GB766567A - Improvements in or relating to electrical measuring bridges - Google Patents

Abstract

766,567. Measuring bridges. WAYNE KERR LABORATORIES, Ltd., and CALVERT, R. April 4, 1955 [April 2, 1954], No. 9683/54. Class 37. A radio frequency impedance measuring bridge comprises a pair of inductive ratio arms formed by two windings of a common core, having a third R.F. energized winding exciting the bridge, and an unbalance detector energized through an output winding connected at one end to the ratio arm junction and connected at the other end through the unknown impedance to a point on one ratio arm remote from the junction thereof; the input of an adjustable attenuator network being connected across the other ratio arm to provide an adjustable output voltage applied through a standard resistance to the output winding. An input transformer 11 is excited at, e.g., 100 mc/s. over winding 12 and has two ratio arm secondary windings 13, 14, of which 14 is variably tapped, whose junction is connected over line 15 and through the primary 16 of output transformer 17 (energizing phasesensitive detector 19) and the unkown impedance 9, 10 to the free end of winding 13. The adjustable tap of winding 14 is connected to the input of an adjustable ladder attenuator 21 energizing potentiometer 22 whose variable tap is connected through adjustable standard resistance 24 to the unknown impedance end of primary 16, which is also connected through adjustable standard reactance 25 to the tapping 20 of winding 14. Resistance 24 and tapping 20 may be fixed, while winding 13 may be variably tapped and the standard reactance connected to adjustable taps on winding 16. The line 15 may also be variably tapped to a point on winding 16. At balance the reactive and non-reactive currents flowing in winding 16 through the unknown impedance 9, 10 are respectively balanced by the reactive current flowing directly through standard impedance 25 and from the attenuator through standard resistance 24, whereby the reactive impedance component is calculable from the turns ratio of the ratio arms, the value of reactance 25, and the setting of the tap 20; and the non-reactive component is calculable from the turns ratio of the ratio arms, the setting of tap 20, the setting of the attenuator, and the value of resistance 24. The measurement of the R.F. values of inductive and resistive impedances of a coil enables its Q to be determined.