JPS5844660Y2 - surface wave device - Google Patents

Description

[Detailed Description of the Invention] The present invention relates to a surface wave device in which a surface wave propagates on the surface of a substrate.

What is generally called a surface wave is one in which energy is concentrated within approximately one wavelength from the surface of the propagation medium through which the surface wave propagates, and in addition to the surface wave, it is also called a surface acoustic wave or an acoustic surface wave.

In a surface wave device that utilizes surface waves, the shape of the transmitting transducer that excites the surface wave when an electrical signal is applied, and the receiving transducer that converts the surface wave into an electrical signal must be carefully designed and propagated. It has the excellent feature that desired frequency characteristics can be easily obtained by controlling it from the surface of the medium.

However, as a characteristic of such surface wave devices, reflected surface waves occur between the transmitting and receiving transducers, and this reflected surface wave causes deterioration of frequency characteristics.

The most notable one is the triple transit echo that propagates between transmitting → receiving → transmitting → receiving transducers (
(hereinafter abbreviated as TTE), and as a method of suppressing this TTE, there is a method of reducing the level of TTE by using it at a high insertion loss of 20 dB or more, and a method of reducing the level of TTE by using it at a high insertion loss of 20 dB or more, for example, There are currently existing methods proposed.

Figure 1 shows a surface acoustic wave device according to the US patent, in which 1 is a substrate made of a piezoelectric material such as lithium tantalate, PZT ceramic, or ZnO thin film, and 2 is a transmitting transducer consisting of an interdigital transducer provided on the surface of the substrate 1. , 3 is a receiving transducer formed from an interdigital transducer disposed opposite to the transmitting transducer 2, and 4 is the receiving transducer 3.
reflective transducers 5 and 6 arranged in parallel to the transmitting transducer 2 with a pitch difference of an odd number multiple of the quarter wavelength (hereinafter abbreviated as λ/4) of the surface wave propagating from the transmitting transducer 2; .7 is a transmission side, reception side, and reflection side load circuit connected to each transducer 2, 3.4.

In the surface acoustic wave device having such a structure, a surface wave excited from the transmitting transducer 2 propagates on the surface of the substrate 1 and is converted into an electrical signal by the receiving transducer 3.

This electrical signal is then led to an externally connected receiving load circuit 6, where the electrical signal is slightly reflected and re-excited from the receiving transducer 3 as an unnecessary electrically reflected surface wave. Being washed away.

On the other hand, a small portion of the surface waves is reflected by the receiving transducer 3 as mechanically reflected surface waves before being converted into an electrical signal by the receiving transducer 3.

This mechanical and electrically reflected surface wave forms a TE.

The reflected surface waves that form this TTE propagate back from the receiving transducer 3 to the transmitting transducer 2, and are propagated again from the transmitting transducer 2 to the receiving transducer 3, where they are converted into unnecessary electrical signals. This is prevented by generating a reflected surface wave in phase and canceling out the reflected surface wave.

That is, since a sound path difference of λ/4 is provided between the transmitting transducer 2 and the reflecting transducer 4, round-trip propagation occurs between the transmitting and receiving transducers 2 and 4 and between the reflecting and receiving transducers 2 and 3, respectively. This is because a phase difference of λ/2 occurs between the two reflected surface waves and they cancel each other out.

In this way, most of the TTE can be suppressed by the reflected surface waves of the reflective transducer 4.

However, if the transmitting transducer 2 or the receiving transducer 3 is weighted to obtain a desired frequency characteristic, or if the maximum crossing width of the transmitting transducer 2 and the reflecting transducer 4 is made to be different, the load circuit of each Even if the load impedance of 5°6.7 was varied, the TTE could not be sufficiently reduced.

The present invention has been devised in view of this point, and will be described in detail below with reference to FIG. 2 and subsequent figures.

FIG. 2 shows a surface wave device of the present invention, most of which is similar to that of FIG.

In FIG. 2, 11 is a substrate made of piezoelectric material, 12 is a transmitting transducer, and this transmitting transducer 12
is the electrode finger 12'.

This is a so-called weighted interdigital transducer in which the intersection width of 12... changes according to a predetermined function.

13 is a receiving transducer which is a regular type interdigital transducer; 14 is a reflection transducer; the reflection transducer 14 is also weighted similarly to the transmitting transducer 12;

Note that the electrode fingers 14' of this reflective transducer 14,
14'...... is assumed to be Wl, and that of the transmitting transducer 12 is assumed to be W2.

