Patent GB-882974-A (GB)
Title: Narrow band rejection filter and tunable monolith for use therein
IPC: H03H1/02
Filed: 1960-09-13
Abstract: 882,974. Circuit arrangements; semi-conductor devices. WESTINGHOUSE ELECTRIC CORPORATION. Sept. 13, 1960 [Jan. 27, 1960], No. 31524/60. Class 37. A narrow-band filter comprises a structure providing between an input and an output terminal distributed resistance and also having adjacent the distributed resistance distributed capacitance which is returned via further resistance to a terminal common to both input and output. In one embodiment the distributed capacitance comprises a wafer 43, Fig. 1, of material of high dielectric constant, e.g. barium titanate and the distributed resistance comprises a thin superimposed layer 40 of resistive material, e.g. chromium, having attached thereto spaced input and output terminals 41, 42 respectively. The other surface of the wafer 43 is provided with a highly conductive coating 44, e.g. silver or gold which is connected via resistance 46 to the line 19 common to output and input. The resistance 46 may take the form of a further wafer of resistive material, attached to the conductive coating 44 and provided with a second conductive coating for connection to the line 19 (Fig. 2, not shown). Another embodiment comprises a wafer 11, Fig. 5, of lightly doped P-type semi-conductive material, forming the distributed resistance and having input and output terminals 14, 15 respectively, attached, superimposed on a wafer 12 of heavily doped N- type material whose other surface bears a highly conductive coating 13 to which terminal 29 is attached. The terminal 29 is connected to the common line 19 via resistance 27 and a biasing potentiometer 24 and battery 23; the element is reverse biased to form a depletion layer 30 which comprises the distributed capacity. The width of the depletion layer and hence the distributed capacity and resistance may be varied so as to tune the filter by variation of the bias. In a modification (Fig. 4, not shown) the resistance 19 is returned directly to the line 19 and the bias is applied across the output terminals via a high resistance. In a modification of the invention the distributed capacitance is connected directly to the common filter terminal and a capacitance is bridged across the other input and output terminals. An embodiment in the form of a monolithic structure comprises a wafer 61, Fig. 10A, of highly resistive material bearing output terminal 62, superimposed on a wafer 68 of dielectric material, the other surface of which bears a highly conductive coating 69 connected to the common line 19. The wafer 61 is provided with a highly conductive extension 63 bearing input terminal 64 and supporting on its upper surface the bridging capacitance in the form of wafer 65 of dielectric material. The upper surface of the dielectric 65 is coated with highly conductive material 66 connected to the output terminal 62. In a modification (Fig. 10B, not shown) the wafer of dielectric material forming the distributed capacitance is extended to form also the dielectric of the bridging capacitance, bearing on the extension a second highly conductive layer which is connected to the output terminal. The structures may comprise suitably doped semi-conductive material.