Patent GB-1076768-A (GB)
Title: Modulation crossover selector
IPC: H03M1/00
Filed: 1964-10-02
Abstract: 1,076,768. Radio navigation. STANDARD TELEPHONES & CABLES Ltd. Oct. 2, 1964 [Oct. 4, 1963], No. 40200/64. Heading H4D. In a radio bearing navigation system of the Tacan type, wherein the bearing information is obtained in the form of the average phase difference between received reference pulse points (e.g.0 or 180 degree points) at which the modulation of the received signal fall to zero (hereinafter termed " crossover points "), and wherein said phase differences are measured in digital fashion, the count starting from a reference pulse and having a discontinuous return to zero at the next reference pulse, 180 degrees bearing errors, caused by jitter of the modulation signal moving the measured crossover point between consecutive measurements from one side of a count discontinuity to the other side, are avoided by means of a circuit which determines whether the 0 degree crossover points or the 180 degree crossover points are to be used for the measurement, by choosing that crossover point which first occurs after a pulse delayed from a reference pulse by the maximum expected amount of jitter. In Fig. 1, the 0 degree crossover point signals (B), Fig. 2 (not shown), are fed on line 1 to inhibited AND gate 20 and to AND gate 16, and the 180 degree crossover point signals (C) are fed on line 2 to inhibited AND gate 21 and to AND gate 15. Reference pulses (A) are fed on line 4 to the reset input of a bi-stable circuit 5. To start the arrangement a start pulse, coincident with every eighth reference pulse is fed on line 3 to set circuit 5, such that a signal (E) is fed to AND gates 10 and 11. The start pulse is also fed to set bi-stable circuit 6, such that clock pulses from generator 7 start to pass through gate 9 into counter 8. When counter 8 registers a count indicative of said maximum amount of jitter (e.g. 80 degrees) a pulse (D) is produced by the counter which is fed to AND gate 10, such that the gate produces a pulse. The crossover pulses occurring before pulse (D) cannot pass through gates 15 or 16, since bi-stable circuit 12 is then in the reset state and does not send any pulses to the AND gates. The pulse from gate 10 sets circuit 12 such that gates 15 and 16 are opened and gate 11 is inhibited, which in turn removes the inhibiting input from gates 20 and 21. The next crossover pulse to occur is a 180 degree crossover and this passes through gate 15 to the set input of bi-stable circuit 25 and to OR circuit 13. The setting of circuit 25 causes its right-hand half to produce a pulse which is fed to the middle terminal of three, to indicate that a 180 degree crossover has been chosen, and to an input of the now uninhibited AND gate 20. The pulse from OR circuit 13 is applied via OR circuit 22 to the upper crossover output terminal, and also resets counter 8 and circuits 6 and 12. The resetting of circuit 12 removes the inhibiting input from gate 11 such that when the next reference pulse resets circuit 5, gate 11 ceases to give an inhibiting output to gates 20 and 21. The resetting of circuit 12 also closes gates 15 and 16. The following 180 degree crossover pulse thus passes through gate 21 to the upper crossover output terminal, via OR circuit 22.