JPS6150536B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a surface acoustic wave filter (hereinafter referred to as SAW) which obtains filter characteristics by providing input and output electrodes on the surface of a piezoelectric substrate and using surface acoustic waves.
Filters) can reduce in-band ripple without increasing insertion loss, especially with regard to transducer configurations.
It provides SAW filters.

By providing an interdigital input/output transducer on a piezoelectric substrate, it is possible to perform mutual conversion between electrical signals and surface acoustic waves of the piezoelectric substrate. The SAW filter was proposed by taking advantage of the property that the structure of the duplexer allows only signals in a specific band to pass through.The SAW filter has a high degree of freedom in its design, and is therefore used as an intermediate frequency filter in television receivers. Various uses are expected.

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, 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.
In this application, the TTE level is required to be −40 dB or less. in this way
If the TTE level is high and the ripples in the frequency characteristics within the band are large, the sharpness of the television image may be lost or on the contrary may be overemphasized, color shifts or color differences may occur, and even worse. This is because the adverse effects on the screen, such as the generation of ghost images, are greater. This ripple and
There is a mutual relationship with group delay, which is a phase characteristic, and although it cannot be generalized, it can be said that fine ripples correspond to ripples in group delay. Therefore, in order to obtain ripple-free group delay characteristics, it is required that there be no ripples in the amplitude-of-frequency characteristics as well.

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.
Second, since a load is connected to the output transducer, the surface waves are radiated again (referred to as re-radiation). 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 a wide band cannot be obtained and the element area increases due to the increase in the number of transducers, and in the latter case, the electrode structure becomes complicated. Furthermore, there are drawbacks such as the need to prepare a two-phase or three-phase power supply and the need for reverse conversion.

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

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, where dimensions are exaggerated for ease of understanding. In the figure, 1 is a piezoelectric substrate made of piezoelectric material, 2 is an interdigital type input transducer formed in the center of its main surface, and 3 and 4 are on both left and right sides with input transducer 2 in the middle. This is an interdigital type output transducer arranged and formed. Each transducer 2, 3, 4
is divided into two segments a and b in a direction perpendicular to the propagation direction of the surface acoustic wave, and the intervals between the input and output transducers are made to be different in these two segments a and b. That is, l ₁ ≠ l ₃ and l ₂ ≠ l ₄ are set. The propagation paths of surface acoustic waves are different between segment a and segment b. 2a ₁ , 2b ₁ , 3a ₁ , 3b ₁ , 4
a ₁ , 4b ₁ and 2a ₂ , 2b ₂ , 3a ₂ , 3b ₂ , 4a ₂ ,
_4b2 is a common electrode and electrode fingers of each transducer. The piezoelectric substrate 1 is made of piezoelectric ceramics (for example,
The transducers 2, 3, and 4 may be formed by vapor-depositing metal such as gold or aluminum on their surfaces. Although the number of electrode fingers in each segment a, b of each transducer 2, 3, 4 is actually several ten, only a small number are shown here for ease of understanding. Also,
In this embodiment, the segments a, b, and c of transducers 2, 3, and 4 are all normal.

In each transducer 2, 3, and 4, these two segments a and b are connected in parallel and used, and an electrical signal is applied from a signal source 5 to the central input transducer 2 to It is converted into a surface wave, reconverted by output transducers 3 and 4 on both sides, and connected in parallel so that an electrical signal can be provided to a load 6.

FIG. 2 shows a comparison of the frequency characteristics of the SAW filter of this embodiment and a conventional SAW filter.

Figure 2 shows the results of calculating the frequency response using the well-known cross-field model in SAW filters, and compares it with that of a conventional SAW filter. The calculation is based on input transducer 2
The number of electrode fingers in the output transducers 3 and 4 was 32, and the resistance of the load 6 was 50Ω, and the internal resistance of the signal source 5 was 50Ω. A 0.1 μH coil is inserted in parallel to the signal source 5 and load 6 for matching, and also to accommodate the increased capacitance during mounting.
A 30pF capacitor was also connected in parallel. Note that even with this, the results are not completely matched. The material constants of the piezoelectric material used for the piezoelectric substrate 1 were those of piezoelectric ceramics (trade name PCM67, manufactured by Matsushita Electric Industrial Co., Ltd.). i.e. coupling coefficient 0.2,
Capacitance per 1 mm of electrode finger 49 pF, sound velocity
The speed was set at 2330.5 m/sec. Further, the transducer aperture (maximum overlapping length of electrode fingers in segment a or segment b in FIG. 1) was set to 0.15 mm. It was assumed that there was no mismatch in acoustic impedance due to propagation loss, diffraction, or electrodes attached to the substrate.

