1. Field of the Invention
The present invention relates to a surface acoustic wave filter. More specifically, the present invention relates to a surface acoustic wave filter adapted to achieve an asymmetrical frequency characteristic with respect to a given central frequency, wherein the interdigital electrode is divided into a plurality of portion electrodes in the propagating direction of the acoustic surface wave and the divided portion electrodes are electrically connected in series.
2. Description of the Prior Art
Recently a surface acoustic wave device has been used as a video intermediate frequency filter in a television receiver, for example. As well known, a video intermediate frequency filter in a television receiver requires a sound trap characteristic as well and hence must have an asymmetrical frequency response characteristic with respect to the central frequency (such as the frequency f intermediate a picture signal frequency and a chrominance signal frequency). Conventionally, a surface acoustic wave filter having an asymmetrical frequency response characteristic with respect to such central frequency has been implemented using the reflection method or a method similar thereto, or a variable pitch method. The present invention is directed to an improvement in a surface acoustic wave filter for achieving the above described asymmetrical frequency response characteristic based on the reflection method or a method similar thereto.
The reflection method and an interdigital electrode structure obtained by such method are described in U.S. Pat. No. 3,968,461, issued July 6, 1976 to Mitchell et al. and entitled "ACOUSTIC SURFACE-WAVE DEVICES". Therefore, such an electrode structure attained by the reflection method will be briefly described to the extent necessary in describing the present invention. According to the reflection method, an electrode pattern is determined based on an impulse response obtained by Fourier transformation or Fourier series expansion of an asymmetrical frequency characteristic. Such impulse response comprises a symmterical component (a cosine component) and an asymmetrical component (a sine component). Exciting sources for exciting the symmetrical component and asymmetrical component are separately provided in such a manner that these are alternately disposed in the transducer in the propagating direction of the surface acoustic wave. One example of a structure of an interdigital electrode thus formed in accordance with the reflection method is shown in FIG. 1.
In the conventional example shown in FIG. 1, electrode fingers 1a, 1b, . . . 5a, 5b and 1c, . . . 5c constituting an interdigital transducer are selected to have a width of 1/8.lambda., where .lambda. is the wave length of the surface acoustic wave for the central frequency, as shown in FIG. 1. The arrangement pitch of these electrode fingers 1a, 1b, . . . 5a, 5b and 1c, . . . 5c is selected to be 1/4.lambda.. The width and the pitch of these electrode fingers are each a half of the ordinary width and pitch. In the case of the FIG. 1 example, the electrode fingers 1a and 1b; 2a and 2b; . . . are each formed as a pair. Then each of the electrode finger pairs is alternately connected to the corresponding one of common portions 10 and 11 which are placed at different potentials. Such electrode structure is rather similar to a surface acoustic wave filter of the so-called split electrode type. Such surface acoustic wave device of the so called split electrode type is disclosed in, for example, U.S. Pat. No. 3,727,155, issued Apr. 10, 1973 to Adrian J. DeVries and entitled "Acoustic Surface Wave Filter". Still referring to the FIG. 1 example, one electrode finger 1a, 2a, . . . of each of paired main electrode fingers 1a and 1b; 2a and 2b; . . . has, on an extention of the free end thereof, an auxiliary electrode finger 1c, 2c, . . . connected to common portions 11 and 12 different from those of these paired main electrode fingers 1a and 1b, 2a and 2b, . . . The symmetrical component is excited at each of the regions (hatched in the left downward direction in FIG. 1) between the adjacent main electrode fingers 1b and 2a; 2b and 3a; . . . of different potentials. Furthermore, the asymmetrical component is excited at each of the regions (hatched in the right downward direction in FIG. 1) between the adjacent auxiliary electrode fingers and the main electrode fingers 1c and 1b; 2c and 2b; . . . Thus the symmetrical component and the asymmetrical component can be separately excited in a single transducer. As a result, the asymmetrical frequency characteristic necessary for a video intermediate frequency filter can be attained.
On the other hand, the electrostatic capacitance of the interdigital transducer becomes a total sum of electrostatic capacitances formed between the respective adjacent electrode fingers. Accordingly, when a piezoelectric material of high dielectric constant is used, the electrostatic capacitance of the transducer becomes large. Accordingly, in incorporating a surface acoustic wave filter in an electrical circuit, it would become difficult to attain proper impedance matching. Therefore, particularly in the case where a relatively large number of electrode fingers is formed, it has been conventional, for the purpose of reducing the electrostatic capacitance, to divide an interdigital electrode forming the transducer into a plurality of portion electrodes in the propagating direction of the surface acoustic wave and to connect the divided portion electrodes electrically in series. Such divided type transducer is disclosed in U.S. Pat. No. 3,600,710, issued Aug. 18, 1971 to Robert Adler and entitled "ACOUSTIC SURFACE WAVE FILTER", for example. However, in the case where an interdigital electrode formed in accordance with the reflection method as depicted in conjunction with FIG. 1 is divided into a plurality of portion electrodes in accordance with U.S. Pat. No. 3,600,710, another problem arises.
More specifically, in the case where such an electrode configuration as shown in FIG. 1 is to be divided, the excitation intensity of the surface acoustic wave at the dividing region becomes larger than the original excitation intensity in the case of non-division, and consequently the resultant frequency characteristic is degraded.
FIG. 2 is an electrode pattern of this type, with reference to which the problem will be discussed. Referring to FIG. 2, the divided portion electrodes are denoted by the reference characters E1 and E2. Meanwhile, the FIG. 2 electrode pattern shows an example wherein the interdigital electrode is divided at the position of the arrow A in FIG. 1. It has been a common practice that such division is made in a region where a relatively large component (in the case of FIG. 1 the symmetrical component) is to be excited. The region between the portion electrodes E1 and E2 in FIG. 2 corresponds to the region shown by the arrow A in FIG. 1. The main electrode finger 3b included in the portion electrode E1 is adjacent to the main electrode finger 4a' included in the portion electrode E2, with the dividing region D therebetween. As seen from FIG. 2, a voltage as high as two times the voltage between the original main electorode fingers (i.e. not divided, as shown in FIG. 1) is applied between the main electrode fingers 3b and 4a'. In accord therewith, due to the relation with other electrode fingers outside the dividing region, the overlapping length of the main electrode fingers 3b and 4a' has been selected to be more or less greater than that of FIG. 1. Therefore, it will be appreciated that in the FIG. 2 example the excitation intensity at the dividing region has become as large as approximately three times that of FIG. 1. (In FIGS. 1, 2 and 3, each arrow symbol denotes a unit amount of the excitation intensity.) Since the excitation intensity at the dividing region thus becomes larger than that of the original one, the frequency characteristic of a surface acoustic wave filter including such transducer becomes more degraded than that desired. Accordingly, one might think of selecting the overlapping length of the adjacent main electrode fingers 3b and 4a' to be less than that of the original one shown in FIG. 1. However, since the excitation intensity of the exciting source positioned at both sides of the dividing region has been made consistent with the corresponding one of the FIG. 1 conventional one, a mere change of the lengths of the adjacent electrode fingers at the dividing region causes a change in the exciting intensity of the exciting sources positioned at both sides thereof. Accordingly, a mere change of the lengths of the adjacent electrode fingers at the dividing region can not achieve adjustment of the excitation intensity to be adaptable to the original one.