Abstract:
A surface acoustic wave filter which comprises a substrate of piezoelectric material and a multistrip coupler. The multistrip coupler comprises a plurality of parallel conductive strips formed on the substrate; at least one input transducer is disposed on one side of the multistrip coupler, and at least one output transducer is disposed on the other side of the multistrip coupler. The surface acoustic wave filter is characterized in that a part of the middle portion of each of the conductive strips of the multistrip coupler is made of a photoconductive material.

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates to improvements in a surface acoustic wave filter. 
     Surface acoustic wave filters have been used in recent years as small-sized filters for waves of ultra-high frequencies. A prior art surface acoustic wave filter is illustrated in FIG. 1. This surface acoustic wave filter comprises: a substrate 1 of a piezoelectric material such as lithium niobate (LiNbO 3 ) or lithium tantalate (LiTaO 3 ); a multistrip coupler 2 comprising a plurality of parallel conductive strips formed on the substrate; an input transducer 3 disposed on one side of the multistrip coupler 2; and two output transducers 4 and 5 disposed on the other side of the multistrip coupler 2. Electric wave signals introduced from a source 6 to the input transducer 3 are converted to surface acoustic wave signals and transmitted to the output transducers 4 and 5 through the multistrip coupler 2. The transmitted surface acoustic wave signals are reconverted to electric wave signals by each of the output transducers 4 and 5 and transmitted to output terminals 7. Each of the input and output transducers 3, 4, 5 has a filtering characteristic which is determined by the shape of the electrodes thereof. Only signals of a frequency corresponding to the filtering characteristic can pass through each transducer when changing from electric signals to acoustic wave signals, or vice versa. Input power from the input transducer 3 is distributed to the output transducers 4 and 5 by the multistrip coupler 2. The distribution ratio changes cyclically in accordance with the change of the number of conductive strips of the multistrip coupler 2, as shown in FIG. 2. In the graph of FIG. 2, the abscissa represents the number of strips of the multistrip coupler and the ordinate represents the output power of each output transducer. The solid line P 1  represents the output power of the output transducer 4. The dotted line P 2  represents the output power of the other output transducer 5. 
     The number of strips of the multistrip coupler 2 cannot be changed after the multistrip coupler 2 is once formed on the substrate. Therefore, the distribution ratio of the input power to the two output transducers 4 and 5 is nonadjustable in the multistrip coupler of the surface acoustic wave filter of the prior art. 
     SUMMARY OF THE INVENTION 
     The present invention was made in order to eliminate the above-mentioned disadvantage. An object of this invention is to provide a multistrip coupler for controlling the distribution of signal power transmitted to selected output transducers. Another object of the invention is to provide a portion of each of the conductive strips of the multistrip coupler with a photoconductive material. 
     A surface acoustic wave filter according to the present invention comprises: a substrate of piezoelectric material; a multistrip coupler comprising a plurality of parallel conductive strips formed on the substrate; at least one input transducer disposed on one side of the multistrip coupler, and at least one output transducer disposed on the other side of the multistrip coupler. The surface acoustic wave filter of the present invention is characterized in that a part of the middle portion of each of the conductive strips of the multistrip coupler is made of a photoconductive material. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a plan view of a prior art surface acoustic wave filter; 
     FIG. 2 is a graph of the power distribution to the two output transducers through the multistrip coupler; 
     FIG. 3 is a plan view of a surface acoustic wave filter of the present invention; 
     FIG. 4 is an equivalent circuit diagram of the multistrip coupler of the surface acoustic wave filter of FIG. 3; 
     FIG. 5 is a graph of the filtering characteristics of bands of different widths to be filtered; 
     FIG. 6 is a graph of the filtering characteristics of different frequencies to be filtered; 
     FIG. 7 is a plan view of a second embodiment of the present invention; 
     FIG. 8 is a graph of the filtering characteristics of the output transducers illustrated in FIG. 7; 
     FIG. 9 is a graph of the delay characteristics of the output transducers illustrated in FIG. 7; 
     FIG. 10 is a plan view of a third embodiment of the present invention; and 
     FIG. 11 is a graph of the filtering characteristics of the output transducers illustrated in FIG. 10. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     An embodiment of the present invention is illustrated in FIG. 3. The surface acoustic wave filter illustrated in FIG. 3 comprises a substrate 8 of a piezoelectric material, a multistrip coupler 9, an input transducer 10 disposed on one side of the multistrip coupler 9, and two output transducers 11 and 12 disposed on the other side of the multistrip coupler 9. The central hatched portion 13 of each conductive strip of the multistrip coupler 9 is made of a photoconductive material such as cadmium-selenium (CdSe). The photoconductive portion 13 serves as a conductor when it is lighted and serves as an insulator when it is not lighted. The relation between the resistance Rl of the photoconductive portion 13 in the light and the resistance Rd of the photoconductive portion 13 in the dark is Rl&lt;&lt;Rd. An equivalent circuit diagram of the multistrip coupler is illustrated in FIG. 4. R represents the resistance of the photoconductive portion, and r represents the impedance of the multistrip coupler, in which Rl&lt;&lt;r&lt;&lt;Rd (Rl≈0). When the photoconductive portion 13 is lighted, ends a, b are short-circuited, allowing signals to be transmitted through the photoconductive portion 13. When the photoconductive portion 13 is not lighted, ends a, b are insulated from each other, preventing signals from being transmitted through the photoconductive portion 13. Thus, the effective number of strips of the multistrip coupler can be changed by changing the number of strips lighted. Therefore, distribution of signal power transmitted to the output tranducers 11 and 12 can be controlled. 
