GB890945A - Improvements in and relating to electric impedances of variable value - Google Patents

Abstract

890,945. Networks of resistances, or capacitances, or inductances. TELEPHONE CABLES Ltd. July 25, 1958 [July 26, 1957], No. 23755/57. Class 40(8). A variable impedance comprises a number of separate equal-value impedances and an impedance of N times that value, the impedances being associated with N two-way switches in such a manner that by operating the switches one at a time in one sense and thereafter one at a time in the opposite sense the total impedance in circuit is varied in 2N-1 equal steps from a minimum to a maximum and then from the maximum to the minimum value in a single final step. The variable capacitance of Fig. I comprises four capacitors Cl of unit value and a five-unit capacitor C5 associated with five two-way switches CA4-CD4 and CE5. In the condition shown, there is no capacitance between terminals x and y. If the contacts of switches CA4-CD4 are closed one at a time in any order upon terminals a, the capacitance between the terminals x,y increases by unit steps to four units. Actuation of switch CE5 brings the five-unit capacitor C5 into circuit and short-circuits the unit capacitors C1. The total capacitance may now be increased from 5 to 9 by returning the switches CA4-CD4 to their initial condition. When the remaining switch CE5 is returned to its initial condition, the value of the capacitance between terminals x and y is again zero. A further section of the circuit comprises four ten-unit capacitors C10 and a fifty-unit capacitor C50 associated with five switches CF4-CI4 and CJ5. Each switching operation of the second section is arranged to occur as the switch CE5 is returned to its initial condition, on the completion of each switching cycle of the first section. By this means the total capacitance is increased in unit steps to 99. A further increase of one unit is obtained by closing switch CK4 which brings the 100-unit capacitor C100 into circuit. Thereafter, a further switching cycle of the second section brings the total capacitance to 199. The variable resistance of Fig. 3 comprises two sections connected in series between terminals x,y, each section comprising two series of four resistors of equal value and a further resistor of five times that value. In addition, a 100-unit capacitor is connected in series with the two sections. The numbers associated with the resistors on the Figure indicate the resistance values. The switches are operated in the same manner as those in the variable capacitance of Fig. 3 to obtain a resistance increasing in unit steps to 199 units. Inductors may replace resistors.