15, 16, 17 are the respective transducers 12, 13
．． In the transmission, reception side, and reflection side load circuits connected to 14,
If the reflection coefficient for the electric signal of the internal reflection side load circuit 17 of these load circuits 15, 16, and 17 is γ1, and that of the transmission side load circuit 15 is 72, then 71 is γl=γ2・W2/W1... ...(1)
The load impedances of the load circuits 15 and 17 are set so as to satisfy the following relationship.

That is, the electrode fingers 12', 12', 14', 14' of the transmitting transducer 12 and the reflective transducer 14.
The ratio W2/W1 of the maximum crossing width of ... and the ratio γ2/γ of the reflection coefficients of the transmitting side load circuit 12 and the reflecting side load circuit 17
There is a reciprocal relationship with □.

To explain the reflection coefficient γ here, γ is the ratio of the surface wave propagating to the transducer and the reflected surface wave re-excited from this transducer, and the conductance of the load circuit connected to the transducer is G.
1. It can be found when the radiation conductance of the transducer is Gt.

FIG. 3 is a characteristic diagram showing the change in γ with respect to Gl/Gt obtained by the above equation (2). Normally, the Gt of a transducer is determined depending on the pattern of the transducer, so it is difficult to obtain the desired γ. For example, the conductance Gl of the load circuit may be varied.

Therefore, the desired maximum crossing width W2, the radiation conductance G
electrode finger 12' according to a predetermined function with t2
, 12', .
A transmitting side load circuit 15 having a conductance G12 that gives .

After that, the reflection transducer 14, which is weighted in a similar shape to the transmitting transducer 12 and having the maximum intersection width W1 and the radiation conductance Gt1, is given a reflection coefficient that satisfies the equation (1) according to the equation (2). A reflection side load circuit 17 having a conductance G1□ that imparts γ is connected.

That is, the reflection level of the reflected surface waves on the transmitting side and the reflected surface waves on the reflecting side is determined by the ratio W2/W1 of the maximum crossing width of both transducers 12.14 and the ratio γ2/γ1 of the reflection coefficients of both load circuits 15.17. By setting it to the reciprocal, the same level is set,
The TTE that ultimately propagates to the receiving transducer 13 can be canceled out.

When a surface wave device having such a structure is used as a video intermediate frequency filter of a television receiver, the above TTE level is -
The required condition is that it is 40 dB or less.

The reason for this is that when the TTE level increases, ghosts occur in the image.

FIG. 4 is a characteristic diagram showing the relationship between the maximum crossing width ratio W2/W1 of both transmitting and reflecting transducers 12, 14 and the TTE level.

In other words, if you want to obtain a TTE of 1 odB or less, W2/W
1 must be 2.5 or less.

On the other hand, when W2/W1 is less than 1, the reflection transducer 14 provided for canceling the reflected surface waves becomes larger than the transmission transducer 12, resulting in an increase in insertion loss.

Therefore, W2/W1 is optimally 1 to 2.5, and γ2/γ1 is preferably 2.5 to 1.

Specific examples of the surface wave device of the present invention will be described below.

A transmitting/receiving/reflecting transducer 12 in which an aluminum vapor-deposited film is formed into a desired shape on the surface of a Mecat lithium tantalate substrate 11 using photolithographic technology;
13.14 shall be provided.

Those transmitting/receiving/reflecting transducers 12, 13 .
14 is 80 pairs, 14 pairs, 80 pairs of electrode fingers 12', 13
', 14'..., their maximum crossing widths are 1.2 mm, 2.1 mm, Q, 6 mm, and the transmitting/reflecting transducer 12.14 has a similar shape. It is weighted.

This weighted transmitting and reflecting transducer 12.14
have radiation contactances of Gt2=0.4 mU and Gt1=0.2 mo', respectively.

Transmitting and reflecting load circuits 15.17 each consisting of a coil and a resistance element are connected to both the transmitting and reflecting transducers 12.14, and tuning with each transducer 12.14 is achieved by the respective coils. It is taken.

When the resistance value of the resistance element of the transmission side load circuit 15 is 3KQ, the conductance G1□ is 1/3rrr6°, so from equation (2), the reflection coefficient 72 on the transmission side is 0.3
becomes.

On the other hand, the reflection coefficient 71 on the reflection side is 0.6 from equation (1).

From this γ = 0.6, the conductance G11 of the reflection side load circuit 17 is found by modifying equation (2), and it becomes 0°059.
m7J- is obtained.

