JPS624890B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a surface acoustic wave filter, and an object of the present invention is to provide a surface acoustic wave filter (hereinafter referred to as a SAW filter) in which ripple is reduced without increasing insertion loss.

Since SAW filters have a large degree of freedom in design, it is being considered that they can be applied to a variety of filters. However, at present, there are many problems when it comes to practical use, and in particular, it has not been used in consumer equipment such as television receivers.

Insertion loss and in-band ripple are particularly important issues when using SAW filters as intermediate frequency filters for television. This ripple is caused by multiple reflected waves (TTE), which will be described later. Depending on the load, SAW filters have
Regarding the TTE level, the following relationship exists.

TTE (dB) ≒ - [2 x insertion loss (dB) + 6 (dB)] In other words, for example, if the insertion loss is 10 dB,
TTE is -26 dB, which is far from the -40 dB required for television filters. If the ripple is large, the sharpness of the television image may be lost or, conversely, it may be emphasized, color shift or color difference may occur, or ghosting may occur.
There is a strong relationship between ripple and phase characteristics (group delay), and although it cannot be generalized, small ripples are
It can be said that it corresponds to the ripple of group delay. Therefore, in order to obtain ripple-free group delay characteristics, it is required that the frequency-amplitude characteristics also be ripple-free.

As is well known, the main cause of ripples in frequency-amplitude characteristics or group delay characteristics is multiple reflections of surface acoustic waves (hereinafter referred to as TTE). In the following description, ripples in frequency characteristics are simply referred to as ripples. There are two causes of TTE. One is the discontinuity in acoustic impedance caused by attaching metal such as electrodes to the surface of the piezoelectric material, and the other is so-called re-radiation, which causes surface waves to be emitted again due to the load on the filter. It is. One method that has been considered to eliminate TTE is, for example, to provide a reflection transducer outside the input/output transducer and transfer the reflected wave from the reflection transducer to the input or output transducer. There is a way to try to cancel it by superimposing it on the reflected wave from Usa. Alternatively, the input may be driven by a two-phase or three-phase power supply, emitting surface waves in only one direction, and the output transducer may also be two- or three-phase.
There is a method of receiving surface waves in a phase and converting them into one phase. However, in the former case, there is a drawback that not only a wide band cannot be obtained, but also the number of transducers increases, resulting in an increase in element area.
Moreover, in the latter case, the electrode structure becomes complicated, and furthermore, it is necessary to prepare a two-phase or three-phase power supply and reverse conversion, which has the disadvantage that the additional circuit becomes large.

The present invention makes it possible to construct a SAW filter with relatively low insertion loss and low ripple with a simple transducer configuration that does not have such problems. That is, the present invention provides an input transducer on a piezoelectric substrate so that the spacing between the electrode fingers differs from one end to the other, and an elastic surface on both sides of the transducer so that the spacing between the electrode fingers does not change. This is a surface acoustic wave filter characterized in that output transducers are arranged so as to have an inclination angle with the direction in which waves should propagate, and with this configuration of the present invention, insertion loss is relatively small. , and a SAW filter with low TTE can be obtained, and the reason for this will be briefly described below.

The present invention attempts to reduce TTE (triple transit echo) by tilting the outer transducer and changing the surface wave propagation distance in each part. This is caused by reflection of surface waves caused by an acousto-mechanical mismatch caused by the provision of a metal different from that of the substrate serving as the electrode. Therefore, TTE appears as ripples in amplitude and phase due to filter characteristics. Here, the distance between the peaks of the ripples is 1/2τ (τ=1/v, where 1: distance between transducers, v: velocity of surface wave) in terms of frequency. The technical idea underlying this invention is to make τ slightly different so that the ripples on the frequency cancel each other out. In this case, acousto-mechanical reflections are easy to calculate, but electrical reflections are less straightforward. However, simulations can be performed using a computer. WR Smith has shown that calculations and reality match well when using a model called the cross-field model. The present inventors used this model to calculate the characteristics of transducers of various shapes, and as a result found that the above-mentioned configuration was superior, and based on this, the present invention was made.

The details will be explained below with reference to the drawings.

FIG. 1 is a plan view of one embodiment of the filter of the present invention, exaggerating the shape, arrangement, and wiring of its transducers. In the figure, piezoelectric substrate 1
An interdigital type input transducer 2 and output transducers 3 and 4 made of a metal film are arranged on the surface of the transducer. One end of the input transducer 2 is grounded and the other end is connected to the power supply 5.
It is connected to the. Output transducer 3, 4
One end of each is grounded, and the other end is connected to a resistor 6.
are connected in series by Of course, consideration is given to the polarity of the transducer so that the output can be extracted. Input transducer 2
is a tapered type, that is, the distance d between the electrode fingers is widened in the Y direction perpendicular to the direction in which the surface waves should propagate. On the other hand, the interval between the output transducers 3 and 4 is constant. The distance l between the transducers 2 and 3 and between them 2 and 4 is not constant in the Y direction. Here, the inter-transducer distances are the distances l _1a , l _2a , l _1b , and l _2b between the electrode fingers of adjacent transducers, although they are shown enlarged in the figure. Also, d _2a , d _3a , d _4a , and d _2b , d _3b , d _4b
are the electrode spacings in the strip-shaped regions between the broken lines in sections a and b, respectively, parallel to the X direction in which the surface waves propagate. Therefore, taking input transducer 2 as an example, section a
The center frequency (o) _2a at is expressed as (o) _2a = v/2d _2a v: surface wave velocity. Therefore, the input transducer 2 will have different center frequencies in the Y direction. On the other hand, for the output transducers 3 and 4, the electrode finger spacing is constant, and d _3a = d _3b , d _4b
= d _4b , and its center frequency is (
o) ₃ , (o) ₄ .

