Patent GB-691344-A (GB)
Title: Improvements in or relating to phase selective networks
Inventor: DOLL HENRI GEORGES
IPC: G01R1/30
Filed: 1951-07-04
Abstract: 691,344. Electrical tests and measurements. SCHLUMBERGER OVERSEAS SOC. ANON. July 4, 1951, No. 15862/51. Class 37. In order to separate the in-phase and out-of-phase components in the unbalance current of a mutual impedance bridge 1 (Fig. 2) comprising an input coil 2 fed from an A.C. source 9, and an output coil 3, the A.C. supply is also fed to a variometer tube circuit 4 which adds an auxiliary voltage to the bridge output to eliminate the unwanted component, the modified signal being amplified at 6, and measured at 8, the amplitude of the auxiliary voltage being determined in a feedback circuit 7. Alternatively, the output of variometer 4 may be made in phase with the desired component and very large, so that the change in readings on a meter 8 represents only the changes in the desired component. In one embodiment (Fig. 3) for use in well-logging apparatus, an oscillator 14 feeds a transmitter coil 11 associated with a receiver coil 13, whose output is initially adjusted to zero by variometers 21, 22. With a change in the coupling medium between the coils 11, 13 a signal appears in coil 13, which is amplified at 19 and fed into headphones 20 or a meter. A variometer tube circuit 27 for neutralizing the unwanted component of the signal comprises a pentode valve 28 whose control grid 31 is fed with a proportion of the oscillator output via condenser 32 and resistor 33, and therefore in phase with the current in transmitter coil 11, whilst the anode circuit includes the primary winding 45 of a transformer whose secondary 47 is in series with the receiver coil 13. The phase-shift between the voltage applied to the grid 31 and the current in winding 47 is adjustable by potentiometer 49 and approximately independent of frequency. This current may thus be made in phase with the reactive component in winding 13, and its magnitude is determined by a D.C. bias on the grid 31 of valve 28 obtained from the output of amplifier 19, and dependent on the value of the unwanted phase component, by a phase selective circuit 61, comprising a valve 60, whose grid 59 obtains a D.C. bias through a winding 56 of a transformer 57 from a portion of the rectified output of the oscillator 14 appearing across a potentiometer 79, 80, 81, together with a fixed portion of the output of amplifier 19, and a large voltage, at 90 degrees to the current in transmitter coil 11, through the transformer 57, the phase of the output of amplifier 19 being adjusted by a condenser 20a so that the phase selective circuit 61 supplies to the control grid of valve 28 a bias which automatically suppresses the unwanted signal component. By changing the resistance 33 to a condenser, the system will neutralize the resistive component instead of the reactive component of the pick-up signal. The magnitude of the suppressed component may be determined by measuring the variations in anode current of valve 28 with a valve voltmeter 91, Fig. 4 (not shown). In a further embodiment (Fig. 5) both components of the signal voltage are initially suppressed by duplicating the phase selective circuit 61, 61<SP>1</SP> and the variometer tube circuit 27, 27<SP>1</SP>, the former initially suppressing the reactive component, variations of which are shown on valve voltmeter 91, and the latter initially suppressing the resistive component whose variations are shown on 91<SP>1</SP>. Minor modifications to the circuits are shown to enable currents and voltages of the appropriate phases to appear where required.