That is, the conductance G12 is applied to the reflection side load circuit 17.
When a resistive element having =0.059 m'U' (resistance value 17 and Ω) is connected, the reflected surface waves reflected by the transmitting transducer 12 and the reflected surface waves reflected by the reflecting transducer 14 have opposite phases. They are on the same level and cancel each other out during propagation, and the receiving transducer 1
It will not reach 3.

Theoretically, as described above, 17 is used as the reflection side load circuit 17.
However, according to the inventors of the present invention, the reflection coefficient 71 of the weighted reflection transducer 14 is 20 to 30 lower than the theoretical value considering the weighting and conversion loss of the transducer 14. It has been experimentally confirmed that it is necessary to increase the amount by about %.

That is, the resistance element of the reflection side load circuit 17 is approximately 30KQ.
By connecting them, a reflected surface wave with a reflection coefficient 71=0.6 can be obtained from the reflective transducer 14, and that of the transmitter 1-transducer 12 can be canceled out.

As a result, such a surface wave device has an insertion loss of 11 dB, T
Exhibits electrical characteristics with a TE level of -40dB.

As is clear from the above description, the surface acoustic wave device of the present invention has two functions: W2/W, which is the ratio of the maximum crossing width of the electrode fingers in both the transmitting and reflecting transducers arranged in parallel so as to face the receiving transducer; Reflection coefficient 72 of each transducer indicating the rate at which the surface wave is reflected by each load circuit connected to the reflective surface transducer and re-excited from both transducers as a reflected surface wave. Since γ2/γ1, which is the ratio of γ1, is set to a reciprocal number, and W2/w1 is set to 1 to 2.5, repropagation from the transmitting transducer to the receiving transducer is possible without increasing the insertion loss of the device. The reflected surface wave can be canceled out during propagation by a reflected surface wave that is in an antiphase relationship with the reflected surface wave to which it re-propagates from the reflected transducer.

Therefore, the unnecessary reflected surface waves do not reach the receiving transducer and exhibit desired frequency characteristics.

In the above description, the reflective transducer is placed next to the transmitting transducer, but the same effect can be obtained even if the reflecting transducer is placed next to the receiving transducer.

[Brief Description of the Drawings] Figure 1 is a front view showing a conventional surface wave device, Figure 2 is a front view showing the surface wave device of the present invention, and Figure 3 is a curve diagram showing the relationship between conductance and reflection coefficient. , FIG. 4 is a curve diagram showing the relationship between the ratio of the maximum crossing width of the electrode fingers of the transmitting transducer and the reflecting transducer and the TTE level. 1.11...Substrate, 2,12...Transmitting transducer, 3.13...Receiving transducer, 4.14...Reflecting transducer, 5,15 ......Transmission side load circuit, 7゜17...
...Reflection side load circuit.

Claims (1)

[Scope of utility model registration request] In a surface acoustic wave device in which a transmitting transducer for converting an electrical signal into a surface wave and a receiving transducer for converting the surface wave into an electrical signal are disposed facing each other on the surface of a substrate, a receiving transducer for receiving a surface wave excited from the transmitting transducer is provided. A reflective transducer that generates a reflected surface wave having an opposite phase to the reflected surface wave that is reflected by the transducer, directed from the receiving transducer to the transmitting transducer, further reflected by the transmitting transducer, and slightly repropagated to the receiving transducer, is mounted on the opposite side. Which of the arranged transmitting and receiving transducers is placed in parallel with one side facing the other with a sound path difference of 1/4 wavelength of the surface wave or an odd number multiple thereof, and each transducer is connected to the other with the electrode fingers intersecting each other. Further, at least one of the transmitting/receiving transducers is weighted so that the crossing width of the electrode fingers changes, and the electrodes of the reflective transducer of the transmitting/receiving and reflective transducers are weighted to change the interdigital transducer. If the maximum intersecting width of the fingers is Wl, and that of the other transducer with which the reflective transducer is juxtaposed is W2, then the ratio W2/W1 is between 1 and 2.5, and these are connected to both transducers. The ratio of the effective reflection coefficient for the electrical signal in each load circuit to be transmitted is set to the reciprocal of the maximum crossing width ratio W2/W1, and the reflected surface waves propagating between the transmitting and receiving transducers are A surface wave device characterized in that the reflection transducer and one of the transducers facing each other are offset by a reflection surface plate having an opposite phase propagating therebetween.