Patent GB-952808-A (GB)
Title: A circuit for measuring the phase angle between two signals of like frequency
IPC: G01R25/00
Filed: 1962-06-01
Abstract: 952,808. Electric selective signalling systems. ALBISWERK ZURICH A. G. June 1, 1962 [June 1, 1961], No. 21147/62. Heading G4H. In an apparatus for measuring the phase difference between two signals S 1  and S 2  of the same frequency in which a measuring gate 15, opened by a bi-stable circuit 13, passes clock pulses from a generator 17 to a counter 16 during a period corresponding to the phase difference and automatically repeats this measurement say 100 times, the bi-stable circuit is arranged to delay its change of state for a whole cycle of the signals if the change occurs almost immediately following the previous change. Signals S 2 , S limited at 1, 2 amplified at 3, 4 differentiated at 5, 6 are fed to bi-stable trigger circuits 7, 8 respectively. One of two rectangular voltage outputs from each trigger is differentiated at 9,10 rectified at 11, 12 and enter the bi-stable circuit 13, the voltage from the rectifier 12 passing through an AND gate 14. The signals S 1  bring up an output E of the bi-stable circuit 13 to open the measuring gate, while signals S 2  bring down this circuit to close the gate and bring up an output F. The measuring gate passes pulses, from a rectangular pulse generator, differentiated at 18 and rectified at 19 to an electronic counter 16. Initially a bi-stable trigger circuit 20 has one of its outputs up to enable an AND gate 36 in a phase comparator P and its other output down thus blocking the AND gate 14. The comparator P determines from the four outputs of the triggers 7, 8 whether the phase difference is in a small predetermined range e.g. <SP>+</SP> or <SP>-</SP># where # is a few degrees and gives a positive output at D if it is. An AND gate 40 passes a rectangular voltage for the period corresponding to the phase difference while an OR gate 41 passes a current during the remainder of the cycle. A pulse generated at an interval r after the signal S1 is combined with each of these outputs in AND gates 42,43 respectively. If the phase difference between the two signals is less than r the output of the AND gate 40 occurs before the pulse thus AND gate 42 is not actuated while an AND gate 43 is enabled switching a bi-stable circuit 44 to bring its output at D up. When a start switch 21 is pressed the input to the AND gate 36 from the trigger 20 is dropped and the output from the comparator P at D is fixed according to the previous cycle of the signals S 1 , S 2 . If there is no output at D the bi-stable circuit 13 acts as a trigger circuit formed by transistors T 1 , T 5  (Fig. 2). A frequency divider 26 passes a pulse to the bi-stable circuit 20 after the gate 15 has been opened a number say 100 of times thus blocking the AND gate 14 to prevent further operation of the gate 15 and re-enabling the comparator P. If the input to the circuit 13 at D is positive then with transistor T 1  non conducting and transistor T 5  conducting under the influence of an input at B from the previous cycle of S 2 , an input at A due to the next cycle of S 1  passes a rectifer G2 to the base of the transistor T5. This renders the transistor T5 non conducting thus dropping its output which was blocking the transistor T 1  through a capacitor C 1  and resistance R 1 . It also has the effect of temporarily rendering the transistor T 2  conducting, a capacitor C 3  being chosen to make the duration of this effect correspond to the period #. The blocking of the transistor T5 by an input at A thus causes the output at F to be dropped and an output at E to be produced. As the transistor T 2  conducts for a period # thus maintaining the transistor T 5  blocked an input at B during this period can have no effect on the system and the circuit maintains an output at E until the next input from S 2  at B. A transistor T 4  similarly ensures that the circuit does not switch back within the period # after bringing its F output up. The transistor T 3  corresponds to the AND gate 14. If for example the counter 16 works on the decimal system then after 100 measurements the digits in the third order and above of the counter will give the average number of pulses passed through the gate 15 per measurement.