Patent GB-696447-A (GB)
Title: Method of and apparatus for indicating amplitude modulation of an alternating carrier voltage
Inventor: WHITWELL ARTHUR LESLIE
IPC: G01R13/20
Filed: 1951-03-02
Abstract: 696,447. Cathode-ray oscillographs. BOULTON PAUL AIRCRAFT, Ltd. March 3, 1952 [March 2, 1951], No. 5175/51. Class 37. Amplitude modulation of an A.C. carrier voltage applied to a cathode-ray oscilloscope is indicated by applying two brightening pulses during each cycle, one occurring when the amplitude of the carrier wave is at or near its maximum in one sense, which may be either positive or negative, and the other when the amplitude corresponds to zero modulation component, or any other datum value. The invention is particularly applicable to the indication or recording of vibration or strain of structures, of electro - medical measurements, and of the cyclic movement of machines and other lowfrequency oscillatory or cyclic phenomena by means of resistance type strain gauges or other pick-ups forming an arm of a Wheatstone bridge supplied with A.C. at carrier frequency. With the bridge normally balanced the output reverses its sign on reversal of the sign of the strain being measured, thereby causing the bright spots 4, Fig. 3, to cross the zero axis 3 and indicate the correct sign of the strain. In the arrangement shown in Fig. 5, the output of the carrier frequency oscillator 6 is transmitted by transmitting means 7 to a phase control network 8. The output from this is fed to a high-gain squaring amplifier 9 and thence to a differentiating circuit 10 which produces two pulses 24, 34 of opposite sign at the two sides of the square half-wave. These pulses are fed to an amplifier 11 including a valve operated at zero grid bias so that the positive pulse 34 is severely limited by grid current, leaving a single pulse 4 corresponding to pulse 24. The negative pulses 4 are fed to the cathode of the cathode-ray tube 12. In the case of a multi-channel recorder these pulses are applied to the cathodes of all the cathoderay tubes 12 so as to serve all the recording channels 13, each of which includes a modulator or transducer 14, such as a bridge pick-up, the output from each of which is fed through transmitting means 15 and amplifier 16 and thence to the corresponding cathode-ray tube 12. The second brightening pulse 5 is obtained by passing part of the output of the differentiating circuit 10 to a controlled relaxation oscillator 17 for generating a square wave when triggered by the brightening pulse 24. The square wave is then passed through a differentiating circuit 18 to produce two pulses 25, 35 corresponding to the two sides of the square, the pulse 35 occurring at a controlled time interval from the pulse 25, according to the adjusted width of the square wave. These pulses are amplified at 19 with suppression of the positive pulse to give a single pulse 5, which is combined with the output of the amplifier 11 and applied to the cathodes of the cathode-ray tubes. By varying the phase shift produced by the phase shifter 8 the first brightening pulse 4 can be made to occur at any desired time interval referred to a fixed point in time of the carrier waveform 1, Fig. 3. By varying the width of the square wave generated by the oscillator 17 the second brightening pulse 5 can be made to occur at any desired time interval after the pulse 4. By producing an initial unbalance of the bridge pick-up, the level of the bright spots 4, 5 may be caused to coincide when the modulation amplitude is zero. The pulse 5 may alternatively be produced by duplicating the phase shifter 8 and the subsequent pulseforming chain 9, 10, 11.