JPS6141448B2 - - Google Patents

Description

Detailed Description of the Invention The present invention relates to a surface acoustic wave filter (hereinafter referred to as
The present invention provides a SAW filter that reduces in-band ripple without increasing insertion loss, particularly with respect to the transducer configuration.

By providing an interdigital type input/output transducer on a piezoelectric substrate, it is possible to exchange electrical signals with the surface acoustic waves of the piezoelectric substrate. proposed the SAW filter by taking advantage of its property of being able to pass only signals in a specific band, and its large degree of freedom in design has led to its use in various applications such as intermediate frequency filters in television receivers. It is anticipated.

However, in conventional SAW filters,
In general, it has been difficult to improve both the insertion loss and the ripple in the frequency characteristics within the band, so it has not yet been put into practical use. Ripples in this band are mainly caused by multiple reflections of surface acoustic waves between the input and output transducers, and among them, the surface acoustic waves emitted at the input transducer are reflected at the output transducer. The main reason for this is the so-called triple reflected wave (hereinafter referred to as TTE), which is reflected at the input transducer section, reflected once again at the input transducer section, and enters the output transducer together with the original surface acoustic wave.

In conventional SAW filters in which input and output electrodes with equal length electrode fingers are arranged in parallel, this
It is known that the TTE level is approximately TTE (dB) ≒ - (2 x insertion loss + 6) (dB). For this reason, even when used in intermediate frequency filters for television receivers, if the insertion loss were to be reduced to about 10 dB, which is the practical limit, the TTE would be about -26 dB, making it completely impractical. It is.
This is because, in this application, the TTE level is required to be -40 dB or less. If the TTE level is high and the ripple in the frequency characteristics within the band is large, the sharpness of the television image may be lost or, conversely, it may be overemphasized, and color shifts or color differences may occur. This is because the adverse effects on the screen will be greater, such as the appearance of ghost images. This ripple and the group delay, which is a phase characteristic, are related to each other, and although it cannot be generalized, it can be said that fine ripples correspond to ripples in the group delay.
Therefore, in order to obtain ripple-free group delay characteristics, ripple-free frequency-amplitude characteristics are also required.

As is well known, there are two causes for the occurrence of multiple reflections in SAW filters as described above. The first is discontinuity in acoustic impedance due to placing metal etc. on the surface of the piezoelectric material. The second problem is that the surface waves are radiated again (referred to as re-radiation) because a load is connected to the output transducer. Various methods have been considered to eliminate this multiple reflection. For example, a reflection transducer is provided in addition to the input/output transducer, and the reflected wave from the reflection transducer is transferred to the input transducer or the output transducer. A transducer that tries to cancel the reflected wave by superimposing it on the reflected wave from the transducer, or one that drives the input with a two-phase or three-phase power supply to emit surface waves only in one direction, and also has an output transducer. There is a method of receiving surface waves in two or three phases and converting them to one phase. However, in the former case, there are disadvantages such as the inability to obtain a wide band and an increase in the element area due to the increase in the number of transducers.In addition, in the latter case, the electrode structure becomes complicated. However, there were disadvantages such as the need to prepare a two-phase or three-phase power supply and reverse conversion.

Therefore, the present invention aims to eliminate such conventional drawbacks and provide a SAW filter that can reduce the ripple in the frequency characteristic within the band without increasing the insertion loss with an extremely simple configuration. This is the purpose.

Therefore, in the present invention, on the piezoelectric substrate,
With the input transducer in the center, output transducers are provided on both sides of the input transducer, and the distance between the input transducer and the two output transducers is set in a direction perpendicular to the propagation direction of the surface acoustic wave. Therefore, it is characterized in that the output transducer is constructed so as to vary linearly, and the two output transducers are connected in parallel, which is further connected to a load.

Hereinafter, one embodiment of the present invention will be described in detail with reference to the drawings.

FIG. 1 shows a schematic plan view thereof, in which dimensions and inclinations are emphasized for ease of understanding. In the figure, 1 is a piezoelectric substrate made of piezoelectric material, 2 is an interdigital type input transducer provided in the center of the main surface, 3, 4
are interdigital output transducers arranged on both the left and right sides of the input transducer 2. 21, 31, 41 and 22, 32, 42 are common electrodes and electrode fingers of each transducer. A piezoelectric ceramic material (for example, PCM: product name of Matsushita Electric Industrial Co., Ltd.) may be used for the piezoelectric substrate 1, and each transducer 2,
3 and 4 are formed by vapor depositing metal such as gold or aluminum on the surface thereof. Note that although several tens of electrode fingers 22, 32, and 42 are actually provided in the transducers 2, 3, and 4, only a small number are shown here for ease of understanding. Further, in this embodiment, each transducer 2, 3,
4 shows the case where both are in normal form.

