JPH05235691A - Surface acoustic wave device - Google Patents

Abstract

PURPOSE:To comprise a device based on simplified design to compensate velocity dispersion property by deciding the time position of an electrode finger by assigning SAW propagation distance considered from the viewpoint of a center frequency and band area upper and lover limit frequencies. CONSTITUTION:Positions to arrange the electrode finger setting a medium provided with the dispersion property in which acoustic velocity is represented in K=K0+alpha(omega-omega0)<2>+beta(omega-omega0)<2> as a substrate and comprised of a chirp transducer formed on the substrate are assumed as Z1-Zn. At this time, the device is comprised so that the positions Z1-Zn can be supplied as the time positions tn expressed in equation I setting the position of the electrode finger to excite the SAW by the center frequency omega0 as reference Z=0. Where, the lower limit frequency of a band area in equation is expressed as omega1, the upper limit frequency as omega2, the propagation distance of omega0 for the SAW as 2d, electrode capacitance as C1, the conductance in the inside of a power source or a load as GL, and coefficients as alpha, beta. The time position tn can be dcided in equation I, and furthermore, spatial position Zn can be found by multiplying by group velocity of a corresponding frequency.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a surface acoustic wave device, and more particularly to an improvement of an in-line type chirp transducer in a surface acoustic wave device using a medium in which the speed of sound has frequency dispersion.

[0002]

2. Description of the Related Art A chirp transducer is a radar
Wide range of useful electrical applications, such as pulse compressors and real-time analog Fourier transforms in systems, where delay time is given as a function of frequency and wide-band characteristics that are relatively easy to obtain. -Acoustic conversion means.

As a basic configuration of the chirp transducer, as shown by reference numeral 1 in FIG. 2, there is a structure in which the electrode finger width and the electrode finger interval monotonically increase or decrease along the surface acoustic wave (SAW) propagation direction. However, the positions of equivalent excitation sources that excite the SAW are distributed in the electrode according to the frequency (instantaneous frequency) of the SAW.

The transducer shown in FIG. 2 is generally called an in-line type, and the intersections of the electrode fingers in the electrodes are arranged such that the centers of the electrode fingers in the longitudinal direction are parallel to the SAW propagation direction. This electrode has the simplest shape of the chirp type and is characterized in that the area occupied by the electrode on the element is relatively small.

As a method of designing the above chirp transducer, W. R. Smith, H.C. M. Gerard, and W.
R. Jones, “Analysis and Design of Dispersive
Interdigital Surface-Wave Transducers ”, IE
EE Trans. Vol. MTT-20, No. 7 (197)
The method shown in the document 2) is widely used in the basic stage of design, and usually, correction for improving the amplitude / phase characteristic is added.

The following equation (2) is an equation which is shown in the above-mentioned document and determines the position of the electrode finger which is the most important in the design of the chirp transducer. However, this equation (2) is the dispersion delay time and the instantaneous frequency (the time in the impulse response of the filter is the dispersion delay time and corresponds to the spatial position where the electrode finger is arranged. An electrode corresponding to a certain dispersion delay time) The SAW frequency at which the finger is most strongly excited is the instantaneous frequency with respect to that time.) Has a linear relationship, and is obtained on the premise of a so-called linear chirp filter. The characteristic is shown in the graph of FIG. BW in the figure
Is the operating frequency bandwidth, ω ₀ is the central angular frequency, ω is the angular frequency, and T is the dispersion delay time.

[0007]

[Equation 2]

By giving Q _L and the desired BW and T for setting the input impedance of the electrode to this equation, the time position where the electrode finger is placed can be determined.

By the way, it is known that when the substrate material of the SAW element has frequency dispersiveness of sound velocity, various characteristic deteriorations occur due to different propagation delay times between different frequency components within the operating frequency band. For example, a monolithic SA having a layered structure in which a piezoelectric thin film is formed on a semiconductor substrate.
In the case of the W convolver, the pulse width of the autocorrelation output and the spurious level increase due to this dispersiveness. Further, the SAW convolver is advantageous to widen the operating frequency band in order to enhance the signal processing capability,
There is a problem that the bandwidth is limited due to the frequency dispersion of sound velocity. For such a velocity dispersive medium, the chirp transducer can change the propagation delay time of a signal according to its frequency, and thus can be used for the purpose of compensating the dispersibility.

