Patent GB-683111-A (GB)
Title: Apparatus for carrying out frequency analysis of transmission systems
IPC: G05B23/02
Filed: 1951-04-26
Abstract: 683,111. Frequency analysis. AKTIEBOLAGET BOFORS. April 26, 1951 [April 28, 1950], No. 9780/51. Class 37. Frequency analysis of a transmission system is carried out by supplying a signal to the system and comparing the signal after transmission, or the difference between the signal before and after transmission, with a reference signal. As shown in Fig. 2, an adjustable low-frequency oscillator 1, containing an adjustable phase shifter, supplies a signal to the amplifier 3 of a servo system 2, 3, 4, 5, 10 which is to be frequency analysed. The generated signal is also supplied as a reference signal through the phase shifter to a wattmeter 6 serving as a comparison means. The servo system consists of the differential 2 which is connected through the amplifier 3 to the controlled member 4. The output from this is connected by a conductor 5 to the differential 2, and the difference between this output and the supplied signal is supplied to the amplifier 3 in order to adjust the controlled member 4. The wattmeter 6 is connected through a demodulator 11 alternatively to contacts 8, 9 by a contact arm 7 so as to compare the distorted signal or the difference signal with the reference signal, the wattmeter indicating the product. By adjusting the phase shifter of the oscillator 1 until the wattmeter reading is a minimum the phase angle can be determined. A carrier frequency oscillator 13 is connected to the demodulator 11 and to a controlling member 12 of the servo system, to the controlled member 4 and to the amplifier 3. In the servo system the low-frequency signal modulates the carrier frequency signal so that the signals operating within the system are modulated carrier-wave signals. In the demodulator 11 the distorted and error signals are separated from the carrier and supplied to the wattmeter 6. In the arrangement shown in Fig. 5, two synchronous elements 24, 26 have their rotors mounted on a common shaft 25. The stator of the element 24 is fixed, while that of the element 26 is adjustable by means of a ring 27 fixed to the stator and carrying an index 29 co-operating with a phase-angle scale on a stationary ring 28. The shaft 25 is driven by a motor 30, the speed of which is varied by an adjusting knob 31 provided with a scale. The oscillator 13 is connected to the stator winding of the element 24, amplifier 3, controlled member 4, and demodulator 11. The stator winding of the element 26 is connected to the demodulator 11, to which is also connected a voltmeter 36. The contact arm 7 is connected to the rotor winding of the element 26. The signal from the oscillator 13 is modulated by the element 24 at a frequency corresponding to the speed of rotation of the shaft 25. The signal from the contact arm 7 is multiplied in the element 26 by a signal similar to that which modulated the carrier-wave signal in the element 24. The carrier wave is removed in the demodulator 11 and the signal is read on the voltmeter 36. To determine the phase angle the stator of the element 26 is turned untilthe voltmeter 36 indicates a minimum, the index 29 then indicating the phase angle.