JPS61290812A - Surface acoustic wave device - Google Patents

Abstract

PURPOSE:To suppress multipath reflection between electrodes as far as possible, to reduce distortion of amplitude and phase characteristic and to obtain good characteristic by determining the relation between matching condition in a one-way electrode and parameter indicating directivity to optimize it over a whole band. CONSTITUTION:When (a) denotes the ratio of energy of radiation of input or output conductance Ga of combined group of reed screen electrodes and a phase-shifting circuit and conductance Gl of power source side or load side from a group of input or output electrodes to another group of output or input electrodes to energy radiated to opposite direction, center frequency of band is made Ga not equal to Gl or a not equal to 0 and at the same time, made to almost satisfy the relation of the expression for any frequency in the band. Groups 7, 8 of input reed screen electrodes and groups 9, 10 of output reed screen electrodes are arranged on a surface acoustic wave substrate 5 at intervals of geometrical phase difference phiM(=2pi.l/lambda, l: distance between reed screen electrodes, lambda: wavelength of surface acoustic wave).

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Application of the Invention] The present invention relates to a surface acoustic wave device having good characteristics within a desired band, low loss, and little distortion in amplitude and phase frequency characteristics.

[Background of the invention]

Conventionally, regarding unidirectional electrodes, for example, there has been a ``flat amplitude surface acoustic wave filter using unidirectional interdigital electrodes'' that uses unidirectional electrodes to achieve both low loss and low ripple. A paper on this topic is included in the Proceedings of the Acoustical Society of Japan, Volume 1-5-14 (Toshibata Meguro et al., October 1976) K.

Here, a unidirectional electrode is one in which an electrical phase difference is given between one or more input or output electrode groups, and unidirectionality is obtained through interaction with the geometric phase difference. This is one method of reducing losses.

With unidirectional electrodes, electrical synergy input from the electrical terminals has surface acoustic wave energy radiated in the direction of the opposing output or input interdigital electrode group (forward direction), and surface acoustic wave energy radiated in the direction of the opposing output or input interdigital electrode group (in the opposite direction). This is converted into surface acoustic wave energy radiated in the opposite direction (in the opposite direction).
98M) zuppl, ) by Yamada et al.
Relationship between insertion loss and TTE in unidirectional SAW J
(@Ralαtionof tAg 1%5ert
ion Lots anti the Tri
pla Transit Ecl Oin Str
JnitLirmational TransmueL1
Similar to the paper entitled ``1''), the ratio of backward energy to forward energy is defined as a parameter α representing directionality. That is, when α is 00, it is completely unidirectional, and when α is 1, it is bidirectional.

Conventionally, in this type of unidirectional electrode, the input or output conductance Ga including the phase shift circuit and the interdigital electrode and the external conductance at are equalized at the center frequency and matched, and the above parameter α is set as wide as possible. The general design approach was to make it smaller. However, as will be explained later, this condition holds true when considering the characteristics of the entire band.
The conditions were not necessarily the best.

[Purpose of the invention]

The purpose of the present invention is to define the relationship between the matching condition in the unidirectional electrode described above and the parameter α representing the directionality so as to be optimal across the metal body in the band. It is an object of the present invention to provide a surface acoustic wave device that suppresses distortion as much as possible, has little distortion in amplitude and phase characteristics, and can obtain good characteristics.

[Summary of the invention]

The unidirectional electrode according to the present invention will be schematically explained with reference to the fa1 diagram. Input interdigital four-pole group 7 on surface acoustic wave substrate 5.
8 and the output interdigital electrode group 9゜10 with a geometric phase difference φ
M (=2π・?, t: distance between the interdigital electrodes, λ: wavelength of the surface acoustic wave), and a phase shift circuit is arranged so that the electrical phase difference between the interdigital electrodes is φE1 and the voltage ratio Vt. Inductance element L1 s Lt s Capacitance element '1 s '! is determined. At that time, the parameter α representing the directionality is given by: Incidentally, a surface acoustic wave absorber 6 is coated on the end face of the substrate in order to suppress reflection from the end face of the substrate.

Here, the input unidirectional electrodes are these interdigital electrodes 7.
．． 8 and Ll s '1 s Lt s't are put together, and as shown in Fig.
, 2 and a pair of electrical terminals 3 can be represented as an equivalent three-terminal pair circuit 11. The mechanical characteristic impedance Z11 is connected to the mechanical side terminal 1.2, and the power supply conductance Gs is connected to the electrical side terminal 3, and the scattering matrix is obtained.Thus, the relationship between the directionality and the reflection at the mechanical side terminal can be obtained. can.

