 8a and b are equivalent electrical circuit diagrams, and Figures 8a and 6b are equivalent electrical circuit diagrams. Figure 9 is a reference diagram applied to axial weighting. 1... Piezoelectric substrate, 2... Electrode finger group, 2a, 31
a...0° side interdigital electrode, 2b, 31b...90° side interdigital electrode, 2c, 31c...common ground electrode, 21a to 21e...projection, 31a', 31b'...
...End portion, 32a, 32b...Dielectric film, 33a,
33b...Input signal electrode, 41a, 41b, 42
a, 42a', 42b, 42b', ... floating electrodes.

Claims (1)

[Scope of utility model registration request] A first electrode to which a reference signal is applied, a second electrode to which a signal whose phase differs from the reference signal by 90 degrees, and a common ground electrode are defined as one group, and the plurality of groups are connected to each of the first and second electrodes. are connected to form interdigital electrodes, and the first electrode of each group
and a two-phase unidirectional surface acoustic wave electromechanical transducer in which the ground electrode is formed so that the surface waves generated from the second electrode are in phase in the traveling direction and out of phase in the opposite direction, The intersecting finger length of the first electrode and the second electrode is changed and weighted for each group, and the common ground electrode is arranged in a zigzag pattern so as to cover the first and second electrodes of each group. , the first electrode and the second electrode of each group covered by the common ground electrode so as to equalize the acoustic power consumed by the first electrode and the second electrode of each group.
An electromechanical transducer for surface acoustic waves, characterized in that the common ground electrode is formed by matching the length of the electrodes.