Patent GB-654212-A (GB)
Title: Improvements in and relating to electrical phase difference measuring devices
IPC: H01J31/12
Filed: 1948-11-03
Abstract: 654,212. Cathode-ray oscillographs phasedifference and frequency measurements. BELLECOMBE, F. M. L. G. DE. Nov. 3, 1948, No. 28547. [Class 37] A cathode-ray oscillograph, for measuring the phase difference between two currents or potentials having the same frequency F, has means responsive to one of the currents or potentials for causing the spot to travel in a closed curve and also means responsive alternately, and at a frequency much higher than F, to the two currents or potentials so that they cause variations in the image to appear at two corresponding moments of the periods of the currents or potentials, the phase difference being measured by the angle between the positions occupied by the spot at these moments. In the cathode-ray oscillograph shown in Fig. 1, for measuring the phase difference between two alternating voltages, one of the voltages is phase split to energize two pairs of deflecting plates 8, 9 by voltages in quadrature so as to produce a circular trace 11 on the screen 3. The other voltage is applied to plates 7 and deflects the beam so that it passes through an aperture 5 in a plate 4 only at zero value of the voltage, so that the spot appears on the screen only at these moments. If there is no phase difference between the two voltages the spot appears at positions 12, 13 corresponding to the zero value of the first voltage. Otherwise the spot appears at positions 14, 15, the angle between the diameters 12, 13 and 14, 15 being equal to the phase difference. This angle may be measured by means of a pointer 16 which may carry a vernier. Alternatively the beam may be deflected by the plates 7 so as to produce peaks on a circle on the screen, the position of which indicates the phase difference. The plates 7 may be alternately and at a frequency much higher than that of the voltages under examination, connected with these voltages so that four spots 12, 13, 14, 15 appear on the screen at positions corresponding to the zero values of the two voltages. Instead of using the plates 4, 7 the intensity of the electron beam may be modified directly, so that it is produced only at the desired moments. This is effected by short impulses controlled by the two voltages to be compared and applied to the electron gun or a grid in front thereof. Fig. 2 shows an arrangement for producing such impulses by one of the voltages, which operates a multivibrator 21 producing a square wave. This is passed through a resistance capacity circuit 22, the output of which is a sharply peaked wave which is applied to the control grid of a hexode 23 biassed so that anode current flows only when the grid is negative. The impulse thus produced is amplified in a valve 24 and produces in an inductance 25 shunted by a resistance 26 a damped oscillation which is applied to the gun or grid of the cathode-ray tube. A switch is provided for changing the resistance in the circuit 22 to provide for different ranges of frequencies. By using a phase-changer having two outputs in antiphase two impulses per period may be obtained. In a modification, Fig. 7, an opaque screen 35 with a circular gap 36 is arranged over the screen of the cathode-ray tube, either inside or outside the tube. One of the voltages to be compared is arranged to give two movements to the electron beam, one a horizontal reciprocating motion at its own frequency and the other a vertical reciprocating motion at a much higher frequency. The path of the spot on the screen is then a sine curve 37 with its loops close together, so that, owing to the dimensions of the spot, the screen is entirely scanned but only two narrow vertical strips 38, 39 are seen through the gap 36. These constitute in effect spots rotating in opposite directions at the frequency of the voltages to be compared. If the beam is allowed to impinge on the screen at only two moments, corresponding to the same fraction of the period of both voltages to be compared, say at one of their zero points, four luminous points α, α<SP>1</SP>, #, #<SP>1</SP>, Fig. 8, are seen. The points α, α<SP>1</SP> are produced by the first voltage and the points, #, #<SP>1</SP> by the second, the phase angle between the voltages being either α# or α#<SP>1</SP>. In order to distinguish between these an approximate value of the phase difference is obtained by means of a milliammeter with a switching device which allows a current to flow after receiving an impulse produced by the first voltage and interrupts it in response to an impulse produced by the second voltage. As the spots #, #<SP>1</SP> approach the ends of the horizontal diameter they become vertically elongated so that the reading becomes inaccurate. To avoid this the gap may be shaped as shown in Fig. 11, each quadrant comprising an arc MP of 30. degrees and a symmetrical arc PQ. Fig. 12 shows a complete circuit for measuring the phase difference between two voltages A, B. The part X comprises the elements for deflecting the electron beam, the part Y comprises the device for measuring the approximate phase difference, and the part Z comprises the devices for producing the impulses to be impressed on the electron gun. The voltage A is divided into two parts of which one, through a potentiometer 41, actuates the part Z and the other, through a potentiometer 42, feeds the parts X, Y. The part X comprises a valve 43 controlled by the voltage A and which controls pentodes 44, 45 connected with the deflecting plates 46, 47 of the cathoderay tube 79 so as to produce horizontal displacement of the spot at the frequency F of the voltages A, B. Vertical displacement at a higher frequency F<SP>1</SP> is imparted to the spot by an alternating field produced by an oscillatory valve 48 feeding the plates 49. A rotating spot thus appears in the annular gap 36 of the screen 35. The part Y comprises two valves 51, 52 constituting a saturated amplifier, a coupling valve 53 and a resistancecapacity circuit 54 with a switch for changing the range of frequencies. The output consists of one impulse per cycle of the voltage A and each impulse acts on a multivibrator 55 and allows current to pass through a milliammeter 56. The multivibrator 55 receives also from part Z, through a wire 57, impulses produced from the voltages A, B, the latter of which interrupt the current. The milliammeter 56 indicates approximately the phase difference between the voltages A, B. The part Z comprises the elements 22, 23, 24, 25, 26, shown in Fig. 2, the inductance 25 being connected to the electron gun and also to the multivibrator 55. The devices corresponding with the multivibrator 21, Fig. 2, are arranged as follows. Voltage A is applied to the control grid of a hexode 58, while voltage B is fed through a potentiometer 50 to the control grid of a hexode 59. The hexodes 58, 59 are alternately rendered operative and inoperative by a multivibrator 61 controlling their third grids C, so that the potential of the point 62 is varied alternately by the anode currents of the valves 58, 59, and controls a multivibrator 63 which, through a valve 64, imparts square waves to the input of the circuit 22. In order to suppress parasitical impulses produced by the multivibrator 61, a multivibrator 65, having a period half that of the multivibrator 61, is arranged to supply short negative correcting impulses to the third grid of the hexode 23. An adjustable condenser 66 allows correction of the phase variations due to variations of capacity caused by the displacements of the sliding contacts of the potentiometers and may be provided with switching devices passing to the electron gun only the impulses due to the voltage B, the angle corresponding to the shift then being read on a dial the zero of which is fixed. In order to increase the accuracy of measurement an auxiliary tube may be provided wherein the spot travels along a graduated scale with such speed that it traverses the whole scale while the spot of the principal tube traverses one division of the scale, the movement of the second spot being started when the first spot is exactly in front of the mark separating two successive divisions of the scale. The apparatus may also be used as a frequency meter by comparing the current, the frequency of which is to be measured with another of known frequency. When the frequencies are equal the spots α, α<SP>1</SP>, #, #<SP>1</SP> or 12, 13, 14, 15 remain steady ; if they are slightly different the spots appear to rotate. If the frequency of one current is a multiple of that of the other there appear a greater number of luminous spots.