JPH0217964B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention provides a surface acoustic wave element that exhibits inductive properties at a specific frequency and is suitable for use as an oscillation element or a surface acoustic wave transducer in combination with ordinary interdigital electrodes. Regarding.

In a typical surface acoustic wave transducer in which a piezoelectric substrate 1 is provided with interdigital electrodes 2 as shown in FIG. 1, the input impedance is capacitive;
It was necessary to add an inductance element 3 externally to match the electric circuit. However, the inductance element 3 consisting of a normal coil
If the inductance element is installed outside the surface acoustic wave transducer, consideration must be given to the installation space and wiring of the inductance element, and when configuring a filter, etc., the overall size becomes large and is disadvantageous in terms of cost. becomes. On the other hand, if an inductance element can be constructed on a piezoelectric substrate, it can be expected to be highly effective in reducing the size, improving characteristics, and reducing manufacturing costs when constructing a filter or the like.

In view of the above points, the present invention makes it possible to construct an element that functions as an inductance element in a predetermined frequency range on a piezoelectric substrate, and provides an extremely effective elastic The aim is to provide a surface wave device.

Embodiments of the surface acoustic wave device according to the present invention will be described below with reference to the drawings.

FIG. 2 shows a first embodiment of the invention. In this figure, a surface acoustic wave element 10 has interdigital electrodes 1
1 and interdigital electrodes 12 arranged on both sides of this
A and 12B are formed on a piezoelectric substrate, and the interdigital electrodes 12A and 12B have the same shape and are electrically connected to each other by conductive wires 13A and 13B. In this case, when the distances between the central interdigital electrode 11 and the interdigital electrodes 12A and 12B on both sides are _L1 and _L2 , _L1 = _L2 +nλ/2+(0.4~0.6)λ...( 1) (where λ is the wavelength at the center frequency of the surface acoustic wave determined by the electrode finger spacing of each interdigital electrode, and n is an even number).

FIG. 3 shows the logarithm N of the electrode fingers of each interdigital electrode 11, 12A, 12B in the configuration of the first embodiment.
50 pairs, crossover width W is 100λ (λ: center frequency 34.5MHz
(wavelength of surface acoustic wave), the piezoelectric substrate is a Y-cut Z-propagation LiNbO ₃ substrate, L ₁ = 1.0λ, and the terminal 14 of the interdigital electrode 11 when L ₂ is changed.
The values of reactances X and Q between A and 14B are shown, respectively. From this figure, it can be seen that when L ₂ =0.44λ, the reactance X is about 50Ω and the Q is about 100.

FIG. 4 shows the logarithm N of electrode fingers of each interdigital electrode 11, 12A, 12B in the configuration of the first embodiment,
The frequency characteristics of the reactances X and Q are shown when the intersection width W and the piezoelectric substrate conditions are the same as in FIG. 3, and L ₁ =1.0λ and L ₂ =0.44λ. As can be seen from this figure, the frequency band in which the reactance X becomes inductive is a narrow range near the center frequency, but
This is even more advantageous when the purpose of use is to compensate for the capacitive impedance of a surface acoustic wave filter. In other words, if the filter is made inductive near the center frequency, energy is converted to surface acoustic waves at other frequencies, which not only reduces insertion loss but also suppresses side lobes of the filter characteristics. can.

FIG. 5 shows the configuration of the first embodiment, in which the number of logarithms N of electrode fingers of each interdigital electrode 11, 12A, and 12B is the same, the crossing width W is 100λ, L ₁ =1.0λ, L ₂ =
When set to 0.44λ, logarithm N and reactance X
and the relationship with Q is shown. Note that the piezoelectric substrate is the same as that shown in FIG. 3. From this figure, it can be seen that since the coupling of surface acoustic waves is used, there is no inductive property unless there are a certain number of logarithms (approximately 20 or more pairs). Furthermore, the value of Q increases as the logarithm N increases.

