Patent GB-644205-A (GB)
Title: Improvements in or relating to electrical phase-splitting devices
IPC: G01R25/00
Filed: 1946-09-25
Abstract: 644,205. Phase-difference measurements; cathode-ray oscillographs. THOMAS, E. L., and SPERRY GYROSCOPE CO., Ltd. Sept. 25, 1946, Nos. 28683 and 28684. [Class 37] A circular trace is obtained on a cathode-ray oscillograph from a reference voltage by means of a phase splitter and is modified by the signal whose phase is to be determined and which is passed through an amplitude-limiting and wavesquaring device so as to give it a steep wave front corresponding to its passage through zero. The sinusoidal reference voltage is supplied through terminals 7, 8, Fig. 1, to an adjustable potential divider 9 so that a sinusoidal input of adjustable amplitude is applied to an isolating amplifier 10. This feeds a bridge having condensers 40, 41 in two opposite arms and ganged variable resistances 39, 42 in the other arms. Outputs for two separate phases in quadrature are taken from a common conductor 14 and conductors 43, 44 and are applied to the grids of triodes 47, 48 of an amplifying and phase-inverting stage 12. The triode 47 has a cathode load 50 equal to the anode load 49. A biassing resistance 51 is also included in the cathode circuit, the grid being connected through a resistance 52 to the junction of the resistances 50, 51. Equal and opposite, sinusoidal voltages are developed at the anode and the junction of the resistances 50, 51 and are applied through blocking condensers 53, 54 to the deflecting plates 2, 4 of a cathode-ray oscillograph 1. The triode 48 is similarly connected and supplies the deflecting plates 3, 5 with voltages equal to but in quadrature with those served to the plates 2, 4 so that a circular trace is produced on the screen 35. The signal whose phase is to be determined is fed through terminals 21, 22 to a potential divider 23, from which a part is tapped off and applied to the control grid of a pentode 57, the anode of which is connected through a condenser 62 and resistance 63 to the anode of a diode 64 and the cathode of a diode 70. The standing potential of the cathode of the diode 64 is determined by a connection to the junction of the resistances 65, 66 of a potential divider 65, 66, 67. The mean or standing potential of the anode of the diode 64 is held lower than the cathode by connection through a resistance 69 to the junction of resistances 66, 67, so that the diode does not conduct until an increase of voltage exceeding the drop in the resistance 66 is applied through a condenser 62 from the anode of the valve 57. The voltage rise of the anode of the diode 64 and cathode of the diode 70 in response to a positive half cycle is precluded from exceeding the drop over the resistance 66 owing to conduction by the diode 64 and the potential drop in the resistance 65. Similarly, the voltage fall of the cathode of the diode 70 and anode of the diode 64 in response to a negative half cycle is precluded from exceeding the drop in the resistance 67. The output at the anode of the diode 64 and cathode of the diode 70 has a steep wave front corresponding with each passage of the signal wave through zero and is passed, either directly or through a second amplitude-limiting or wavesquaring device 27, to a wave-shaping device 30. This comprises a pentode 72, the anode of which is connected to earth through a condenser 78 and resistance 79, the output developed on a conductor 31 taken from their junction being alternate positive and negative pulses of very short duration at every passage through zero of the signal input wave. This output is applied to the grid 33 of the cathoderay tube 1 to control the intensity of the beam. The mean potential of the grid 33 is established by means of a resistance 81 connected between the grid and an external adjustable source of potential. Positive pulses on the grid 33 brighten the trace as shown at 85, Fig. 2, while succeeding negative pulses extinguish the trace as shown at 86. The reference potential is applied also to the terminals 21, 22 and the scale 34 of the oscillograph is rotated until its zero coincides either with the dark spot 86 or bright spot 85. The signal potential is then applied to the terminals 21, 22 and the phase difference read off on the scale. In a modification, the square waveform output of the amplitude limiting device 30 is employed to cause modulation of the amplitude of the quadrature voltages producing the circular trace. This is effected by the interposition of a symmetrical balanced modulating stage. The resulting display is shown in Fig. 4, in which the trace 101 corresponding to one half-cycle is of lesser diameter than the trace 100 corresponding to the other. Owing to the application of alternate positive and negative pulses to the grid, one radial portion of the trace 102 is brightened and the other 103 is suppressed. One or both semicircular traces 100, 101 may be masked so that only the radial parts .102, 103 of the trace are visible. Fig. 5 shows a phasesplitting device which avoids the necessity for manual adjustment with change of frequency and produces from a sinusoidal voltage of any frequency within a wide band a pair of equal sinusoidal voltages in quadrature. This device is substituted for the phase-aplitting device in Fig. 1 by connecting the primary winding 105, Fig. 5, of a transformer 106 between the condenser 38, Fig. 1, and earth so as to be energized by the output of the triode 36. The transformer 106 has two secondary windings 107, 108 connected to two circuits which have a common earthed point 109 forming the neutral output terminal for two phases in quadrature. Two condensers 110, 114 correspond to the condensers 41, 40, Fig. 1, while the anode-cathode paths of two triodes 111, 113 correspond to the resistances 42, 39. The amplitudes of the two output voltages on the conductors 43, 44 are compared and any difference between their amplitudes is employed to correct the resistances of the valves 111, 113. To do this the two voltages are applied through condensers 117, 118 to rectifiers 119, 120 shunted by resistance loads 121, 122 connected in series opposition. The junction of the rectifiers 119, 120 is connected to the common conductor 109 and to the cathodes of the valves 111, 113 through a condenser 123, and the mean or standing potential of the rectifier circuit is determined by a connection of the condenser 118 and rectifier 120 to the common conductor 109 through a resistance 124. The condenser 123 remains uncharged as long as the amplitudes of the voltages applied to the rectifiers 119, 120 are equal, but becomes charged if they are unequal and the potential of the whole circuit varies accordingly. The junction of the rectifier 119 and its load 121 is connected through a resistance 125 to the grids of the valves 111, 113, this connection being shunted to the conductor 109 through a smoothing condenser 126. If there is any inequality in the amplitudes of the sinusoidal voltages developed on the conductors 43, 44, the potential of the grids of the valves 111, 113 is changed through the resistance 125 so as to reduce this inequality to zero and bring the voltages into quadrature. A variable frequency oscillator and a modulator may be employed in conjunction with the reference and signal voltages so that by adjustment of the oscillator frequency the actual measurement of phase may be carried out always at one frequency. In order to obtain a fine measurement of phase angle the input terminals of the reference circuit may be connected through a two-way switch so that a frequency multiplier may be included when required. The frequency of the reference voltage is thus raised to an integral multiple of that of the signal voltage.