Patent GB-1031113-A (GB)
Abstract: 1,031,113. Testing electromagnetic transducers; transistor oscillator circuits. BRITISH PETROLEUM CO. Ltd., and B. A. PALMER. Nov. 29, 1962, No. 45145/62. Headings H3T and H4J. [Also in Divisions H2 and G1] In a method of testing electromechanical transducers, e.g. a moving coil geophone, the transducer is connected in series with a variable resistor across a variable frequency supply, the voltages developed across the resistor and transducer are distorted, amplified, added, and displayed; the frequency of the supply and the resistance of the variable resistor are then varied until the displayed signal contains no fundamental component of supply frequency, in which condition the frequency of the supply is equal to a resonant frequency of the transducer and the value of the resistor is equal to the impedance of the transducer at resonance. In the described embodiment the display comprises a Lissajous Figure on a C.R.T. obtained by applying the 2nd harmonic distorted, added, signal to one beam axis and the variable frequency supply to the other. Circuits are described for the oscillator, distorting amplifier and adder, and the transistorized power supply unit. Oscillator.-Fig. 5 shows the Wien bridge oscillator having transistors T1 and T2 connected as common emitter amplifiers feeding a linearized emitter follower comprising transistors T3 and T4, the output from which is fed to the Wien bridge in the input of the amplifier, via a stabilizing thermistor Th, and to the output terminal A via a further amplifier transistor T5. Amplifier and adder, Fig. 6. The oscillator output is applied via terminals A and B, to the series combination of variable resistor R1 (comprising R1A, R1B and R1C for coarse, medium and fine adjustment) and the transducer, connected across terminals 14. The voltages developed across the resistor and transducer are applied to the bases of transistors T6 and T7 respectively where they are added, in addition second harmonic distortion is introduced by biasing the transistors to the curved portion of their characteristic. The distorted, added, signal is applied to a phase splitter, transistor T8, and a push-pull amplifier, transistors T10 and T12, before applying to the deflecting circuit of the C.R.T. at terminals Y1 and Y2. A similar amplifier T9, T11 and T13 amplifies a portion of the oscillator output at terminal C for the X deflection circuits of the C.R.T. at X1, X2. Power supply circuits.-Fig. 7 comprises an oscillator, transistors T14 and T15, the oscillations of which are initiated by the momentary closure of contact RX2 due to the charging current of capacitor C8 flowing through the relay coil when the supply is switched on by switch S4. Relay RY is provided in the supply circuit to ensure that the battery supply is correctly poled before power is connected to the oscillator circuit. Saturable transformer TR1 is connected in the oscillator circuit and the output feeds the C.R.T. heater from secondary S 1; unstabilized and Zener diode stabilized supplies to the amplifier from secondary S2; Zener diode stabilized supply to the oscillator from secondary S3; and potentials for the C.R.T. from secondary S4.