In addition, in all figures 3 to 5, the intersection width W is
Although the case of 100λ is shown, the reactance X is inversely proportional to W, and the value and frequency characteristics of Q remain unchanged. Therefore, in order to obtain the necessary inductance, W may be determined appropriately.

As explained above, according to the first embodiment, it is possible to easily configure a surface acoustic wave element that works as an inductance element in a certain frequency band, and it can be used for matching a surface acoustic wave filter or as an oscillation element. If you do, the effect will be great.

Note that the interdigital electrodes 12A arranged on both sides,
12B may be made symmetrical, and in this case L ₁ ,
The relationship of L ₂ is shown in the above equation (1) when n is an odd number.

FIG. 6 shows a second embodiment of the invention. In this figure, the surface acoustic wave element 20 includes an excitation interdigital electrode 21 electrically connected in series to the interdigital electrode 11 in addition to the interdigital interdigital electrodes 11, 12A, and 12B shown in FIG. The configuration of a surface acoustic wave transducer provided on a substrate is shown. In this case, the excitation interdigital electrode 21 is capacitive, but if the interdigital electrodes 11, 12A, and 12B are set to be inductive so as to match at the center frequency as described above, the terminal Since the input impedance between 22A and 22B is substantially only a resistance component, good matching with the electric circuit can be achieved.
Furthermore, at frequencies outside the center frequency, it does not function as an inductance element, and energy is radiated from the interdigital electrode 11, resulting in less energy reaching the excitation interdigital electrode 21. This means suppression of side lobes of the filter characteristics. Then, such a surface acoustic wave element 20
By providing two on the same piezoelectric substrate, a surface acoustic wave filter can be constructed. In this case, insertion loss can be reduced,
It is possible to suppress side ropes of filter characteristics.

As described above, according to the present invention, an element that functions as an inductance element in a predetermined frequency range can be constructed on a piezoelectric substrate, and when applied to a surface acoustic wave filter, etc., it is extremely effective in improving characteristics. .

[Brief explanation of drawings]

Fig. 1 is a configuration diagram showing a case where conventional surface acoustic wave elements are matched, Fig. 2 is a plan view showing a first embodiment of the surface acoustic wave element according to the present invention, and Fig. 3 is a diagram showing reactance, Q, and distance. A graph showing the relationship with L ₂ ,
FIG. 4 is a graph showing the frequency characteristics of reactance and Q, FIG. 5 is a graph showing changes in reactance and Q with respect to logarithm N, and FIG. 6 is a graph showing the frequency characteristics of reactance and Q.
FIG. 2 is a plan view showing an example. 1...Piezoelectric substrate, 2, 11, 12A, 12B,
21... Interdigital electrode, 10, 20... Surface acoustic wave element, 13A, 13B... Conducting wire.

Claims (1)

[Claims] 1. A first interdigital electrode and second and third interdigital electrodes arranged on both sides of the first interdigital electrode and electrically connected to each other on a piezoelectric substrate. , the second and third interdigital electrodes have the same or symmetrical shape, and the distances between the first interdigital electrode and the second and third interdigital electrodes are L ₁ and L ₂ , respectively.
_{When the} The logarithm of the electrode fingers of the shaped electrode is
2. A surface acoustic wave element, characterized in that the number of pairs is set at 20 or more so that the impedance between the first interdigital electrode terminals exhibits inductive properties. 2. A first interdigital electrode and second and third interdigital electrodes arranged on both sides of the first interdigital electrode and electrically connected to each other are provided on a piezoelectric substrate, , the third interdigital electrode has the same or symmetrical shape, and the distances between the first interdigital electrode and the second and third interdigital electrodes are L ₁ and L ₂ , respectively.
_{When the} The logarithm of the electrode fingers of the shaped electrode is
The number of pairs is set to 20 or more so that the impedance between the first interdigital electrode terminals exhibits inductive properties, and the excitation interdigital electrode is electrically connected in series to the first interdigital electrode. A surface acoustic wave element provided on the piezoelectric substrate.