Patent GB-632776-A (GB)
Title: Improvements relating to electrical phase responsive circuits
IPC: G01S13/42
Filed: 1944-06-09
Abstract: 632,776. Phase difference measurements. SPERRY GYROSCOPE CO., Inc. June 9, 1944, No. 11125. Convention date, May 25, 1943. [Class 37] [Also in Group XL (c)] Phase detecting apparatus comprises a pair of opposed rectifiers serially connected with a voltage divider between them across one pair of input terminals, the other pair of input terminals being connected respectively to the mid-point of the above - mentioned voltage divider and to the mid-point of a further voltage divider connected across the first pair of input terminals. The invention is described as applied to a micro-wave pulse type of radio locator wherein a radiator 11, Fig. 1, is driven by a motor 15 to produce a conical scan of the transmitted pulses. The echo pulses from a target 12 are modulated at the frequency of rotation of the radiator 11 in such a way that the phase of the modulation envelope varies according to the orientation of the target relative to the scanner axis 21. The modulation envelope is compared as to phase with two reference voltages derived from a two-phase generator 16, rotating synchronously with the motor 15, in phase detectors 24, 25. The two reference voltages, of the same frequency as the modulation envelope, are in quadrature, and the output voltages from the phase detectors 24, 25 which are a measure of the respective phase displacements, are applied to cathode-ray oscillographs 22, 23 which indicate the orientation of the target 12 in azimuth and elevation respectively. Alternatively, the outputs from both phase detectors may actuate a single indicator 28 of the rectangular co-ordinate type which indicates both azimuth and elevation. The phase detectors 24, 25 each comprise a pulse lengthening stage 32, Fig. 3, phase detector stage 34, a D.C. amplifier stage 35 and cathode-ray indicator 36. The received modulated pulses are applied through input terminals 14 to a cathode follower 32. The time constant of the cathode resistor 45 and condenser 46 combination is chosen so as to lengthen the duration of the pulses which are applied across a high impedance 113 in series with blocking condensers 99, 114. The reference voltage derived from winding 116 of the two-phase generator 16 is applied across series-connected equal resistors 117, 118 whose junction 119 is connected to the high potential end of choke 113, and also across rectifiers 126, 127 connected in opposition and in series with variable resistors 128, 129 and load resistors 131, 132. These load resistors and their byepass condensers 133, 134 are at their, junction terminal 135 connected to the low potential end of the choke 113. The time constants of the resistance-capacitance circuits 126, 128, 133 and 127, 129, 134 are relatively small to permit substantially instantaneous charging of condensers 133, 134. The time constants of circuits 133, 131 and 134, 132 are, however, large in order to maintain the charge on condensers 133, 134 between successive pulses and between successive half-cycles of the reference voltage, i.e. scanner frequency. Thus, with zero input from stage 32 the rectifiers 126, 127 serve as half-wave rectifiers for the equal scan frequency reference voltages applied thereto from resistors 117, 118. The condensers 133, 134 are thereby charged equally but oppositely so that the resultant output voltage across the series-connected load resistors 131, 132 at terminals 37, 38 is zero. Pulses applied from stage 32 across choke 113 will affect the two rectifier circuits differentially so that a D.C. output voltage is produced across terminals 37, 38 corresponding to the phase displacement of the pulse modulation envelope relative to the reference voltage. Filter networks 136 ... 139 remove scan frequency and higher frequency ripple-components after which the output voltage is applied to the push-pull D.C. amplifier 35. The output voltage across load resistors 146, 147 is further filtered at 154, 156, 157 and applied through coupling resistors to the deflecting plates 161 of the cathode-ray tube indicator 36. Superposed upon this D.C. voltage is a suitable alternating voltage derived from source 162. The other deflecting plates 167 of the indicator 36 are connected to a similar source of alternating voltage 1621 of the same frequency and in quadrature with that of source 162 to produce in known manner a circular beam trace of the spot which is deflected in one sense or the other according to the D.C. output from the phase detector. The azimuth and elevation indicators are both operated as above, the only difference being that the reference voltages for the indicators are obtained from the respective windings 115, 116 of the two-phase generator 16 and are in quadrature. In a modification, an adjustable buffer amplifier stage 33, Fig. 4, is interposed between the pulse lengthening stage 32<1> and the phase detector to ensure sufficient charging current for condensers 133, 134. A grid resistor 112 prevents overdriving of the stage 33 and serves to round off the pulse peaks from stage 32<1>. The reference voltage is adjustable by a potentiometer 178 to a value substantially equal to that of the pulse envelope and is applied through a transformer 171 to the opposed rectifiers 126, 127. The pulse input is applied across the junction 120 of the rectifier load resistors 131, 132 which is connected to the centre tap of the secondary of transformer 171 and a point 170 connected through condensers 168, 169 to the secondary ends. A double stage filter 174 is preferably connected between the phase detector output 37, 38 and the D.C. push-pull amplifier 35<1>, the output of which is applied to the deflecting plates 161 of the indicator. This indicator is of the co-ordinate type indicating both target azimuth and elevation so that in this case the output of the other similar phase detector 25 is connected to the plates 167. A cathode follower stage may be interposed between the buffer amplifier 33 and the phase detector stage, Fig. 4A (not shown).