Patent GB-938728-A (GB)
Title: High input impedance circuit for measuring apparatus
Inventor: ROSENSTOCK DAVID
IPC: G01R19/00
Filed: 1960-07-11
Abstract: 938,728. Voltage measurement; phase measurement; high impedance input circuit. NORTH AMERICAN AVIATION Inc. July 11, 1960, No.24144/60. Class 37. [Also in Group XL (c)] A circuit for presenting a high input impedance to a high impedance voltage source 10 to be measured comprises the primary winding 12 of a transformer 13 connected between source 10 and a further variable voltage source 16, both sources being compared with a source 11. The arrangement is such that when source 16 is adjusted until the reading on meter 17 connected across transformer secondary 14 is zero, the bucking voltage supplied by source 16 is equal and opposite to that of source 10, so that no current flows from source 10 through primary 12 or through meter 15 so that the measuring circuit presents a very high impedance to the voltage to be measured. In the embodiment of Fig. 2, the voltage to be measured is applied at 22 and an amplifier A is connected to transformer secondary 25 to apply a bucking voltage at the output end of transformer primary 23, it being proved that the effective input impedance of meter 27 is increased by a factor equal to the product of the amplifier gain and transformer turns ratio. In an alternative embodiment (Fig. 3, not shown), only a fraction of the amplifier output is provided as a bucking voltage. A circuit for measuring the voltage and phase of an A.C. signal from source 37 by comparing it with one from a reference source 38 is shown in Fig. 4 and comprises a high impedance input circuit (as in Fig. 2) consisting of a transformer 42, 51 and amplifier 47 and also comprises a source 70 of variable phase and magnitude including two phase shifters feeding voltages of opposite phases and mutually in quadrature, to summing resistors 71, 72, 73 to provide a voltage of variable magnitude which is opposed to that from tap 54 until meter 53 reads zero. A reference voltage of variable phase and magnitude is provided by a source 70 including two phase shifters, the first comprising a transformer 74, the ends of the secondary 76 thereof being selectively connected to potentiometer 78 through switch 77. The voltage magnitude from potentiometer 78 is indicated on a dial (not shown) and the phase by the setting of switch 77. The output fed to resistor 72 from the first phase shifter is of variable magnitude and has one of two mutually opposite phases such as 0 degrees or 180 degrees with respect to the reference phase. A second, similar, phase shifter comprising a transformer 79 feeds a voltage of variable magnitude and one of phases 90 degrees or 270 degrees with respect to the reference phase to a resistor 73. The summed voltages on resistors 72, 73 thus provide the reference voltage of variable magnitude and phase with which the unknown voltage on resistor 71 is compared. In operation, switches 77, 81 are set to produce quadrature components which are of opposite phase relative to the quadrature components of the unknown voltage and the tapping on potentiometer 78, 82 simultaneously varied until indicator 53 reads zero when the quadrature components of the unknown voltage and the reference voltage are equal and the magnitudes indicated by the dials on the potentiometers 78, 82.