Patent GB-778090-A (GB)
Title: Electric phase difference measuring instrument
IPC: G01R25/00
Filed: 1954-10-22
Abstract: 778,090. Phase measurements. LINK AVIATION, Inc. Oct. 22, 1954 [Oct. 23. 1953], No. 30492/54. Class 37. A phase difference measuring apparatus comprises means for vectorally adding or subtracting two alternating voltages derived, e.g. from the input and output of a transformer, amplifier or like apparatus and for measuring such sum or difference, means for varying either input signal until the vector sum or difference is a minimum and in quadrature with the varied input signal, and means for producing and measuring an output signal proportional to the magnitude of the vector which value is proportional to the sine of the phase angle between the input signals. Fig. 1 shows a phase difference measuring apparatus wherein a reference signal is applied to input 10 and a test signal of identical frequency to input 30. The signals are each coupled over a resistance and capacitance in parallel to the respective inputs of high gain amplifiers 13, 33 (Fig. 2, not shown) which each comprise three pentodes in cascade whose outputs are developed across potentiometers 24, 35 from the sliders of which retroactive feedback signals are fed to the inputs over plural parallel resistance capacitance channels selectable by ganged multiway switches S1 and S2 ; so that gain is held constant and phase shift is reduced in each amplifier ; the proportion of feedback being adjusted coarsely by the switches and finely by the sliders of potentiometers 24, 35. The output of amplifier 13 is fed through a 2 way 4 gang changeover switch S3 in the position shown to summation resistance 5 in the input of amplifier 71 similar to amplifiers 13, 33 but having fixed degenerative feedback over resistance 73, while the remaining summation resistances 52, 53 are earthed with switch S4 as shown, and the output of amplifier 71 is connected to an A.C. indicating valve or other voltmeter 90 either direct or through noise-reducing bandpass filters 80, 81 selectable by 2 gang 4 way switch S5. The 2-way switch S4 is operable to connect the output of amplifier 33 to switch S3 in the position shown so that such output combines additively with that of amplifier 13 over summation resistances 51, 62, while S3 in its alternaive position enables the outputs of amplifiers 13 and 33 (through S4) to be connected in opposition across the primary of unity ratio transformer 56 whose output is applied over summation resistance 53 to the input of amplifier 71. Initially, switch S4 is set as shown, and switch S3 is set in the position alternative to that shown, while the reference voltage E R  is applied to input 10, and switch S1 and potentiometer 24 are adjusted to obtain a predetermined arbitrary reading on indicator 90. The test voltage Ex is then applied to input 30 and switch S4 set to its alternative position, so that the indicator reads the vector difference EM (Fig. 3) between ER and Ex. Switch S2 and potentiometer 33 are adjusted to minimize the difference reading, at which setting EM is in quadrature with Ex so that the phase angle between ER and Ex is given by and the indicator may be calibrated directly to read phase angle. For measuring variations from a phase difference of 180‹ between ER and Ex, switch S3 is placed in the position shown and after setting the reference reading, switch S4 is closed so that the indicator then reads the vector sum of ER and Ex which is minimized by adjustment as before to indicate the angle of phase variation.