Patent GB-978506-A (GB)
Title: Improvements in receiver for phase shift keyed signals
IPC: H04L27/227
Filed: 1961-03-21
Abstract: 978,506. Telegraphy. ROBERTSHAW CONTROLS CO. March 21, 1961 [April 8, 1960], No. 10338/61. Heading H4P. A receiver for phase-shift signals comprises means for deriving an output pulse at each transition from one to the other polarity of the input signal, these pulses being fed to a plurality of " and " gates, each providing output pulses corresponding to the transitions occurring in portions of the inputsignal of a given respective phase. A gating generator applies the second gating pulse signal to each " and " gate, and is synchronized by the output of said gates. As shown, the intelligence signal B, Fig. 1, modulates the carrier A by shifting its phase 120 degrees. The phase-shift keyed signal C is applied to a limiter-amplifier 1, Fig. 2, the square wave output D of which is differentiated at 2, to give waveform E. The negative-going portions F, Fig. 1, derived from clipper 3 are applied to " and " gates 4, 5. Gating signals L, M, Fig. 1, derived from a gating generator 6 (see below) are also applied to gates 4, 5 so that respective outputs G, H are obtained from these gates, corresponding in time to the 0 degrees and 120 degrees phase portions of the keyed signal. These outputs G, H trigger a bi-stable multivibrator 11 to provide a replica O of the intelligence signal B. The output G of gate 4 is delayed by one-third of the carrier period in circuit 7, the output of which I, Fig. 1, is applied to an " or " gate 8 together with the output of gate 5. The " or " gate output J is a train of equi-spaced pulses at the tarrier frequency. These pulses lock an oscillator 9 to the frequency and phase of the carrier. Oscillator 9 drives an oscillator 10 tuned to three times the carrier frequency producing an output K controlling the gating generator 6, which produces a series of equally spaced negative pulses L at carrier frequency, and a second similar output M of the same frequency but phase displaced - 120 degrees. These pulses are one-third of the period of oscillator 10 in duration and are centred respectively on the positive to negative transitions of the respective 0 degrees and 120 degrees phase portions of the keyed signal to permit a maximum amount of pulse position displacement due to noise &c. When synchronization is lost, one or other gate will give no output, and oscillator 9 will drift, in the absence of synchronizing pulses, until synchronism is automatically re-established. Oscillator 9 is tuned to a frequency slightly below the driving frequency for this purpose and can be tuned to give resynchronization within a specified time. The delay circuit 7 and one oscillator may be dispensed with, the output of gates 4, 5 being fed as locking pulses to a single oscillator tuned to three times the carrier frequency for controlling the gating generator. Instead of using a 120 degrees phase shift system, phase positions corresponding, for example, to shifts of 0 degrees, 72 degrees, 144 degrees, 216 degrees may be transmitted, in which case three delay circuits (for the 72 degrees, 144 degrees, 216 degrees positions) will be required with delays of <SP>1</SP>/ 5 , <SP>2</SP>/ 5 , <SP>3</SP>/ 5  of the carrier period, respectively. The " or " gate will then have four inputs and the second oscillator will be tuned to 5 times the carrier frequency. Four gating signals will be developed and fed to four " and " gates.