In the present invention, the distance between the input transducer 2 and the output transducers 3 and 4 is set so that the distance between the input transducer 2 and the output transducers 3 and 4 is changed linearly in the direction perpendicular to the propagation direction of the surface acoustic wave (the direction of the broken line in the figure). The present invention is characterized in that the two output transducers 3 and 4 are connected to an output load 5. That is, the spacing l ₁ , l ₂ at one end in the direction perpendicular to the propagation direction of the surface acoustic wave is different from the spacing l ₃ , l ₄ at the other end, and the spacing is diagonally changed between them so that the spacing changes linearly. It is formed by And the center input transducer 2
An electrical signal is applied from the signal source 6 to the surface acoustic wave, which is converted into an electrical signal by the output transducers 3 and 4 on both sides, and taken out in parallel to provide an output signal to the load 5. I'm trying to be able to do that.

FIG. 2 shows a comparison of the frequency characteristics of the SAW filter of this embodiment and a conventional SAW filter. The solid line A is the characteristic of the above embodiment, and the broken line B is the characteristic of the conventional SAW filter. The characteristics shown are the results of frequency response calculations using a cross-field model that is commonly used for characteristic analysis of SAW filters, and it is well known that the calculated values match the actual measured values extremely well. be.

Here, the calculation conditions are that the number of electrodes in each of input/output transducers 2, 3, and 4 is 32, and the resistance of load 5 is 50.
Ω, and the internal resistance of the power supply 5 was 50Ω. Also, for matching, a 0.44μH coil is connected in parallel to the load 5, and a 0.2μH coil is connected in parallel to the signal source 6.
Connect the H coil. Note that even this is not completely matched. As the material constants of the piezoelectric material used for the piezoelectric substrate 1, those of piezoelectric ceramics (trade name: PCM, manufactured by Matsushita Electric Industrial Co., Ltd.) were used. That is, the coupling coefficient was 0.2, the capacitance per 1 mm of each electrode finger was 4.9 pF, the sound velocity was 2330.5 m/sec, and the transducer aperture was 0.3 mm. It was assumed that there was no propagation loss, no diffraction, and no mismatching of acoustic impedance due to the deposition of electrodes on the substrate. In addition, the center frequency was set to 56.5MHz.

In this example, l ₁ = l ₂ = 26.0λ ₀ , l ₃ = l ₄ =
26.3λ ₀ (However, λ ₀ (However, λ ₀ =υ/ ₀ :
Characteristic A was obtained by assuming that υ is the speed of sound and ₀ is the center frequency), and that the interval changes linearly between them. In various attempts, l ₃ −l ₁ =l ₄ −l ₂ =0.15
Good results were obtained with ~045λ ₀ . In the conventional model, when l ₁ = l ₂ = l ₃ = l ₄ = 26.0λ ₀ , characteristic B
It was hot. As is clear from the figure, in the above example, although the insertion loss is increased by about 1 dB compared to the conventional example, the ripple within the band is significantly reduced, and a SAW filter that can be used almost practically is obtained. I understand that. In this case, the number of parameters required to obtain the desired frequency characteristics is significantly reduced, making the design easier.

Note that in SAW filters for television receivers, etc., the center frequencies of the input transducer and the output transducer are made different to widen the bandwidth, but the present invention does not apply to such a type. It has also been confirmed that it is effective when applied.

In addition, in SAW filters, transducers with different lengths of electrode fingers (electrode length weighted transducers) or transducers with different electrode finger spacing along the X direction are often used in one transducer. Although a deducer is used, the present invention can only be applied in such a case. In addition to this, well-known split electrode fingers may be used, but the present invention can of course be applied to such cases as well.

Regarding the configuration shown in FIG.
and the resistance value of load resistance 5 to 75Ω, and the value of load resistance 5 to 1KΩ.
When the calculations were made by changing each of
It was confirmed that a more significant effect was obtained than in the case of . These specifications may be set according to the constants of the materials used, the weighting of the electrode fingers, etc.

As is clear from the above description, the present invention provides a SAW filter with excellent characteristics that can significantly reduce in-band ripple without significantly increasing insertion loss compared to conventional SAW filters. This can be obtained with a simple configuration.

[Brief explanation of the drawing]

FIG. 1 is a plan view of a surface acoustic wave filter according to an embodiment of the present invention, and FIG. 2 is a characteristic diagram showing the frequency characteristics of the same surface acoustic wave filter and a conventional surface acoustic wave filter. DESCRIPTION OF SYMBOLS 1... Piezoelectric substrate, 2... Input transducer, 3, 4... Output transducer, 5... Load, 6... Signal source.

Claims (1)

[Claims] 1 On the piezoelectric substrate, output transducers are provided on both sides of the input transducer, with the input transducer in the center, and the distance between the input transducer and the output transducer is set perpendicular to the propagation direction of the surface acoustic wave. What is claimed is: 1. A surface acoustic wave filter that varies linearly in a direction, and is characterized in that the above two output transducers are connected in parallel, and further connected to a load.