The gist of the present invention is that in the conventional well-known electrode configuration, ie, in FIG. 1, d _2a = d _2b , d _3a = d _3
b , d _4a = d _4b , l _1a = l _1b , l _2a = l _2b , but d _2a ≠ d _2b , d _3a = d _3b , d _4a
= d _4b and l _1a ≠ l _2b .

The electrode indices of the input and output transducers are respectively
32 wires, the load resistance is 50Ω, the power supply resistance is 50Ω, the parallel inductance for matching is 0.1μH, the parallel capacitance considered to be added during mounting is 30pF, and a piezoelectric ceramic substrate with a coupling coefficient of 0.2 and a dielectric constant of 750 is used as the delay medium. Then, the characteristics were determined by setting the transducer aperture, that is, the interval between sections a and b to 0.3 mm. The speed of sound in that medium is 2330.5
m/sec. It is assumed that there is no propagation loss, diffraction, or mismatch in acoustic impedance due to the provision of electrodes. The results are shown in FIG. Diagram A
is the result from the conventional regular transducer, (o) _2a = (o) _2b = (o) ₃ = (o) ₄
= 57MHz, l _1a = l _2a = l _1b = l _2b = 0.419 mm. Figure B shows the results according to the present invention, (o) _2d =
(o) ₃ = (o) ₄ = 57MHz, (o) _2b = 57・
675MHz, l _1a = l _2a = 0.419mm, l _1b = l _2b =
This is the result of setting it to 0.414mm. l _1a , l _2a , l _1b , l ₂
Regarding _b , good results were obtained up to l _1b = l _2b = 0.419 mm. This range is considered preferable. (o) _2b was varied from 57.45 to 58.35MHz, and good results were obtained. From the results shown in FIG. 2, in the case of the conventional normal type (FIG. A), the difference between the peak and valley of the characteristic curve is about 0.8 dB, which is about -21 dB with respect to the magnitude of the output signal. On the other hand, in the case of the present invention, as shown in Figure B, there are no peaks or valleys in the characteristic curve, so it cannot be determined as ripple, but the deviation from the smooth curve is about 0.2 dB.
, which is approximately - relative to the output signal magnitude.
It is 33dB. Therefore, this can be said to be an excellent characteristic.

As a result of various studies, we found that the distance l between transducers is determined by the center frequency (o) _2y of each section of the input transducer: 1 _1y = 1 _2y = n x v/(o) _2y n: Preferably, it is a constant. That is, for each section the input transducer center frequency (o) ₂
It is preferable to use l _1y and l _2y determined from the above formula using _y .

In intermediate frequency filters for television, the input transducer and output transducer often have different center frequencies. That is, for example, (o) _2a ≠ (o) ₃ = (o) ₄ , and in this case, the same results as in the above example were obtained.

In the above example, the center frequency (o) _2y of each section of the input transducer and the distance between the transducers are changed linearly in the Y direction, but of course this need not be the case. For example, in the case of the above equation, since the amount of change in l is small, it is almost a straight line, but to put it more precisely, l does not change linearly in the Y direction.

The values used in the above example are merely examples, and the center frequencies (o) _y , (o) ₃ , (o) ₄ and the inter-transducer distance l can be varied. Naturally, the preferred values mentioned above are subject to change.

Furthermore, in SAW filters, transducers with different electrode finger lengths (electrode length weighted transducers) and transducers with different electrode spacing are often used, and the present invention can be applied to such cases as well. . In addition, split electrodes are used to eliminate reflected waves due to well-known discontinuities in acoustic impedance, but the present invention can of course be applied to such cases as well.

FIG. 3 shows another embodiment in which the input transducer 10 and the output transducers 11 and 12 are each divided into two parts. Such division of transducers is known. Stage A and stage B each correspond to the input/output transducer of FIG. In this case as well, good results similar to those in the above example were obtained.

In the examples shown in Figs. 1 and 3, even if the load resistance was changed (power supply 75Ω, output resistance 1KΩ), the ripple was very good. The difference between the present invention and the conventional transducer was greater than when the power source was 50Ω and the output load was 50Ω.

As is clear from the above explanation, the present invention has great effects and has overcome the relationship between insertion loss and ripple, which has conventionally been a problem in terms of practical application.
This is what you can get from a SAW filter.

[Brief explanation of the drawing]

FIG. 1 is a plan view of an embodiment of the surface acoustic wave filter according to the present invention, and FIG. 2 shows its frequency characteristics, where A is that of a conventional example and B is that of a filter according to the present invention. be. FIG. 3 is a plan view of another embodiment. 1... Piezoelectric substrate, 2, 10... Input transducer, 3, 4, 11, 12... Output transducer, 5... Power supply, 6... Load.

Claims (1)

[Claims] 1. An input transducer is provided on a piezoelectric substrate so that the spacing between electrode fingers varies from one end to the other, and the spacing between electrode fingers on both sides of the transducer remains the same. A surface acoustic wave filter characterized in that output transducers are arranged so that the surface acoustic waves propagate in a direction and have an inclination angle. 2. A surface acoustic wave filter as set forth in claim 1, wherein the input/output transducer is divided into two in parallel to the direction in which surface acoustic waves are to propagate. 3. A surface acoustic wave filter according to claim 2, wherein the input/output transducer is formed symmetrically with respect to the propagation direction of the surface acoustic wave. 4. A surface acoustic wave filter according to claim 1, 2, or 3, characterized in that the output transducer is arranged symmetrically with respect to the input transducer.