Assuming that the scattering matrix is lossless and real, 15+r l"+ISI@I"+15181"
= 1+5. , ＋! +15tR1”+15.+1
= 115.11+ISn l”+ 15*sl”
= 1. Furthermore, S°=Sji (reversible), and from the no-loss condition, S11 SII+Sl! StB+51151
1" O. This is because the loss in the circuit system is due to mismatch.

h = Gs/Ga as A-1 533° Shigo sl, -1-s latitude J: I-芊GEO 5u = "-Seitate Ztan U a-)-1 However, de = h-f-1. ...(41A = G5
/ to α ...... (as mentioned above, the equivalent circuit l) was calculated assuming that there is no passive loss and each element of the scattering matrix is a real number, but in actual equipment, the loss of the phase shift circuit is usually set to be sufficiently small with respect to the radiated energy, and the combined susceptance of the device and load at the electrical side terminal is set sufficiently small with respect to Ga within the desired band, so the above assumption is sufficient for image reception. be.

Here, the reflected wave at the machine side terminal, represented by I5.11'', undergoes multiple reflections between the input and output interdigital electrodes, appears as an unnecessary delayed signal at the output end, and causes ripples in the amplitude and phase characteristics within the band. Normally, this reflection is called interelectrode multiple reflection (TT
It is called E). FIG. 3 shows the relationship between 4 and the degree of rrx suppression obtained from equation (3). In this way, the direction α that minimizes &CTTE is expressed as a function of h as “=7=dq°−−−−−(e).G
, are approximately constant within the desired band, whereas am is usually a function of frequency. Therefore, it is possible to suppress TTE within the band by setting the parameter α representing the directionality to a value close to equation (6) without approaching 0 throughout the desired band. For example, if the desired interelectrode TTE suppression ratio in the interdigital electrode group on one side of the input or output is 1/C2, then (3
According to equation 1, if the directionality C of the entire band is ra-1rC+1 e-)-1≦a≦o-1 (7),
Reflected waves within the band can be suppressed to a desired suppression ratio or less.

In this way, it was conventionally thought that TTE could be suppressed by setting the parameter α, which represents the directionality within the band, as small as possible and setting Gα = aS at the center frequency. It has been found that by setting the directionality to an appropriate value that satisfies equation (7), it is possible to suppress TTE throughout the band.

Here, if G is replaced with the load conductance GL added to the output terminal 4, the relationship between loss and multiple reflection in the output interdigital electrode group 9 and 10 can be considered in the same way as above.

[Embodiments of the invention]

Hereinafter, the present invention will be explained in detail using FIGS. 4 to 7. FIG. 4 schematically shows a unidirectional electrode according to an embodiment of the present invention. The interdigital electrodes use group type unidirectional normal type electrodes of 2 pairs and 10 groups as the same input and output electrode structure, and each group has an interdigital shape. The geometric phase difference of the electrode group 7.8 was set to π/2.

The substrate was a lithium niobate single crystal with 128 degree rotation on the Y axis and propagation on the X axis, the center frequency was 10 dE, and the aluminum electrode film thickness was 500A.

In addition, the phase shifter constants are C,=11F, LA=2
I chose 20%B. The power supply and load resistance at of the circuit system were both about 29 m5.

For reference, FIG. 5 shows the frequency characteristics of a unidirectional electrode according to a conventional method. The directionality is designed to be as wide as possible within the band, and R8 = so that S becomes Ga = at the center frequency.
It was set at 50. In the figure, 12 is a forward loss, 13 is a reverse loss, 14 is a degree of rrx suppression, and 15 is a parameter a1.1 representing the directionality that gives the maximum degree of rrx suppression according to equation (6).
6 is an actual measured value of the parameter a representing directionality. The reverse characteristic can be easily measured by connecting the power supply and load terminals to the interdigital electrode 8 side. Further, the parameter α representing directionality can be determined from the difference between forward and reverse losses. The degree of TTE suppression at the center frequency is large, but the α curve becomes h around 98MHz and 1ozMHz.
The degree of TTE suppression is also 3 near that frequency.
It can be seen that it has deteriorated to about 6dE. At this time, the loss was 4 ttE, amplitude ripple rJ, 5 dB% group delay time ripple 26 ns, -, and P-P.

In Fig. 6, cL, 2o and α21 are made close to each other throughout the band.

