Patent GB-454052-A (GB)
Title: Improvements in electric attenuators
IPC: H03H7/24
Filed: 1935-10-11
Abstract: 454,052. Variable resistances ; current meters. FORRESTER, H. J. C., 88, Chancery Lane, London.-(Electrad, Inc. ; 173, Varick Street, New York, U.S.A.) Oct. 11, 1935, No. 28172. [Class 37] [See also Groups XXXIX and XLJ An attenuator which introduces a variable amount of series and shunt resistance into a circuit so that its input and output impedances remain within predetermined limits and in some cases are substantially constant at all values of attenuation, comprises a series resistance 12 engaged by a slider 18 and a shunt resistance 16 connected substantially continuously between the series resistance and a conductive bar 14. Preferably, the resistance elements are in the form of a continuous film, but they may be wire-wound. Constructional forms in which the input impedance varies slightly. In one arrangement, Fig. 3, a metal casing lined with insulation 22 has the usual mounting bush 24 through which the adjusting shaft 26 passes. A strip of insulation 28 is coated on both faces and the edge 34 with resistance material. Contact is made with the inner face by means of a coil of fine wire 38, the turns of which are severed as at 40. This part of the resistance element forms the series resistance, and a variable amount is inserted by means of an arm 44 which makes contact with the coil 38 and is connected to an arm 56 coacting with a continuous strip 58. Both arms are carried by an insulating disc on the shaft 26. The remainder of the resistance film constitutes the shunt resistance and connection is made with it by means of a metallic ring 14. A uniform resistance element gives a logarithmic variation of attenuation. In a modification, Figs. 4 and 5 (not shown), a sliding contact is made directly with the resistance film. Metallic paint may be used for connecting the film to the terminal lugs and for providing the continuous conducting member 14. Modified constructions giving constancy of input impedance as well as output impedance. In the construction shown in Figs. 9 and 10, this is achieved by making the conductive strip 14 make contact with a portion of the parallel resistance corresponding to the amount of series resistance in circuit. The resistance element is straight and the strip 14 is springy and tends to leave the resistance material. A variable portion is forced into contact with the resistance by a roller 162 carried by the contact-carrying slider. The input impedance can be made nearly constant by using a resistance element tapered as regards both series and shunt resistances as indicated in Fig. 12 with a permanently connected strip 14. A better approximation is given by having two tapered elements as indicated in Fig. 13 and sliders 170, 172 moving simultaneously in opposite directions. Application to oscillator circuits for testing. Fig. 7 shows a test set oscillator for a radio repair man. Tube 100 oscillates at a highfrequency determined by the tuning of circuit 102,104. Tube 112 generates audio-oscillations in virtue of the back-coupling 122 and tuned circuit 126, 120 and also acts as a mixer. The A battery is of only 22¢ volts. Modulated H. F. currents are taken off at 136, their volume being controlled by an attenuator 138 of the kind described in the earlier paragraphs. Audio energy can be drawn from terminals 140.