FIG. 9 is a top view of a prior art SAW convolver of typical structure. In the figure, reference numeral 1 is a piezoelectric substrate or a piezoelectric film/insulator/semiconductor multi-layered substrate; 2 is a gate electrode; 3 is an interdigital electrode; 4 is an input terminal; 5 is an output terminal; and 6 is a sound absorber. The structure, in which a piezoelectric substrate is used, has a feature that, since the propagation loss of the surface acoustic wave propagating in the substrate is small and the frequency dispersion of the group velocity is also small, the gate electrode 2 can be long and therefore a convolution integration of long time width can be effected. On the other hand, the structure, in which a piezoelectric film/insulator/semiconductor multi-layered substrate is used, has a feature that the non-linearity constant of the substrate is great and therefore a high convolution efficiency can be obtained.
However, in the prior art structure indicated in FIG. 9, unnecessary signals called self convolution signals are produced apart from the convolution signal (convolution integration signal) between inputted signals through the two interdigital electrodes (hereinbelow abbreviated to I.D.T). A self convolution signal is a signal produced by the fact that a surface acoustic wave generated by an I.D.T is reflected by the other I.D.T opposite thereto. FIG. 10 shows this aspect. In the figure, surface acoustic waves S1 and S2 are generated by electric signals F1(t) and F2(t) applied to the respective I.D.Ts and these surface acoustic waves are reflected by the other I.D.Ts, which gives rise to reflected waves S1r and S2r. In this case, it will be obvious that a convolution signal between S1 and its reflected wave S1r and that between S2 and its reflected wave S2r are also produced apart from the aimed convolution signal between S1 and S2. The former two signals S1r and S2r are self convolution signals. In this case, the output signal C(t) obtained at the output electrode 2 is expressed by the following formula; ##EQU1## t denotes the time; L the gate length; v the propagation velocity of the surface acoustic wave; K a constant; T is a reflection coefficient of the surface acoustic wave at the I.D.T; and .alpha. the attenuation constant of the surface acoustic wave.
Further, as expressed by Equation (2), T corresponds to the in-gate delay time. In Equation (1), the first term represents the convolution signal between the input signals F1(t) and F2(t). The second and third terms are terms which do not exist, in the case where there exist no reflected waves, and represent self convolution signals.
Now such self convolution signals appear as spurious noise for the output of the convolver and give undesirable influences that they decrease the dynamic range of the convolver. As an example, the output signal, when the input signals F1(t) and F2(t) expressed Equations (3) and (4), respectively, are inputted, is shown qualitatively in FIG. 11. ##EQU2##
In FIG. 11 is shown the aspect that self convolution signals appear other than the convolution signal, which raises the spurious level L. When the spurious level is so remarkably raised, it can be a serious problem, when the SAW convolver is applied. For example, in the case where the SAW convolver is applied to a spread spectrum communication apparatus (hereinbelow called simply SS communication apparatus), the rise of the spurious level as described above increases the rate of errors at the data reception, which decreases effectively the date transfer speed or reduces the distance, for which the communication is possible.
In order to reduce the influences of the self convolution signal as described above, heretofore various methods have been proposed. As such a method, there is known a method, by which e.g. a dual gate convolver (refer to Literature 1) or a one-directional transducer (refer to Literature 2) is used.