Patent GB-1012131-A (GB)
Title: Improvements in transmitter and receiver for phase modulated signals
IPC: H04L27/20
Filed: 1961-11-13
Abstract: 1,012,131. Phase shift systems. ROBERTSHAW CONTROL CO. Nov. 13, 1961 [Nov. 17, 1960], No. 40536/61. Heading H4P. In a phase-shift communication system for transmitting an even number n of different items of information a carrier oscillation having n + 1 evenly spaced phase angles is used, and each item of information is transmitted by selection of a phase angle differing from the next preceding phase angle by an angle representative of the item. A logic circuit effects the phase angle selection at the transmitter and a similar logic circuit is used at the receiver for demodulation. Figs. 1, 2 show, respectively, the transmitter and receiver of a system for transmitting binary signals, e.g. " mark " and " space " signals from a teletypewriter. These signals are fed over circuits 25, 26, 27 and separate " space " and " mark " lines 2, 3 to the logic circuit 1. The carrier oscillator 17 provides three separate signals at phase angles 0 degree, 120 degrees, 240 degrees which are passed to the output 24, selectively, by " and " gates 20, 21, 22 controlled by respective signal generator elements 16, 14, 15. Transmitter operation, Fig. 1. Assuming that generator element 16 is " on " when transmission starts, then gate 20 will be open and a carrier signal of 0 degree phase fed to the output for transmission. If a " space " signal is now applied to line 2, gate 4 opens (its second input being derived from the " on " generator element 16). The gate 4 output via " or " gate 10 switches on generator element 14 which turns off element 16 and opens gate 21 (as 20 closes) causing an output signal at 24 of 120 degrees phase. The next signal input, a " mark " over line 3, operates " and " gate 9 which via " or " gate 12, operates generator element 16 to shift the phase of the output 24 to 0 degree. Thus a " space " signal shifts the phase of the carrier forward by 120 degrees and a " mark " signal shifts it forward by 240 degrees and with the logic circuit shown this always happens regardless of which gate 20, 21, 22 is operative when the first " mark " or " space " signal arrives over lines 2, 3. The " phases " transmitted are thus any two of the three possible phases. Receiver, Fig. 2. The received phase shift modulated signals are applied to a limiter 28, the square wave output of which is differentiated at 29. The negative or positive outgoing pulses at 29 are used to synchronize a local oscillator 35 tuned to three times the carrier frequency. These pulses are also fed to " and " gates 31, 32, 33 one for each " phase." The second inputs to these gates are derived from a signal generator 34 having output circuits A, B, C energized sequentially from the oscillator 35 so that the outputs of the gates 31-33 correspond, respectively, to the three possible phase positions transmitted. The phase correspondence may vary, however, dependent on the initial phase relationship established. The outputs of gates 31-33 are fed through circuits 36, 37, 38 &c. which apply corresponding pulses to inputs 46, 47, 48 of a logic circuit 45. Logic circuit 45; operation. Assume that the 0 degree and 240 degrees phase portions are the two being transmitted and that output circuit A of generator 34 is in phase with the 0 degree phase portions. Then if the last phase portion received was a 0 degree portion, a pulse would have been fed over input 46 to delay circuit 55, which, after a time sufficient for the input at 46 to produce an appropriate output at 64, turns on its associated signal generator element 59. This turns off the previously operative signal generator element 58 or 60 and supplies one input to " and " gates 52, 53. The next input signal will be of 240 degree phase, and applied to point 48, will operate " and " gate 53 the output of which via " or " gate 62 triggers output circuit 64 to one of its two levels representative of a 240 degree phase shift. The input at 48 is also applied to delay circuit 57 which will turn on generator element 58 (and generator 59 off) to supply one input to gates 50, 51. The next signal input will be over 46 and consist of a 0 degree phase portion and will operate gate 50 the output of which, will, via " or " gate 63, trigger output 64 to its other level representative of a 120 degree phase shift. The logic circuit 45 ensures that the output circuit 64 responds in this way to the amount of phase shift regardless of the phase relationships initially established (see above). Should this phase relationship, initially established between the signal generator elements A, B, C and the phase portions of the received carrier, alter during reception, only one bit of the transmitted signal is passed incorrectly to the output 64 before the circuit 45 resumes correct operation. A similar system for transmitting the outputs of two synchronously operated teletype signal generators, i.e. where n=4, is described, Figs. 3-6 (not shown).