Patent GB-662923-A (GB)
Title: Impedance measuring apparatus
IPC: G01R27/04
Filed: 1949-12-10
Abstract: 662,923. Impedance measurements. AIRBORNE INSTRUMENTS LABORATORY, Inc. Dec. 10, 1949 [Dec. 11, 1948], No. 31775/49. Class 37. Apparatus for determining the impedance characteristics of components or circuits at high frequencies includes voltage-responsive coupling means for producing a first signal which is a function of the voltage at the point of measurement, current-responsive coupling means for producing a second signal which is a function of the current at the same point, and adjustable means for equalizing the magnitude of the first and second signals, the adjusted position of which means indicates the impedance at the measuring point. In the arrangement shown a coaxial line 4B extends between a signal generator (not shown), and a load (not shown), the impedance of which is to be measured. A loop 22B induces a current, proportional to the current in the line 4B at the reference point, in a transmission line 14B which is terminated by a resistor 24B, the value of which is equal to the characteristic impedance of the line 14B. A capacity probe 34B induces a current, proportional to the voltage in the line 4B at the reference point, in a coaxial line 26B which is terminated by a resistor 36B, the value of which is equal to the characteristic impedance of the line 26B. The outer conductors 18B, 32B of the lines 14B, 26B are slidable in sleeves 42, 44 joined to the outer conductor 6B of the transmission line 4B at the reference point. The lines 14B, 26B are secured to a carriage 46 which is movable laterally by a screw 52, while the transmission line 4B and sleeves 42, 44 are anchored to a stationary support. Motion of the carriage 46 towards the left causes an increased coupling between the loop 22B and the inner conductor 8B of the line 4B and a decreased coupling between the probe 34B and the conductor 8B. If the carriage 46 is adjusted so that the currents in the lines 14B, 26B are equal, a scale 56 co-operating with a pointer 58 can be calibrated directly in terms of the impedance of the line 4B at the point of reference, which is proportional to the ratio of voltage and current on the line 4B at the reference point. In order to determine when equality of current between the lines 14B, 26B has been obtained, and at the same time to determine the phase difference between these currents and therefore the phase angle of the impedance of the line 4B, a rigidly-supported portion of the line 14B is provided with a slot 64 in the outer conductor 18B. A similar, but oppositely-disposed, slot 65 is provided in the outer conductor 32B of a -parallel rigidlysupported portion of the line 26B. Probes 66, 72 carried by an insulating yoke 68 are movable longitudinally in these slots by rotation of a screw 70. The probes 66, 72 are joined by the primary winding 74 of a transformer, the secondary winding 78 of which is connected to a null indicating device 82. As the yoke 68 is moved along the slotted portions of the lines 14B, 26B, a point can be located where the two signals are exactly 180 degrees out-of-phase. At this point, if the carriage 46 has been adjusted so that the currents in the lines 14B, 26B are equal, no signal will be induced in the secondary winding 78. Such balance may be obtained by alternately adjusting the carriage 46 and the yoke 68. When the null condition is obtained, the magnitude of the impedance on the line 4B at the point of reference can be read on the scale 56 and the phase angle of the impedance can be read from a scale 84 cooperating with a pointer 86 extending from the yoke 68.