[0010]

In order to design the chirp transducer for the purpose of compensating for the dispersibility described above, the arrangement of the electrode fingers must be determined so as to satisfy the desired instantaneous frequency-dispersion delay time relationship. However, since the expression (2) is intended for the linear chirp filter as described above, it cannot be applied to the design of electrodes having an arbitrary frequency-time relationship. This is clear from the fact that only BW and T are required as conditions for indicating the frequency-time relationship given in the above equation (2).

To solve the above problem, as indicated by reference numeral 2 in FIG. 3, signals are distributed according to their frequency components in a direction orthogonal to the SAW propagation direction, and the oblique electrodes can control the propagation delay time on different propagation paths. An example using a type transducer is known [for example, JP-A-2-13809 (Japanese Patent Application No. 63-276664)].

However, since the basic principle that the portion where the electrode fingers intersect serves as the excitation source of the SAW is the same, the effective electrode finger intersection length corresponding to a certain frequency component is determined by considering the diffraction phenomenon. There is a lower limit, and there is a drawback that the required substrate area increases as the operating frequency bandwidth expands. For devices such as SAW convolvers that require higher power density in the interaction region to increase efficiency. Is not suitable for

[0013]

SUMMARY OF THE INVENTION An object of the present invention is to provide a SAW element using a medium in which the speed of sound is frequency dispersive, and a surface acoustic wave which can be constructed based on a simple design for compensating the speed dispersiveness with an in-line chirp transducer. The purpose is to provide a device.

[0014]

According to the present invention, the speed of sound has frequency dispersibility, and the dispersibility is k = k ₀ + α (ω-ω ₀ ) +
β (ω−ω ₀ ) ² [where ω = angular frequency, ω ₀ = center angular frequency, k = angular frequency of SAW with respect to angular frequency ω, k
₀ = SAW wavenumber with respect to center frequency, α, β = coefficient]
Is used as a substrate, and the positions Z _{1 to} Z _n where the electrode fingers l _{1 to} l _n formed by the chirp transducer formed on the substrate are arranged are the positions of the electrode fingers that excite the SAW at the center frequency. With reference Z = 0, the time position t _n (= Z _n / v _g (ω _n )) represented by [Equation 3], ω _n is the frequency of the SAW excited most strongly at the electrode l _n , and v _g (ω _n ) Is ω _n
The group velocity of the SAW with respect to is the gist.

[0015]

[Equation 3]

[0016]

According to the present invention, if the SAW propagation distance 2d considered by the center frequency component and the band upper / lower limit frequencies ω ₁ and ω ₂ are given in the above equation, the time position t _n of the electrode finger l _n of the transducer can be calculated. The spatial position Z _n is obtained by determining and then multiplying by the group velocity v _g (ω _n ) of the corresponding frequency.

[0017]

FIG. 1 shows an embodiment of the present invention. In the present embodiment, only one transducer of the SAW filter is a chirp type to compensate for dispersion, 3 is an in-line chirp transducer for dispersion compensation, and 4 is a normal type transducer. First, FIG. 5 shows an example of the relationship between the angular frequency ω (hereinafter simply referred to as frequency) in the velocity dispersive substrate and the group velocity v _g (ω). In such a medium, the SAW wave number k of the frequency ω is expressed as a function of ω, but if approximated around the operation center frequency ω ₀ ,

[0018]

[Equation 4]

Can be written as Also, the group velocity v
_g (ω) is _calculated from equation (4)

[0020]

[Equation 5]

Can be related to Here, k ₀ is the wave number with respect to the center frequency ω ₀ , α and β are respectively linear,
Is a quadratic coefficient,

[0022]

[Equation 6]

[Equation 7]

Is represented by Now, assuming that the required SAW propagation distance is 2d as shown in FIG. 1, the propagation delay time τ
_g (ω) is expressed by the following equation (8).

[0024]

[Equation 8]

Assuming that the upper and lower limits of the required operating frequency bandwidth BW are ω ₁ and ω ₂ , respectively, in a dispersive medium, τ
_g (ω ₁ ), τ _g0 , and τ _g (ω ₂ ) are generally all different, and the purpose of the dispersion correction is

[0026]

[Equation 9]

[0027] In order to compensate the delay time approximated by the above equation (8) with the electrodes, it is necessary to appropriately position the electrode fingers where the SAW is excited according to the frequency thereof. The position of the electrode finger that excites the center frequency component is the reference (Z = 0, the SAW propagation direction is positive)
And then, the time required for the frequency omega _n relative position Z = Z _n should be placed electrode fingers l _n for exciting components, also components of omega _n is Z _n propagation, i.e. dispersive delay time t _n (which For the spatial position Z _n , we call the time position)

[0028]

[Equation 10]

[Equation 11]

To obtain This is the relational expression of ω-t which is the basis of the chirp transducer design treated in the present invention.
Generally, there is a non-linear relationship as shown in the graph portion of FIG. By the way, W. R. The first term on the left side of the equation (2) by Smith corresponds to the phase component of the impulse response of the chirp transducer, and the second term in () corresponds to the instantaneous frequency. On the other hand, in the case of the chirp transducer to be realized by the present invention, the instantaneous frequency ω _n is given by the above equation (11), and the phase θ _n is given by the following equation (11).