     As mentioned above, the surface acoustic wave filter according to the present invention comprises a multistrip coupler having strips, each of which has a photoconductive portion. Therefore it is possible to change and adjust the distribution of signal power transmitted to the output tranducers. Also, it is possible to enlarge the field of application of the surface acoustic wave filter. 
     In using the surface acoustic wave filter of the present invention, the desired amount of signal can be transmitted to each of the output transducers by selectively lighting the strips of the multistrip coupler. 
     The photoconductive portions of the surface acoustic wave filter of the present invention can be used as a switch for selecting output transducers thereof by lighting all the photoconductive portions simultaneously. In this case, the number of strips of the multistrip coupler is preset so that all of the signal power from the input transducer 10 is transmitted to the output transducer 12 when all of the photoconductive portions are lighted, while all of the signal power from the input transducer 10 is transmitted to the other output transducer 11 when none of the photoconductive portions are lighted. When the photoconductive portions are used as a selection switch, as mentioned above, the output transducers 11 and 12 may have different filtering characteristics A and B, respectively, as illustrated in FIG. 5. The characteristic A has a wider filtering band than the characteristic B. In use, signals from the input transducer 10 are first transmitted to the output transducer 11 which passes A wide range of signals for rough signal treatment. Then, the photoconductive portions 13 are lighted to change the effective output transducer from 11 to 12 which passes a narrow range of signals for precise signal adjustment. 
     The output transducers 11 and 12 may also have completely different filtering characteristics A and B, respectively, as illustrated in FIG. 6, so that of the output transducers 11, 12 passes a different frequency band of signals. In this case, the photoconductive portions are used as a switch for changing the signal channel. 
     The surface acoustic wave filter may comprise only one output transducer 12. In this case, the entire group of the photoconductive portions 13 is used as a switch for turning on or off the output transducer 12. 
     Another embodiment of the present invention is illustrated in FIG. 7. Horizontal distances l 1  and l 2  in the drawing between the input transducer 10 and the output transducers 11 and 12 differ from each other. The transmission time of signals from the input transducer 10 to the output transducer 11 is more than the transmission time from the input transducer 10 to the output transducer 12, as can be seen from FIG. 9, which illustrates the delay characteristics A&#39;, B&#39;, of the output transducers 11 and 12. In this case, both output transducers 11 and 12 may have the same filtering characteristic A, B, as illustrated in FIG. 8. On of the output transducers 11 or 12 can be selectively used by lighting or not lighting the photoconductive portions 13. 
     A further embodiment of the present invention is illustrated in FIG. 10. Three output transducers 11, 12 and 14 are disposed side by side facing the multistrip coupler 9. The output transducers 11, 12 and 14 have different filtering characteristics A, B and C, respectively, as illustrated in FIG. 11. Two portions 13a and 13b of each strip of the multistrip coupler 9 are made of a photoconductive material. The photoconductive portions 13a and 13b are located at the positions which correspond to the spaces between two adjacent output transducers. The output tranducer 11 can be selected by lighting the portions 13a and not lighting the portion 13b. The output transducer 12 can be selected by not lighting the portions 13a and 13b. The output transducer 14 can be selected by not lighting the portions 13a and lighting the portions 13b. 
     The many features and advantages of the above invention are apparent from the detailed specification and thus it is intended by the appended claims to cover all such features and advantages of the system which fall within the true spirit and scope of the invention. It is not desired to limit the invention as shown and described in the above specification. Further, since numerous modifications and changes will readily occur to those skilled in the art, it is not desired to limit the invention to the exact construction and operation shown and described, and accordingly all suitable modifications and equivalents may be regarded as falling within the scope of the invention.