Patent GB-843532-A (GB)
Title: Improvements in or relating to method and apparatus for measuring wave propagation time
IPC: G01N29/34
Filed: 1956-07-17
Abstract: 843,532. Radar; ultrasonic testing. ELECTROCIRCUITS Inc. July 17, 1956 [July 18, 1955], No. 13948/56. Class 40(7). Relates to a frequency or phase modulated radar or corresponding ultrasonic distance measuring system in which the echo signal is heterodyned with the transmitted signal or with a locally generated frequency or phase modulated signal to give a heterodyne signal and some parameter of the system is varied to make the frequency of the heterodyne signal equal to that of a narrow-band filter, the magnitude of said adjustment being indicative of the distance of the reflecting object. The variation of the parameter may be effected manually or cyclically; in the latter case the distance is indicated by applying the output from the filter to modulate the electron beam of a cathode-ray tube which is scanned synchronously with said variation. In a first type of system, Figs. 1-8, the transmitter frequency is modulated by a waveform, Fig. 2D, produced by combining a sawtooth, Fig. 2B, of recurrence period tj with a square wave, Fig. 2C, of recurrence period 2tj and the echo signal is heterodyned with the transmitted signal. The variable parameter may comprise:- (1) the time interval tj, Fig. 1 and Fig. 4 (not shown), the filter being tuned to the frequency corresponding to the amplitude of the waveform C so that the heterodyne signal is passed by the filter when tj is equal to the echo time te. (2) the amplitude of the square wave C, Figs. 3 and 6 (not shown). (3) the time interval tj and the amplitude of the waveform C, Fig. 7 (not shown). (4) the position of the transducer, Fig. 8 (not shown). In a second type of system, Fig. 9, the transmitter is modulated by a waveform, Fig. 10J, of recurrence period 2tj and the echo signal is heterodyned with a signal whose frequency is modulated by a waveform, Fig. 10K, comprising the waveform J delayed by a period tj and superimposed on a D.C. voltage. In this system the variable parameter is tj and the filter is tuned to the frequency corresponding to the difference in D.C. levels between waveforms J and K so that the heterodyne signal is passed by the filter when tj is equal to the echo time te. In the embodiments described, Figs. 1-8 relate to ultrasonic testing systems and Figs. 9 and 10 relate to a radar system with a type B C.R.T. display. Although the embodiments described use sawtooth modulation, other wave shapes, e.g. sinusoidal, may be employed. FIRST SYSTEM (1) Varying time interval tj. As shown in Fig. 1, the waveforms B and C, Fig. 2, are produced by generators 22 and 24 which are triggered by the output A from a pulse generator 12 whose recurrence period tj is varied cyclically by the sawtooth timebase voltage of a C.R.T. 62 produced by a potentiometer 16 driven by a motor 20. The waveforms B and C are combined in an adding circuit 26 giving a resultant waveform D which is applied to a reactance valve 36 to modulate the frequency of an oscillator 38 whose output is applied to a mixer 54 and through an amplifier 40 to a hybrid network 42 coupled to a transducer 44 and to a receiving amplifier 52 coupled to the mixer 54 whose output is applied through the narrow-band filter 56, detector 58 and video amplifier 60 to the Y plates of the C.R.T. 62 to give a type A display. In a modification, Fig. 4 (not shown), the hybrid network 42 and mixer 54 are omitted and the gain of the amplifier 40 is controlled by the waveform C so that the amplitude of the transmitted signal is considerably reduced during alternate tj periods, Fig. 5 (not shown). Effective echo signals will be produced only by the transmitted signals of large amplitude so that when tj is adjusted to equal the echo time te, heterodyning between the transmitted and echo signals will occur only during the periods of weak signal transmission. (2) Varying amplitude of waveform C. In this arrangement, Fig. 3 (not shown), the recurrence frequency of the pulse generator 12 is fixed and the output C from the generator 24 is applied to the adding circuit 26 through a potentiometer which is varied cyclically by the motor 20. In a modification, Fig. 6 (not shown), the output from the generator 24 is applied directly to control the gain of the amplifier 40 as described above with reference to Fig. 5 (not shown). (3) Varying time interval tj and the amplitude of the waveform C. In this arrangement, Fig. 7 (not shown), the two adjustments are effected manually. (4) Varying position of transducer. In this arrangement, Fig. 8 (not shown), the position of the transducer is varied cyclically in the direction of transmission by means of a rack and pinion driven by the motor 20 and the gain of the amplifier 40 is controlled by the waveform C. SECOND SYSTEM As shown in Fig. 9, the waveforms J and K, Fig. 10, are produced by generators 82, 95 triggered by alternate pulses H and I from a generator 80 whose recurrence period tj is cyclically varied by a sawtooth timebase generator 14 coupled to the X plates of a C.R.T. 62 whose Y plates are energized by the cyclically varying output from a potentiometer 100 which is rotated with the transmitting and receiving transducers 90, 92 by a motor 99. The waveforms J and K are applied to reactance valves 84, 96 to modulate the frequency of oscillators 86 and 98 coupled respectively to the transmitter 88 and a mixer 54 which is coupled through an amplifier 94 to the receiving transducer 92 and also to the narrow-band filter 56 whose output is applied through a detector 58 and amplifier 60 to intensity modulate the C.R.T. 62 to give a type B display.