Patent GB-608016-A (GB)
Title: Improvements in or relating to impedance measuring devices
IPC: G01R17/00
Filed: 1946-02-14
Abstract: 608,016. Impedance measurements. STANDARD TELEPHONES & CABLES, Ltd., and SECKER, B. Feb. 14, 1946, No. 4631. [Class 37] An admittance or impedance bridge comprises two equal ratio arms formed by two equal, balanced, and closely coupled inductive windings, means for connecting one of these to the admittance or impedance to be measured, one or more standard admittances or impedances, at least one of which is connected to the other balanced winding through an attenuator, means for applying test voltages to the test admittance (or test currents to the test impedance), and to all the standard admittances or impedances, and means for indicating when the algebraic sum of the currents in all the admittances (or the voltages across all the impedances) is zero. Fig. 2 shows a bridge for measuring low admittances (or high impedances). The ratio arms AB, BC are formed by the two balanced windings 14, 15 of a threewinding transformer, the primary winding 16 of which is connected to input terminals 17, 18 for a test oscillator. A variable differential condenser 19 has the movable plates connected to the corner D of the bridge and the sets of fixed plates connected to the corners AC through a balanced attenuator 20, the neutral conductor of which is connected to the corner B. A variable differential conductance element 21 has the movable contact connected to the terminal D and its ends to the corners A, C through a second balanced attenuator 22, which has its central conductor connected to the corner B. Both halves of both attenuators should have the same characteristic impedance Z, and if the attenuators are adjustable, Z should be constant. The corners A, C are connected to the attenuators through resistances 23, 24, 25, 26, each of resistance Z. Terminals 27, 28 for the test admittance to be measured are connected to the corners A, D through a resistance 31 of Z ohms. Terminals 29, 30 connected to the corners B, D are provided for the detector. The bridge is balanced by setting the attenuators and then adjusting the elements 19, 21 until the detector shows no current. Fig. 7 shows a simplified bridge for measuring the capacities of small condensers in cases where no conductance balance is necessary. A double-screened threewinding transformer 32 has closely coupled and balanced secondary windings 14, 15. The detector is connected to the corners B, D by a transformer 33. Terminals 27, 28 for the condenser under test are connected to the corners A, D. An unbalanced attenuator 34 has its input circuit connected to the terminals B, C, and an adjustable output tap is connected to the variable condenser 35 which is connected to the corner D. Fig. 9 shows a bridge for measuring very low impedances. A balanced adjustable attenuator 33 has the neutral conductor connected to the earthed corner B and both input conductors connected to the corner A through equal resistances 45, 46. The output of one half of the attenuator 44 is connected to a fixed inductance L0, and the output of the other half to a variable inductance L1. An unbalanced variable attenuator 49 of constant impedance has its neutral conductor connected to the corner B and its input conductor connected through a resistance 50 to the corner C. An adjustable standard resistance R is connected to the output of the attenuator 49, and the low impedance 52 to be measured has one terminal connected to earth and the other to the corner C through a resistance 53. Two identical output transformers 54, 55 have their secondary winding connected in series to terminals 29, 30 for the detector.