Surface Acoustic Wave Device

This invention relates to a surface acoustic wave device for use in a television receiver set or the like.

With the recent trend of practicing sound multiplexing in television broadcasting, it has become desirable to improve sound quality. An approach to this end is disclosed in "Funkschau, 6/1981", according to which a surface acoustic wave filter is used to extract a sound signal component and a picture carrier component from a tuner output while suppressing a picture signal component relatively, and sound reproduction is effected on the basis of the two extracted components. Meanwhile, Japanese Utility Model Publication No. 22435/79 discloses a surface acoustic wave filter adapted for separation of a tuner output into a picture channel and a sound channel, which filter comprises a piezoelectric substrate on which are disposed one input transducer connected to a tuner, and two output transducers for receiving surface acoustic waves and converting them into electrical signals. In this example, the two output transducers are designed to have the same electrode aperture length probably because it is intended to make the impedance of one output transducer for the sound signal identical with the impedance of the other output transducer for the picture signal, the latter impedance being set for suppression of multipath reflection. For this reason, the area of the substrate is approximately doubled as compared to that of a substrate carrying a single output transducer. Accordingly, this example of a composite surface acoustic wave filter inevitably requires a doubled quantity of substrate material, resulting in an increase not only in production cost but also in the size of a packaged device, and can not be applied to conventional packaged devices used.

In addition, the generation of multipath reflection is inevitable in the surface acoustic wave filter. Multipath reflective wave interferes with a main signal and is delivered out of the filter. To suppress such multipath reflective wave, it is common practice to couple the filter to a succeeding amplifier in an impedance mismatching fashion. The impedance mismatching coupling can suppress the multipath reflection wave but it will adversely attenuate the main signal. Thus, when in the example of the aforementioned Japanese Publication the sound and picture signal channels are brought into the same impedance coupling relationship, the suppression of multipath reflective wave can be accomplished only at the cost of the undesirable attenuation of the sound signal component.

An object of the present invention is to provide a surface acoustic wave filter capable of transmitting a sound signal without causing attenuation.

Another object of the present invention is to provide a surface acoustic wave filter without an increase in the area of the filter substrate.

According to the present invention, in a surface acoustic wave filter for separating a picture signal component and a sound signal component, multipath reflective wave is suppressed in a picture signal channel only, and an output transducer provided for a sound signal channel is so spaced apart from an input transducer that the delay time of the multipath reflective wave can be reduced to an extent that occurrence of the multipath reflective wave can not be discriminated and is conditioned to have a short electrode aperture length by making its output impedance high to thereby prevent attenuation of the sound signal due to impedance mismatching with a succeeding amplifier.

In the surface acoustic wave device, there occurs a multipath reflective wave called a triple transit echo (TTE) between the input and output transducers. This reflective wave is delayed with respect to a desired surface acoustic wave signal to reach the output transducer and turns into a ghost. Therefore, in a conventional surface acoustic wave device, circuit constants of a filter and a succeeding circuit are so selected as to purposely establish an impedance mismatching therebetween and hence the unwanted TTE can be suppressed only at the cost of an increased loss in the device. However, in a system for separation of a picture signal and a sound signal components, the TTE must be suppressed, like the conventional device, in the picture signal channel whereas the occurrence of the TTE in the sound signal channel is not serious if the delay time of the TTE is so short that human ears may be insensitive to the TTE. More particularly, when the period of comb like characteristics for the TTE as defined by a reciprocal of the delay time between the desired signal and TTE is very large, amounting to, for example, 500 KHz of the TTE's interval for a sound signal of a band width of about 15 KHz, the frequency characteristics for the sound signal are not affected by the TTE comb like characteristics so that the sound signal can be substantially freed from distortion. Accordingly, for the sound signal channel of a band width which is far narrower than that of the picture signal channel of the order of 4 MHz, the establishment of impedance matching between the output transducer and the succeeding amplifier is significantly effective for both the size reduction of the surface acoustic wave device and prevention of the signal attenuation.