[0030]

[Equation 12]

Is given by Therefore, by replacing these ω _n and θ _n with the corresponding components of the equation (2), the notation of the equation (2) according to the object of the present invention is as follows (where the equation (2) is The time position at n = N / 2 is t (N
/ 2) = 0, so the right side is (n-N / 2)
Although a [pi, here using a constant C ₀ t (N /
2) = 0 condition is excluded).

[0032]

[Equation 13] In this case, the number of electrode fingers N is

[Equation 14]

Is given by This equation shows that the number of electrodes increases even if the coefficient β indicating the degree of dispersion, the propagation length 2d, or the bandwidth BW (= ω ₂ −ω ₁ ) increases. Constant C
Regarding _0, it is set to − (N / 2) π in the equation (2),
This means that N = N / 2 and t _n = t (N
/ 2) = 0. However, this condition is inappropriate in this case, and the conditions given in this case are t _n = t ₁ when _n = ₁ and t _n = 2 (t ₁ ,
t ₂ is the dispersion delay time for ω ₁ and ω ₂ , respectively. C ₀ obtained from this condition is given by the following equation.

[0034]

[Equation 15]

From the above equations (13) and (15), the time position determining equation of the electrode finger becomes the following equation (16).

[0036]

[Equation 16]

As for β, if an approximate expression of the group velocity v _g (ω) within a desired operating frequency band is obtained, the above (7)
The SAW propagation distance 2d and the band upper / lower frequency limit ω ₁ ,
If ω ₂ is given, the time position t _n of the electrode finger l _n of the transducer can be determined from the above equation (16), and the spatial velocity can be determined by multiplying the group velocity v _g (ω _n ) of the corresponding frequency. It is possible to obtain various positions Z _n .

[0038]

As described above, according to the present invention, by using the above-mentioned method, the occupied area on the device surface is small,
In addition, it becomes possible to easily design and configure an in-line type chirp transducer having a simple electrode shape for compensating the frequency dispersion of sound velocity. Of course, in the case of constructing a filter, for example, one pair of input and output transducers are required, and when both transducers compensate equal velocity dispersion of the medium, as in the embodiment, Is considered to be a normal type and the other is compensated for, but in the former case, the propagation distance given to the equation (16) is half the propagation length between both transducers when the center frequency is targeted, In the latter case, it will be the full length.

[Brief description of drawings]

FIG. 1 shows an embodiment of the present invention, and is an explanatory diagram which constitutes an electrode finger of an in-line type chirp transducer.

FIG. 2 is a configuration diagram of a conventional in-line type chirp transducer.

FIG. 3 is a graph showing characteristics of operating frequency width-dispersion delay time.

FIG. 4 is a configuration diagram of a conventional oblique electrode type transducer.

FIG. 5 is a graph showing group velocity-angular frequency characteristics.

[Explanation of symbols]

1 In-line chirp transducer 2 Oblique electrode type transducer 3 Dispersion compensation in-line chirp transducer 4 Normal type transducer

Claims (1)

[Claims] 1. The speed of sound has frequency dispersibility, and the dispersibility is k = k ₀ + α (ω−ω ₀ ) + β (ω−ω ₀ ) ² [where
ω = angular frequency, ω ₀ = central angular frequency, k = angular frequency of SAW for angular frequency ω, k ₀ = S for central frequency
AW wave number, α, β = coefficient] is used as a substrate, and the positions Z _{1 to} Z _n where the electrode fingers l _{1 to} l _n formed by the chirp transducer formed on the substrate are arranged are center frequencies. With the position of the electrode finger that excites the SAW of the reference Z = 0, the time position t _n (= Z _n /
v _g (ω _n ), ω _n is the SAW excited most strongly at the electrode l _n
, V _g (ω _n ) is the SAW group velocity for ω _n )
A surface acoustic wave device characterized in that: [Equation 1]