Patent GB-1307665-A (GB)
Title: Electric impedance
IPC: H03G5/12
Filed: 1971-05-13
Abstract: 1307665 Variable capacitors SPRAGUE ELECTRIC CO 13 May 1971 [14 May 1970] 14814/71 Heading H1M [Also in Division H3] A variable capacitance network comprises at least two lower electrodes separated from at least two upper electrodes by a layer of dielectric material, one of the upper electrodes having a continuous conductive surface and another of the upper electrodes being constituted by a plurality of isolated islands, the lower electrodes each having an associated terminal and the continuous upper electrode being connected to a terminal of the network, a wiper type contact engaging part of the continuous electrode surface and selected ones of the isolated islands to electrically connect then to the continuous electrode and its terminal. As shown, two lower electrodes 12, 14 with terminal portions 24, 26 are deposited on substrate 16, suitably alumina, and are covered with a dielectric layer (28), (Fig. 3, not shown), of barium titanate glass. Continuous electrode 30 is deposited on this layer to partially overlap electrode 14 and provide fixed capacitance 64, (Fig. 5), and two sets of isolated islands 32, 34 are deposited, which partially overlap electrodes 12 and 14 respectively. Electrode 30 is electrically connected via conductive strip 38 and deposited resistor 48 to terminal 46 which is electrically connected to terminal 24. A rotor for the stator unit 20 formed may comprise a disc about 120 degrees of the surface of which is a wiper consisting of phosphor-bronze mesh (52), (Fig. 4, not shown), the remainder being P.T.F.E. A stop is provided so that the rotor may move from a position in which it covers islands 32 over electrode 30 to a position in which it covers islands 34. Electrode 30 occupies more than 120 degrees so that at least a marginal portion is always in contact with the wiper. The variation in the numbers of islands of sets 32 and 34 which become electrically connected to electrode 30 and thus to terminal 46 provide variable capacitances 60, 62 respectively (Fig. 5). The conductive material may be silver, tantalum, gold, platinum, rhodium, indium, copper or molybdenum, and the resistive material platinum-silver cermet or carbon-filled varnish. The RC network 58 formed (Fig. 5) may be used as a variable feedback for tone control of an audio amplifier. Electrode 12 is shaped as shown to provide non- linear capacitance change. The same network may be obtained in a rectilinear form (Fig. 7, not shown). In modifications a three-terminal stator assembly also includes variable resistance, an arcuate tapering resistor (92), (Fig. 9, not shown), being deposited outside one of the sets of isolated islands for contact by the same wiper, and the resistor being electrically connected to the underlying lower electrodes and its associated terminal (Figs. 8, 10, not shown). The substrate 16 may support additional fixed resistors and capacitor electrodes to provide network 124 (Fig. 11, not shown). A continuous upper electrode (162) may be connected to an underlying lower electrode through a fixed resistor (184), (Fig. 13, not shown), forming circuit (190), (Fig. 14, not shown), suitable for tone control operating on negative series voltage feed-back from an amplifier output.