3 shows frequency characteristics of an embodiment of the present invention that improves the degree of TTE suppression within the band. In order to make α and eL− close to each other throughout the band, R8 was set to 150. At that time, loss 1. tLB,
Amplitude ripple Q, 2dE, -, 2nd group delay time, pull 2
1 s z p -p. 17 is forward loss, 1
8 is the reverse direction loss, and 19 is the TTE suppression degree. A TTE suppression degree of 5f3dB or more was obtained throughout the entire band.

FIG. 7 shows the frequency characteristics of the embodiment of the present invention using the phase shifter shown in FIG. The interdigital electrode has the same structure as the above embodiment, but as a phase shifter, Li='L3μ'eL
t=230nlξC,=2.2pF,C,=.

'lpF value was selected. With this structure, it is possible to make a, 25 and eL 26 close to each other within the band without using the variable resistor R1. Here, 22 is the unidirectional loss,
n is the reverse direction loss, and s is the TTE suppression degree.

In this example, the variable resistor R8 is not used, so the loss is reduced to 5.
It can be improved to below rLB, and it is 44cLB throughout the entire band.
The degree of TTE suppression above is obtained, and the amplitude repggle α1tLBP−
, 0 value of the first group delay time ripple 11r' was obtained.

The explanation above has been about group type unidirectional electrodes, but it goes without saying that similar effects can be obtained with three-phase excitation type unidirectional electrodes (if they are designed to satisfy Equation 71). stomach.

〔Effect of the invention〕

As explained above, according to the present invention, it is possible to suppress crosstalk, inter-electrode multiple reflection, insertion loss, amplitude ripple, and group delay time ripple throughout the entire desired band, and a low-loss surface acoustic wave device. This is effective in flattening the frequency characteristics within the band.

[Brief explanation of the drawing]

FIG. 1 is a schematic explanatory diagram of a unidirectional electrode according to the present invention, and FIG.
The figure is an equivalent three-terminal pair circuit diagram of a unidirectional electrode, FIG. 3 is a diagram showing the relationship between directionality and TTE suppression degree, FIG. 4 is a schematic diagram of a unidirectional electrode according to an embodiment of the present invention, and FIG. The figure is a frequency characteristic diagram of a unidirectional electrode designed using a conventional method, and FIGS. 6 and 7 are frequency characteristic diagrams of an embodiment of the present invention. 12.17.22...Forward loss characteristics 13.18.2
3... Reverse loss characteristics 14.19, 24... TTE
Suppression degree 15.20, 25... Parameter representing the directionality that gives the maximum TTE suppression degree 16.21.26... Parameter R5 representing the directionality
...Variable resistance L1a'1 m"* mC*...Inductance element and capacitance element that constitute the phase shift circuit.
("way'i") Grass 4 Fig. ni Period wave number (r'lHz) Measure 6 Fig. i'l iK number (MH:r) '7th WJ Shu precept number (Jjfh)

Claims (1)

[Claims] 1) At least one input/output interdigital electrode group is disposed on a piezoelectric substrate or a surface acoustic wave substrate including a piezoelectric body,
In addition, in a surface acoustic wave device in which a circuit is provided to create an electrical phase difference between each interdigital electrode forming the input or output interdigital electrode group to make a unidirectional electrode, one interdigital electrode group and The combined input or output conductance Ga of the phase shift circuit and the conductance Gl on the power supply side or load side are in the opposite direction to the energy radiated from one input or output electrode group to the other output or input electrode group. When the ratio of energy radiated to is a, Ga≠Gl or a≠ at the band center frequency.
0, and the relationship a=(Gl-Ga)/(Gl+Ga) is substantially satisfied at any frequency within the band. 2) Set the desired suppression ratio of multiple reflected waves between the input and output interdigital electrodes to 1/c^2 at one side of the interdigital electrode group, and always accurately set r=(Gl-Ga)/(Gl+Ga) at all frequencies within the desired band.
), the Ga, Gl, a are within the desired band (r
c-1)/(c+1)≦a≦(rc+1)/(c-1)
A surface acoustic wave device according to claim 1, which satisfies the following relationship. 3) As a phase shift circuit, an inductance element and a capacitance element are electrically connected in parallel with each interdigital electrode of the interdigital electrode group, and an inductance element is connected between the non-grounded electrodes of each interdigital electrode group. The surface acoustic wave device according to claim 1, wherein the element and the capacitance element are inserted in series.