The present invention will be apparent from the following detailed descriptions taken in conjunction with the accompanying drawings, in which:

FIG. 1 is a plan view showing an embodiment of a surface acoustic wave device according to the present invention;

FIGS. 2 and 3 are graphic representations showing the characteristics of transmission channels in the device of FIG. 1; and

FIG. 4 is a plan view showing another embodiment of the surface acoustic wave device according to the invention.

Referring to FIG. 1, a preferred embodiment of a surface acoustic wave device according to the present invention comprises an input transducer 1, a picture signal output transducer 2 and a sound signal output transducer 3. The input transducer 1 has 15 pairs of unapodized electrodes in which the finger width is 8.6 .mu.m and the finger spacing and finger overlapping length are fixed, and its electrode aperture length is 2260 .mu.m. The picture signal output transducer 2 has 60 pairs of apodized electrodes in which the finger spacing and finger overlapping length are not uniform, and the sound signal output transducer 3 has 70 pairs of apodized electrodes in which the finger spacing and finger overlapping length are also not uniform. The output transducer 2 has an electrode aperture length W.sub.2 of 1280 .mu.m and the output transducer 3 has an electrode aperture length W.sub.3 of 680 .mu.m. The electrode finger of each transducer is of a double finger configuration (split type) and is made of aluminum deposited films of 5000 .ANG. thickness formed by a photolithographic technique on a surface of a piezo-electric substrate 8 made of a lithium niobate single crystal of 128.degree. rotated Y-axis cut. Interposed between the input and output transducers is a shield electrode 4.

The input transducer 1 cooperates with the output transducer 2 to provide frequency characteristics are shown in FIG. 2 wherein a self channel sound carrier fs, an upper adjacent picture carrier fp and a lower adjacent sound carrier fs are suppressed. The input transducer 1 is also in cooperation with the output transducer 3 to provide frequency characteristics as shown in FIG. 3 wherein other frequency components than the sound carrier fs and a picture carrier fp are suppressed. A picture signal component from the output transducer 2 is supplied to an amplifier 9, and a sound signal component and a picture carrier both delivered out of the output transducer 3 are supplied to an amplifier 10. Output signals of the amplifiers 9 and 10 are respectively subjected to necessary signal processing and then supplied to a picture tube and a loudspeaker, respectively. Each of the amplifiers 9 and 10 has an input impedance of about 300 .OMEGA. and on the other hand, the output transducer 2 has an output impedance of about 150 .OMEGA. and the output transducer 3 has an output impedance of about 300 .OMEGA.. Therefore, while in the picture signal channel the TTE can be suppressed because of the impedance mismatching, a main signal and the TTE can not at all be suppressed in the sound signal channel where the impedance matching is completely established. However, the output transducer 3 is disposed in close proximity of the input transducer 1 so that the delay time of the TTE may be of the order of several micro seconds at the most and human ears may be completely insensitive to the interfering TTE. The transducer can be conditioned to assume a high impedance by reducing the electrical capacitance between paired fingers, and to this end, the duty factor defined by a ratio of finger width to structural period of interdigital electrodes can be reduced. For example, when the electrode aperture length is 980 .mu.m, the finger width is 8.5 .mu.m and the finger spacing is 8.5 .mu.m with the duty factor measuring 50%, the electrical capacitance is 13 pF. For the same electrode aperture length, however, the electrical capacitance is 7 pF when the finger width is 4.5 .mu.m and the finger spacing is 13.5 .mu.m with the duty factor measuring 25%. Therefore, it is possible to realize a high impedance by reducing both the electrode aperture length and the duty factor.

FIG. 4 shows, in plan view, another embodiment of the present invention.

This embodiment is the same as the FIG. 1 embodiment except that an input transducer 1 is divided into sub-transducers 1A and 1B and the sub-transducer 1B has characteristics which are more fitted for the sound signal channel.

As described above, according to the present invention, the chip size of the piezo-electric substrate carrying the filter electrodes can be reduced in the longitudinal direction so as to measure 3.2 mm which is smaller by 0.8 mm than 4 mm of the prior art device and hence the output number of chips produced from a single substrate can be increased to greatly contribute to cost reduction. Further, the output voltage level of sound signal can be approximately doubled, thereby improving the electrical characteristics.