Patent GB-1010944-A (GB)
Title: Phase demodulation
IPC: H04L27/227
Filed: 1961-10-25
Abstract: 1,010,944. Phase shift radio signalling; automatic phase control. ROBERTSHAW CONTROLS CO. Oct. 25, 1961 [Oct. 25, 1960], No. 38252/61. Headings H3A and H4P. Relates to a synchronizing arrangement in a receiver for a communication system in which the information to be transmitted is impressed upon a carrier signal of a given frequency by effecting corresponding predetermined phase shifts in the carrier signal. As shown in Fig. 1 the carrier A is modulated by the information signal B to provide the transmitted signal C in which the carrier has a phase shift of 0 degrees or 240 degrees, i.e. -120 degrees. A further possible phase position of 120 degrees is not used for signalling but is reserved for the detection of incorrect synchronising at the receiver. At the receiver, Fig. 2, the signal C is supplied to a limiter amplifier 1 providing an output D which is differentiated at 2, Fig, 1E, clipped at 3, Fig. IF, and supplied to AND gates 4, 5, one for each phase portion transmitted, and to a further AND gate 6. A gating signal generator 7 supplies pulses G of one-third the period of the carrier to gate 4 at the same time that pulses F corresponding in time to the polarity transitions of the 0 degrees phase portions occur, pulses H to gate 5 when pulses F corresponding to the - 120 degree portions occur, and pulses I to gate 6 that would cause gate 6 to pass pulses corresponding to 120 degrees phase portions were they present. The output J and K from gates 4 and 5, respectively, are supplied to an output circuit 19 producing the demodulated signal 0, no output being provided by gate 6 if the system is operating correctly. The circuit 19 may include a bi-stable circuit or may include a pair of one-shot multivibrators respectively triggered by gates 4, 5. A signal generator 8 providing a signal L of three times the carrier frequency controls the gating signal generator 7 and since the pulses F are displaced relative to any one pulse at points which are whole multiples of <SP>1</SP>/ 3  of the period of the carrier signal they are used for synchronizing the oscillator 11. Both oscillators 11 and 12 provide signals at three times the carrier frequency which are supplied to a phase detector 13 which via reactance valves 14, 15, controls the frequency of both oscillators. Should synchroniza tion be lost one or other of gates 4, 5 will provide no output and an output signal will appear or gate 6. An output from gate 6 is applied via ar integrator-rectifier 9 to operate a one-shot multi vibrator 10 and each time a pulse is provided from 10 the next element in the ring counter 7 is made conductive until synchronism is restored By varying the time constant of the integrator 9 the delay in operation of the multivibrator 10 may be adjusted to prevent operation by spurious pulses. In a modification, Fig. 3 (not shown), the connection between the clipper 3 and oscillator 11 is dispensed with and the outputs of gates 4, 5, 6 are combined so as to provide an uninterrupted train of equally spaced pulses N for synchronizing the signal generator 8 whenever a signal is received, the output of gate 4 being delayed as at M and combined with the pulses K from gate 5. Any output from gate 6 is also suitable delayed so that a continuous train of synchronizing pulses is formed. Automatic phase control. Fig. 10 shows details of the control signal generator 8. The oscillator 11 consists of a conventional oscillator circuit 114, 120 controlled by a conventional reactance valve 14, the synchronizing signal being supplied to the control grid of valve 114. The oscillator 12 is a conventional circuit 131, 137, also controlled by a conventional reactance valve 15. The outputs of the two oscillators are supplied in opposite phase via respective resistors 143, 144 to the phase detector 13, any difference signal being amplified at 145, rectified at 146, 147, and supplied via a cathodefollower 150 to the reactance valves 14, 15.