JPS5834973B2 - acoustic surface wave device - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to an acoustic surface wave device that reduces triple transit echoes.

A surface acoustic wave device is a device that uses surface acoustic waves SAW that propagate on the surface of a piezoelectric substrate, and is an integrated device that has an output transducer and an input transducer that transmit and receive SAWs installed on the surface of the substrate. It is a solid-state functional device.

The output and input transducers are usually crossed phase transducers IDT consisting of a pair of interdigitated comb-shaped electrodes.

The input electrical signal is converted to a SAW at the input IDT, propagates across the substrate surface, and reaches the output IDT.

Part of the above SAW that has reached the output IDT is converted into an electrical signal and becomes a seasonal signal, but the other part of the SAW is output
The remaining part is reflected by the output IDT.

The latter two signals are unnecessary signals and need to be removed, but the former is absorbed by the sound wave absorbing material provided at the edge of the board, and the latter propagates toward the input IDT and is reflected once again by the input IDT. A part of it is detected as an electrical signal by the output IDT.

This signal is called a triple reflection TTE because it has a delay time three times the delay time of the desired signal.

The cause of this TTE is W.R. Smith (Ul.
traso-nics synposivm proc
eedings, l 973゜pp, 410-41
3), it has been clarified that it consists of reflected MEL due to acoustic impedance mismatch between the electrode part and the propagation path part, and re-radiation RW at the load, and the former is caused by the SAW as shown in Figure 1. Approximately 1 wavelength at the center frequency
It is stated in US Pat. No. 3,727,155 that the electrode width can be made sufficiently small for practical use by using a so-called split electrode method having an electrode width of /8.

In addition, the latter is closely related to insertion loss, and it is impossible in principle to reduce the insertion loss without increasing it.

As a rough estimate, it is well known that RW is twice the insertion loss measured in decibels.

That is, in order to make the TTE less than -40 dB, the insertion loss necessarily becomes more than 20 dB.

In electrical signal processing devices such as acoustic surface wave devices, such a large insertion loss is a serious problem in practice.

Conventionally, to improve this point, two output IDTs were provided,
A method of canceling the reflection caused by one side (U.S. Patent 3,
596, 211) has been proposed, but it is not practical because it increases the number of IDTs and therefore increases the board size.

The present invention aims to eliminate the above-mentioned drawbacks of the prior art and provide an acoustic surface wave device with a small TTE effect.

The gist of the present invention and its principle will be explained using the drawings.

As a result of detailed analysis of the causes of TTE, the inventors found that the MEL of the so-called solid electrode 3 has a width of about 1/4 of the acoustic surface wave at the center frequency, and the MEL of the so-called split electrode 1 has a width of 1/8 of the wavelength of the acoustic surface wave. It was found that the phase difference with RW is given as follows.

Here, Ga and GT are the conductance and capacitance of the IDT, and GL and BL are the conductance and susceptance of the load.

is the deviation between the periodic position 2 (indicated by the dotted line) of the split electrode 2 and the position of the solid electrode 3, as shown in FIG.

The size at the center frequency of the MEL of M solid electrodes (M/2 pairs) is based on the paper by E.K.
EE Trans, S U-15, 42.

According to the results of the inventors who developed the paper (1968, pp. Ill-119), it is given by the following equation.

K is a constant proportional to the electrode length, V is the speed of SAW, △V
is the magnitude at which the SAW speed changes due to the presence of the electrode metal.

The frequency dependence of MEL is given by the following equation.

On the other hand, the reflection of M/2 pairs of split electrodes at the center frequency is expressed by the following equation.

Here, Yr and Yi are given by equations (2) and (3).

Ga, CT is double--R. Smith et al. (IEE
E Trans, Vol, MTT-17, l I
.

1969, pp. 856-864), as follows:

Here, k2 is the electromechanical coupling coefficient Ci for the SAW and is the capacitance between the pair of electrodes.

The frequency dependence of R and W is determined by A.G.O. de Vries (
Ultrasonics Symposium Pro
eeedings, 1972. pp. 35:3-358
), it can be approximately expressed by the following equation.

From this equation and equation (5), the relationship between RW reflected from II)T with the same logarithm and MER is as if MEL had a frequency dependence equivalent to the RW reflection from an IDT with twice the logarithm. understood as suggesting.

The main points of the present invention are the above-mentioned MEL and R,
Due to the properties of W, by using an IDT with a structure consisting of M/2 pairs of solid electrodes and M pairs of split electrodes, reflections generated from both can be canceled out using (1), (4), and (5). In addition to this, TTE is eliminated over a wide band.

The present invention will be explained in detail below using examples.

FIG. 2 is an enlarged perspective view showing the arrangement of the IDTs 4 and 5 of the surface acoustic wave device.

FIG. 3 is a partially omitted pattern of the detailed structure of II) T4 and 5 shown in FIG. 2 according to the present invention.

The structure of the device will be briefly described below. The sample is an ID made of aluminum material on a lithium niobate (LiNb03) substrate 6 using a normal semiconductor integrated circuit processing method.
TifU poles 4 and 5 were installed.

The wavelength at the center frequency is 60μ, and the electrode pitch is 6
It was set to 0μ.

The input IDT has a crossing width of 1.000 mm, and the output IDT has a crossing width of 1.077 mm, and is a so-called regular type IDT consisting of 15 pairs of electrodes having a uniform crossing width for both input and output.

In this embodiment, the present invention is applied only to the input IDT, and among the 16 pairs of electrodes, as shown in FIG. A pair of electrodes consists of only a split electrode 1.

In the eight pairs of electrodes 1 in the center part, one part 1' is made of solid and the other part is made of split, the ratio of solid and split electrodes is 1:13, and the relative positional displacement is 4.3μ. did.

As a result of driving the sample device of this example described above with a 50Ω signal source and operating it with a 50Ω load added, TT
E was approximately 48 dB.

This is an improvement of about 6 dB compared to a surface acoustic wave device consisting of a normal split-structure IDT, and the improvement effect is large.

As described above, according to the present invention, TT
Since we have obtained an acoustic surface wave device with significantly less E effect,
The present invention is expected to expand into a wide range of applications that could not be used due to TTE or other obstacles, and the contribution of the present invention to the acoustic surface wave device is extremely large.

[Brief explanation of the drawing]

Fig. 1 is a top view of a partial structure of two types of electrode shapes constituting the interdigital transducer, Fig. 2 is a schematic diagram showing the arrangement of the interdigital transducer, and Fig. 3 is a schematic diagram showing the arrangement of the interdigital transducer. It is a partially omitted diagram of a pattern structure applied to the electrodes of an input finger transducer as a practical example of a finger transducer. 1.2...Split electrode, 2...Periodic position of split electrode, 3...Solid electrode, 4...Input finger transducer , 5...
・Output sentence finger converter, 6...L t N b
Os substrate, 7... Electrode consisting of a combination of solid and split to which the present invention is applied, 1'... Solid part of electrode 7, γ"... Split of electrode Department.

Claims (1)

[Claims] 1. In a surface acoustic wave device in which at least one of the plurality of crossed phase converters arranged on a piezoelectric substrate has N pairs of crossed phase converters, the crossed phase converter is divided into three electrode parts. The N/2 pairs of electrodes in the center are composed of solid electrodes and split electrodes, and the N/4 pairs of electrodes on both sides are split electrodes.The electrodes have a symmetrical structure. An acoustic surface wave device having a characteristic crossed phase converter.