Abstract:
A dispersive surface acoustic wave delay line device consists of a plurality of delay line elements which are disposed parallely so that a propagating path of a delay line element is not co-owned by the other delay line elements, in order to reduce secondary effects caused by electrode fingers.

Description:
FIELD OF THE INVENTION 
     This invention relates to a surface acoustic wave device, and more particularly to an improvement of a delay line device using dispersion type surface acoustic wave transducers. 
     BACKGROUND OF THE INVENTION 
     A delay line device shown in FIG. 5, one of the most basic existing arrangements using dispersion type surface acoustic wave transducers (chirp transducers) formed on a surface acoustic wave substrate 1 is practically used in a high-speed analog Fourier transformer for use in a real time spectrum analyzer or in a pulse expanding/compressing apparatus for use in a chirp radar. It usually has a wide-band property and a large delay time for purposes of improving the processing capacity and the resolving power. The substrate 1 is in the form of a monocrystal piezoelectric substrate such as lithium niobate, lithium tantalate, etc. or a multi-layer piezoelectric substrate such as AlN/sapphire, ZnO/Si, ZnO/SiO 2  /Si. The arrow mark in FIG. 5 indicates the propagating direction of a surface acoustic wave. 
     Since the chirp transducers of the above-indicated delay line device have a large number of electrodes and a surface acoustic wave passes under a large number of electrode fingers before exiting from the transducer end, a large loss caused by internal reflections in the transducers themselves or mode transformation into a bulk wave deteriorate the characteristic of the device. 
     Particularly when the bands of the chirp transducers include sets of frequencies F 1  and F 2  in the relationship of f 2  =2f 1 , deterioration of the characteristic by their effects cannot be disregarded. Further, a change in the sonic speed by the large number of electrodes on the propagation path causes a turbulence in the group delay and the phase characteristic. 
     FIG. 6 shows chirp transducers having a generally called slanted electrode arrangement in which the propagation path of an excited surface acoustic wave is gradually moved in a direction across the propagation direction, depending on the frequency of the surface acoustic wave, so as to substantially decrease the number of electrode fingers, for purposes of reducing influence of internal reflections inside the transducer and mode transform. 
     The above type transducers suppress the influence of internal reflections inside the transducer and mode transformation into a bulk wave and realize a good characteristic of the device. However, since the minimum size of the interdigitating width of the electrode fingers is limited and the entire electrode width in the vertical direction with respect to the surface wave propagating direction is therefore increased in order to prevent an increase in the loss caused by diffraction of a surface acoustic wave radiated from a narrow opening, the manufacturing cost per surface acoustic wave element is increased. Further, when it is used in a dispersive substrate, the design for correcting the group delay and the phase characteristic over a wide band in a single transducer is a difficult technology even with the slanted electrode arrangement. 
     OBJECT OF THE INVENTION 
     It is therefore a main object of the invention to provide a chirp transducer eliminating the above-indicated drawbacks of the existing arrangement and suppressing the dimensional increase of the element. 
     SUMMARY OF THE INVENTION 
     In order to overcome the above-indicated problems, a first invention is so arranged that dispersive delay line elements whose inputs and outputs are dispersive surface acoustic wave transducers are disposed in a parallel alignment without sharping their propagation paths, and respective inputs of all the delay lines and respective outputs thereof are electrically connected, respectively. 
     A second invention is so arranged that dispersive delay line elements whose inputs and outputs are a dispersive surface acoustic wave transducer and a non-dispersive surface acoustic wave transducer are disposed in parallel alignment without sharping their propagation paths, and respective inputs of all the delay lines and respective outputs thereof are electrically connected, respectively. 
     In each such dispersive surface acoustic wave delay line device, a desired continuous frequency characteristic is obtained by the arrangement where a plurality of delay line elements whose respective frequency characteristics are involved in a desired characteristic are connected in parallel and so that they do no share common propagation path. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a fragmentary plan view of a dispersive surface acoustic wave delay line device embodying the invention; 
     FIG. 2 is a graph showing frequency characteristics of two delay line devices; 
     FIG. 3 is a graph showing the frequency characteristic of an inventive delay line device; 
     FIG. 4 is a fragmentary plan view of a delay line device taken as a further embodiment of the invention; 
     FIG. 5 is a fragmentary plan view of an existing dispersive surface acoustic wave delay line device; and 
     FIG. 6 is a fragmentary plan view of an existing delay line device having a slanted electrode arrangement. 
