GB690158A - Improvements in or relating to electrical frequency meters - Google Patents

Abstract

690,158. Bridge circuits. SANGAMO WESTON, Ltd. Oct. 10, 1951 [Nov. 18, 1950], No. 23606/51. Class 37. A frequency sensitive bridge comprises a pair of impedances forming two adjacent arms at a bridge input junction, a load connected across these bridge arms, and an opposite pair of bridge arms connected through a reactor to a frequency input terminal in which opposite arms the mutual inductance of the reactor is balanced against capacitance whilst the self-inductance of the reactor secondary is balanced out at the bridge balance frequency. As shown, two adjacent arms are formed by equal resistors 10, 11 at one input junction, whilst the opposite arms include a capacitor C and the two secondary windings 13, 14 of a reactor 15, balanced for the condition of equal reactance. An indicating instrument 16 is connected across one diagonal and the frequency to be measured is applied across the other diagonal through input terminals 17, 18 and the primary winding 20 of the reactor. The secondary windings 13, 14 are in mutual opposition so that the selfinductance of the secondary cancels at bridge balance, leaving only the primary-secondary mutual inductance effective in the bridge. The temperature coefficient of the capacitor C may be corrected by the choice of a dielectric giving an appropriate coefficient of resistance, and the level coefficient of the reactor may be compensated by a non-linear resistor. The reactor 15 may comprise a two-legged core, each leg carrying portions of the primary and secondary windings, corresponding portions being of equal turns and connected in resistive and reactive symmetry. The resistors 10, 11 may be replaced by equal capacitors. The instrument 16 may be replaced by a dynamometer instrument having crossed movable coils connected in series across the bridge diagonal, and a field coil connected in series between the frequency source and a bridge input junction. A resistor may be connected in parallel with the field coil for compensating the phase shift due to bridge reactance. The instrument 16 may be a D.C. instrument, rectifier elements being connected in a pair of bridge arms and further rectifier elements being cross connected between these arms and the D.C. instrument to form a series loop constituting a modulator bridge. A resistor may be connected in series with the secondary coil 14 to compensate for residual leakage reactance.