Patent GB-638318-A (GB)
Title: Improvements in or relating to arrangements for comparing two alternating voltages
Inventor: SECKER BEN
IPC: G01R19/10
Filed: 1947-09-24
Abstract: 638,318. Voltage measurements. STANDARD TELEPHONES & CABLES, Ltd. Sept. 24, 1947, No. 25928. [Class 37] Equality of two primary alternating voltages having a given frequency is indicated by deriving two secondary voltages proportional to the vector sum and difference of the primary voltages and applying the secondary voltages to a quadrature detector so as to produce a zero output when the secondary voltages are in quadrature. A test oscillator 1, Fig. 2, supplies two parallel paths 2, 3 containing respectively a filter or other transmission device whose attenuation is to be measured, and an adjustable attenuator by which the measurement is made. The output voltages V1, V2 from the paths 2, 3 are supplied to a combining device 4 which supplies to a quadrature detector 5 over conductors 6, 7 the vector sum V1+V2 and vector difference V1-V2 of the output voltages. A meter 8 is connected to the output of the quadrature detector 5 and gives a zero indication when the voltages supplied over the conductors 6, 7 are in quadrature. The attenuator 3 is adjusted until zero reading is obtained. The voltages V1, V2 are then equal and the reading of the attenuator 3 gives the insertion loss of the filter. The combining device 4 comprises a three-winding transformer or hybrid coil 9, Fig. 3, and a second transformer 10. The junction point of the two line windings 11, 12 of the transformer 9 is connected to earth through the primary winding 14 of the transformer 10. The voltages V1, V2 are applied to the line windings 11, 12. The secondary winding 15 is centre-tapped and is connected to one pair of diagonal terminals of a ring modulator 16 comprising four rectifiers. The other pair of diagonal terminals is connected to a D.C. instrument 17 which is shunted by two equal blocking condensers 18, 19 in series. The secondary winding 20 of the transformer 10 is connected between the centre tap of the winding 15 and the junction point of the condensers 18, 19. The transformer 10 may be omitted, the junction of the line windings 11, 12 being connected directly to the centre tap of the winding-15, and the junction point of the condensers 18, 19 being connected directly to earth. Both transformers 9, 10 may be omitted and the condensers 18, 19 replaced by resistances, or the transformers 9, 10 and condensers 18, 19 may be omitted and the meter 17 provided with a centre-tapped differential operating winding. Fig. 4 shows a modification in which the voltages VI +V2 and V1 - V2 are amplified. Modulators 21, 22, filters 23, 24, and amplifiers 25, 26 are included in the leads 6, 7. The local carrier wave for the modulators 21, 22 is derived from a frequency changer comprising an auxiliary oscillator 27 supplying a fixed frequency #0 to a modulator 28, which is also supplied from the test oscillator 1. This provides a frequency ft which may be adjustable. The upper sideband fo+ft is selected by a filter 29 and is applied as a carrier wave to both modulators 21, 22. The filters 23, 24 are set to select the lower sideband, which will then be at the fixed frequency #0. The frequency of the voltages V1+V2 and V1 - V2 is thus changed to a fixed frequency which is independent of the test frequency used. Specification 540,579 is referred to.