GB812129A - Improvements in or relating to bridge circuit arrangements especially for indicatingthe intensity of high frequency electric fields - Google Patents

Abstract

812,129. Power measurements; measuring bridges. ELECTRIC & MUSICAL INDUSTRIES Ltd. Aug. 5, 1955 [Ang. 7, 1954], No. 23009/54. Class 37. [Also in Group XXXVIII] A measuring circuit comprises a first electric bridge containing a temperature-sensitive first impedance responsive to a parameter producing a temperature change, and a second similar bridge containing a second temperature-sensitive impedance responsive to the ambient temperature of the first impedance but irresponsive to the parameter; the bridges being connected in series in an energizing circuit passing direct and oscillatory components of current and means being provided responsive to the latter for deriving a signal representing balance variations of the second bridge, together with means responsive to such signal for regulating the direct component so as to nullify such variations and stabilize the first bridge for ambient temperature changes. Power in a waveguide or coaxial line is wholly or in proportional part applied to heat a first thermistor connected at BD in a bridge 1 comprising fixed resistances 2, 3, 4 and unbalance meter 7 whose deflection is a measure of the power. A second bridge 10 energized in series with the first comprises fixed resistances 12, 13, variable resistance 14 adjustable for similarity to bridge 1, and across AC a second thermistor exposed to the same ambient temperature as the first but in which no power is dissipated. Excitation for the bridges is derived from a D.C. source in series with a triodeconnected pentode 22 whose control grid is energized by the adjustable 1 kc/s. output of Colpitts oscillator triode 26a so that the excitation current contains a 1 kc/s. component, and the unbalance output of bridge 10 (varying in amplitude and phase sense in accordance therewith) is coupled over windings 20, 21 to pentode amplifier 45, double triode balanced cathodecoupled phase splitter 56, and double diode tuned phase discriminator 93, 95 which is also supplied with a 1 kc/s. reference signal from oscillator 26a over triode amplifier 26b, capacitance 83, resistance 84, and the cathode circuit of the balanced phase splitter, so that variations of ambient temperature produce variations of balance of bridge 10 with consequent variations of phase sense and amplitude of its 1 kc/s. output, which are converted by the discriminator to corresponding variations of magnitude and sense of a direct voltage superimposed on the 1 kc/s. grid excitation of pentode 24, whereby the D.C. excitation of the bridges is varied until bridge 10 is rebalanced and bridge 1 is compensated for ambient temperature variations. The device is applicable to measurement of other parameters which produce temperature change of a thermistor or other temperature-sensitive resistive elements.