FIG. 2A shows the characteristics of a conventional normal form with only one segment. It can be seen that there are many ripples.

On the other hand, FIG. 2B shows the example of FIG. 1, where d ₂ = d ₄ = d ₆ = V/2fo ₁ (V: speed of sound, fo ₁ : center frequency of segment a, 56.80 MHz) d ₃ = d ₅ = Results when d ₇ = V/2fo ₂ (fo ₂ : heart frequency of segment b 56.50MHz) l ₁ = l ₂ = 26λo ₁ = 26V/fo ₁ l ₃ = l ₄ = 26λo ₂ = 26V/fo ₂ It is. There were almost no ripples, clearly demonstrating the remarkable effects of the present invention. That is, the effect is most significant when l ₁ =l ₂ =nλ ₀₁ and l ₃ =l ₄ =nλ ₀₂ . Next, in Figure 3,
Another embodiment of the invention is shown. In this method, the two segments a and b are not separated using independent common electrodes as shown in FIG. 1, but are formed by bending the electrode fingers in the middle. Of course, it is equivalently the same as that in FIG. In FIG. 3, the intermediate portion between the two segments a and b is exaggerated for clarity, but in reality, this intermediate portion is very small.

Figure 4B shows the characteristics actually measured for the SAW filter having the shape shown in Figure 3. In addition, the fourth
Figure A shows the characteristics of a conventional one-segment type device in which the center transducer is weighted. From this figure as well, it can be clearly understood that the SAW filter of the present invention has less ripple and is excellent.

In addition, in the SAW filter for television receivers, the center frequency of the input transducer and the center frequency of the output transducer are different, that is, d ₂ ≠ d ₄ = d ₆ and d ₃ ≠ d ₅ = d _7. Good results were obtained even in such cases. three transducers 2,
It is possible to vary the total of six center frequencies of each of the third and fourth segments, and the present invention can be applied to such cases as well.

In the above calculation example, the spacings l ₁ , l ₂ , l ₃ , and l ₄ between the transducers were determined as described above, but these may vary depending on the load, material constants of the piezoelectric substrate, shape of the transducer, etc. It will change somewhat. Preferably l ₁ = l ₂ and l ₃ = l ₄ , but of course this may not be the case.

In order to obtain better frequency characteristics, well-known electrode length weighting is sometimes used in SAW filters. Even in such a case, the above l ₁ , l ₂ ,
Although l ₃ , l ₄ etc. may vary somewhat, the present invention can still be practiced as shown in FIG. Of course, any transducer may be weighted. It goes without saying that not only the electrode length weighting but also other weighting, such as weighting based on electrode width, electrode spacing, etc., may be used.

Furthermore, in order to prevent reflections due to acoustic impedance mismatching caused by the provision of metal electrodes, the well-known split electrode is sometimes used, and it is obvious that the present invention can be applied even in this case. .

Furthermore, although in the above embodiments the apertures of the two segments of the transducer were identical, this is again not limiting. However, in this case, the above l ₁ , l ₂ , l ₃ , and l ₄ change.

As is clear from the above explanation, the effects of the present invention are significant, and the present invention can be used to easily replace conventional SAWs.
It is possible to obtain an excellent surface acoustic wave filter with low insertion loss and low in-band ripple through substantially the same manufacturing process as the filter without changing the additional parts.

[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, FIGS. 2A and B are characteristic diagrams showing the characteristics of the conventional filter and that of the filter shown in FIG. 1, and FIG. 3 is another embodiment of the present invention. A plan view of the surface acoustic wave filter in the example, and FIGS. 4A and 4B are characteristic diagrams showing the characteristics of the conventional filter and the filter of FIG. 3. 1... Piezoelectric substrate, 2, 3, 4... Input/output transducer, 5... Signal source, 6... Load.

Claims (1)

[Claims] 1. Output transducers are provided on the left and right sides of the input transducer on the piezoelectric substrate, and the input transducer and the output transducer are arranged in the direction of propagation of surface acoustic waves. The transducer is divided into two segments in a direction substantially perpendicular to the transducer, and the spacing between the electrode fingers of the transducer is different between the two segments, and within each segment, the spacing between the electrode fingers is determined by the elastic surface. The two segments of each transducer are connected in parallel without changing their position perpendicular to the direction of wave propagation, the two output transducers are connected in parallel, and then connected to the load. A surface acoustic wave filter characterized by: 2. The surface acoustic wave filter according to claim 1, wherein the intervals between the input transducer and the output transducer are all different. 3. According to claim 1 or 2, the spacing between the transducers in the two segments is made different by bending the electrode fingers of the input transducer and the output transducer. surface acoustic wave filter.