GB662707A - Improvements in or relating to impedance measuring equipment - Google Patents

Abstract

662,707. Impedance measurements. STANDARD TELEPHONES & CABLES, Ltd. July 6, 1949, No. 17855/49. Class 37. A high frequency A.C. impedance comparator comprises a generator of high frequency currents continuously variable in frequency over a selected frequency range but having stable output level over this range, a fixed standard resistor, means for supplying the generator output in series with the fixed resistor to input terminals of a network whose impedance is to be determined and D.C. means for comparing the voltage at these input terminals with that at the input to the resistor, whereby successive maximum or minimum values of impedance of the network occurring with continued variation of frequency may be determined with reference to the resistor. At radio frequencies the impedance-frequency characteristic of a long coaxial cable or equivalent network has the form shown in Fig. 1, shown in full lines for a short-circuited cable and in dotted lines for an open-circuited cable. The impedance Z is complex but becomes purely resistive at the extreme values Z1, Z2. From measurements of Z1, Z2, and the frequency separation of adjacent peaks of the same curve, together with a knowledge of the lengths of the cable, the characteristic impedance, attenuation constant, phase constant, and velocity ratio may be calculated. Fig. 3 shows a circuit by which the impedance Z may be determined in terms of a constant resistance R and the ratio of the voltages e/E. The generator voltage E is rectified at W1 and produces a D.C. potential drop across a resistance r1 The cable voltage, rectified at W2, produces a potential drop across a resistance r2 which is balanced by a detector Det against a tapped-off portion a of the drop across r1. The slider of the resistance r1 may be calibrated in terms of Z for a particular value of R. The high frequency generator is stabilized by amplified D.C. voltage feed-back derived from the rectified voltage E, which is differentially compared with a fixed reference voltage.