1. Field of the Invention
The present invention relates to a transversal filter in which the stages of an analog charge transfer device (CTD) shift register integrated on a doped semiconductor substrate is provided with parallel inputs and evaluation circuits assigned thereto, and more particularly to such a filter in which the evaluation circuits each exhibit an area which is doped opposite to the substrate, first and second input gates and a transfer gate, the transfer gate being arranged directly adjacent to the transfer channel of the CTD shift register, and in which the one input gate is connected to an input signal, the other input gate is connected to a constant DC voltage, the oppositely doped area is connected to a first pulse voltage and the transfer gate is connected to a second pulse voltage, and in which the output signal can be tapped from an output of the CTD shift register.
2. Description of the Prior Art
A transversal filter of the type generally set forth above is disclosed in the publication "Electronics Letters", Vol. 13, No. 5, Mar. 3, 1977, pp. 126-127. The magnitude of the individual coefficients with which the input signal is individually evaluated at each parallel input is thereby provided by the capacitance of the second input gate. Transversal filters of this type are also the subject of the German patent application No. P 26 43 704.7. A disadvantage arises, however, in that large amounts of the evaluation coefficients correspondingly enlarge the semiconductor surface to be provided for the transversal filter.
From the "Proceedings of the 8th Conference (1976 International) on Solid State Devices, Tokyo, 1976", and printed in the "Japanese Journal of Applied Physics", Vol. 16 (1977) Supplement 16-1, pp. 387-390, such a transversal filter is known in which a charge coupled device (CCD) shift register has several transfer channels insulated from one another by separating diffusions, the channels being respectively individually assigned to the parallel inputs. The transfer gate thereby provided respectively consists of the first displacement electrode of the respective transfer channel in the evaluation circuits. The transfer channels are conveyed together in a common output level in whose range the separating diffusions mentioned above are omitted. Then, a summation of the signal portions occurs to provide an output signal, the signal portions being separately transmitted by way of the channels and arriving with different delays. The evaluation coefficients with which the signal fed to the parallel outputs is occupied, are provided by the gate surfaces in the individual evaluation circuits. A disadvantage also arises with this construction, however, in that the necessary semiconductor surface greatly increases with the number of parallel inputs and the desired amounts of evaluation coefficients.
From the above-mentioned "Proceedings", printed in the "Japanese Journal of Applied Physics", Vol. 16 (1977), Supplement 16-1, pp. 391-396, an additional transversal filter is known which differs from the type initially mentioned by a different design of the evaluation circuits and the parallel inputs. In this structure each of the individual evaluation circuits contains metal-oxide-semiconductor (MOS) field effect transistors connected in series, and a resistance diffusion, whereas each parallel input includes a level of a four-phase CCD shift register provided with an input diffusion. The input diffusion is connected to a terminal of the resistor and the last displacement electrode is arranged adjacent to a displacement electrode of a CCD shift register which undertakes the summation of the signal portions. The gate of one of the MOS field effect transistors of each evaluation circuit is connected to the input signal, the gate of the other MOS field effect transistor is respectively connected to a DC voltage which determines, by its magnitude, the current flowing through the transistors, and thereby determines the respective evaluation coefficient. Thereby, the transmitting properties of the transversal filter can be electrically adjusted. However, the disadvantage with this structure is that only in a small alteration range is the current linearly dependent on the DC voltage.
In the earlier German patent application No. P 26 44 284.2 it has already been suggested to form charge amounts twice in succession in order to realize an evaluation coefficient in the individual evaluation circuits and to input the amounts of charge into the respective stages of the CTD shift register in order to design the evaluation (input weighting) circuits in a space-saving manner.