    
    
     DETAILED DESCRIPTION 
     FIG. 1 shows an inventive delay line device so arranged to obtain the substantially same frequency characteristic as that of the dispersive surface acoustic wave delay line device of FIG. 5 and consisting of a pair of parallel delay line elements. 
     In this drawing, transducers A and A&#39; in one of the delay line elements and transducers B and B&#39; in the other delay line element both formed on a substrate 1 as used in the existing delay line device have mirror relationships, respectively. A region la of the transducer A and a region lb of the transducer B are identical in pitch of electrode finger arrangement, and a region la&#39; of the tranducer A&#39; and a region lb&#39; of the transducer B&#39; are identical in such pitch. 
     Region-to-region distances (la-to-la&#39; distance and lb-to-lb&#39; distance) are selected so that delay times τa and τb in frequencies having wavelengths corresponding to the same electrode finger pitches in the regions are substantially equal. 
     FIG. 2 is a graph which shows in an overlapped configuration a frequency characteristic C of the delay line element having the transducers A and A&#39; as its input and output and a frequency characteristic D of the delay line element having the transducers B and B&#39; as its input and output. The frequency characteristics C and D have adjacent frequency bands sharing a frequency fc approximately as the maximum and minimum limits respectively in their 3dB bands. Therefore, electrode fingers with a pitch corresponding to the frequency fc are involved in all the regions la, la&#39;, lb and lb&#39;. In FIG. 2, L indicates the insertion loss, and f shows the frequency. 
     For purposes of suppressing the loss L caused by internal reflections in each transducer and mode transform into a bulk wave, the frequency bands are shared so that frequencies f 1  and f 2  in relationships at least of one being double or half the other never co-exist in the frequency band of either single delay line element. 
     A graph of FIG. 3 shows the frequency characteristic of the embodiment in which the said two delay line elements are connected electrically in parallel. Since delay times of the two delay line elements in the frequency fc are substantially equalized, responses near the frequency fc are summed in the same phase, and the loss at the frequency fc in the characteristics C and D is decreased by about 3dB from their maximum values as shown in the graph of FIG. 2. Therefore, in the summed characteristic in the graph of FIG. 3, the response in the vicinity of the frequency fc is smoothed. 
     FIG. 4 shows a further embodiment of the invention which uses two delay line elements having chirp transducers A and B and wide band regular transducers A&#34; ahd B&#34; as their inputs and outputs. By giving to the frequency characteristics of the respective delay line elements the same conditions as those in the first embodiment, the substantially same effects are obtained. 
     As described above, according to the invention, a dispersive surface acoustic wave delay line device comprises two or more delay line elements which have frequency characteristics involved in a desired characteristic and are connected in parallel so that they do not share propagation paths. Therefore, a desired continuous frequency characteristic is readily established, and the number of electrode fingers per input or output transducer can be decreased as compared to an arrangement using a single delay line element to obtain the final frequency characteristic of the device. This permits a decrease in the loss caused by internal reflections in each transducer itself and mode transform, and ensures suppression in turbulence in the group delay and phase characteristic caused by changes in the sonic speed under the electrodes. 
     In particular, by selecting their respective bands so that sets of frequencies in relationships of one being double or half the other do not co-exist in a transducer, it is possible to reinforce the effect of suppressing internal reflections in the transducer. 
     Further, as compared to a device using a slanted electrode arrangement to realize these effects, the inventive arrangement can reduce the increase in the element size by selecting the least required number of combined delay line elements. 
     Further, in case of realizing a wider band on a substrate whose sonic speed has a frequency dispersive property, it is very difficult to correct the dispersion throughout a desired band by means of a single transducer. However, the inventive arrangement can divide a desired band into smaller regions and correct the dispersion in respective small bands. Therefore, it is very effective particularly for use in a monolithic surface acoustic wave convolver, etc. using a multi-layer piezoelectric substrate.