A COHERENT CARRIER TECHNIQUE FOR SELF-TIMED DIGITAL REGENERATIVE REPEATERS a thesis submitted for the degree Doctor of Philosophy at The University of New South Wales by William Carroll, B.E., M.I.R.E.E. October 1970 UNIVERSITY OF N.S.W. 02056 29.JUN.71 LIBRARY PREFACE This thesis is primarily an experimental investigation concerned with the fundamental problem in long distance digital communication systems of maintaining synchronisation of the digital data after its passage through many hundreds of regenerative repeaters and dispersive transmission lines. Existing self-timed regenerative repeaters derive the timing wave from the base band digital data and are subject to systematic timing jitter ac­ cumulation because of their data pattern dependency; hence communication systems employing these techniques are limited to contain a maximum of 100-300 repeaters. By utilizing a simple technique of phase-locking the data pulse repetition frequency to the carrier frequency a new type of self-timed digital regenerative repeater has been designed, implemented and tested which derives a pattern insensitive timing wave from the carrier frequency thus preventing the occurrence of the systematic timing jitter accumulation. Also the spectral purity of the timing wave frequency impulse function can be controlled by appropriate filtering in each regenerator. This reduces the noise power in the vicinity of the carrier frequency by an amount which produces an overall convergent signal-to-noise ratio for the system of such a value that the average number of zero crossings of the carrier frequency remains constant. This allows synchronisation to be maintained in extremely long systems con­ taining large numbers of repeaters.. Wherever possible suitable analytic material is presented so that the experimental results may be appropriately evaluated, with emphasis being placed upon prediction and performance analysis from measurements rather than from pure mathematical models. To serve this purpose a simulation network was constructed and the manner in which its performance differs from a real system is detailed. Envelope Bi-stable Detector Filter - Self - Adapt Limiter SampleTime Network Modulator Regenerated AM Output FIGURE 1. Block diagram of the Coherent Carrier Self-timed Regenerative Repeater Front View, all test points shown connected. Rear View, cover panels removed. FIGURE 2. Front and Rear Views of the racks containing the Test/Simulator Control Equipment and the two Regenerators with the Delay Line Network ACKNOWLEDGEMENTS I am indebted to Professor A.E. Karbowiak for his introduction to the problem area and criticisms during the investigation. Discussion with other members of staff, particularly Mr. G.T. Poulton, and fellow research students is also recognised as being essential to the progress and success of the project. Professors A.E. Karbowiak and M.W. Allen, Associate Professor R.M. Huey together with Dr. E.H. Fooks are thanked for their recommendations which enabled me, through my employer, the Postmaster-General's Department, to obtain sufficient study leave from the Commonwealth Public Service Board to conduct the investigation. LIST OF PRINCIPAL SYMBOLS a = Any odd number greater than 2. A = Arbitrary constant. A = Amplifier voltage gain. A.G.C. = Automatic Gain Control. A.P.C. = Automatic Phase Control. B = Communication channel half bandwidth. c = Timing filter half bandwidth. = Effective bandwidth of cascaded filter networks. = Jitter cumulation factor. = Synchronisation code group. = Carrier frequency. = Digital data transmission speed. = Combined noise factor of limiter. = Noise factor of first limiter stage. = Noise factor of second limiter stage. = Power gain of first limiter stage. G = Filter conductance. H(s) = Timing filter transfer function. K = Reciprocal of filter inductance (I/L). m = Modified noise reduction ratio. M = Positive integer equal to number of carrier cycles per data pulse. N r = Number of repeaters. = r.m.s. random jitter accumulation rate. = r.m.s. systematic jitter accumulation rate. r N o = Average number of zero crossings of sine wave plus noise per second. NRZ = Non-return to zero digital data format. ON/OFF - Signal peak-to-carrier leak ratio. P = Average number of time slots containing pulses P = Positive integer. PCM = Pulse Code Modulation. PST = Paired Selected Ternary code. q = Effective noise increase due to mistuning. Q = Timing filter Q-factor. r = Noise reduction ratio. = Maximum number of early/late section data bits SM SASTN = Self Adaptive Sample Time Network. T = Total loop delay time (measured). D T Transducer Gain. g = v . = r.m.s. noise input voltage. ni V r.m.s. noise output voltage. no = Vi(s) = Regenerator input signal voltage. V (s) Regenerator input noise voltage. n = V (s) Regenerator output signal plus noise voltage. o = = Input noise spectral power density. a = Standard deviation for noise spectrum. Factor by which r.m.s. timing deviation at nth repeater Yn = than that at first repeater. T = Total loop delay time (theoretical). 03 = Filter center frequency. O 0) = Carrier frequency. c l = Signal loss due to mistuning. CONTENTS Chapter Page 1 INTRODUCTION 1.1 Area of Research........................................... 1 1.2 Arrangement of Thesis......................... 2 1.3 Originality and Significance of Thesis .................. 3 2 LITERATURE SURVEY 2.1 Introduction ............................................... 6 2.2 Self-Timed Digital Regeneration .......................... 6 2.3 Simulation Networks .................................. .. 22 2.4 Conclusions ................................................. 39 3 SYSTEM DESCRIPTION 3.1 Introduction............................................... 40 3.2 The Coherent Carrier Self-Timed Digital Regenerative Repeater 40 3.2.1 Design Criterion .................................. 42 3.2.2 Functional Description .............................. 43 3.2.3 System Synchronisation .......................... .. 56 3.3 The Simulation Network .................................. 60 3.3.1 Design Criterion .................................. 60 3.3.2 Control and Test Functions............................. 68 3.4 Conclusions ............................................... 4 THEORETICAL CONSIDERATIONS 4.1 Introduction................................................. 72 4.2 Timing Filter Characteristics ............................... 73 4.2.1 Filter Transfer Function .......................... 74 4.2.2 Noise Reduction Ratio r................................. 76 Chapter Page 4.3 Signal-to-Noise Analysis ....................................... 79 4.3.1 Cascaded Repeater System .............................. 79 4.3.2 Noise Convergency Criterion .......................... 80 4.4 Number of Zero Crossings of a Sine Wave plus Narrow Band Gaussian Noise ............................................... 82 4.5 Signal-to-Noise Ratios in Band Pass Limiters.................... 86 4.6 Mistuning ...................................................... 87 4.6.1 Modified Noise Reduction Ratio m ...................... 87 4.6.2 Frequency Spectra ...................................... 90 4.7 Simulation Loop Transfer Function .............................. 92 4.8 Conclusions ..................................................... 99 5 EXPERIMENTAL RESULTS 5.1 Introduction.................................................... 102 5.2 Simulation Network Characteristics .......................... 102 5.2.1 Natural Response ...................................... 102 5.2.2 Forced Response ........................................108 (a) Alignment 108 (b) Synchronisation ................................... 112 (c) Implementation ................................... 115 5.2.3 Loop-Induced Spectral Characteristics .................. 118 5.3 Timing Filter Characteristics ............. 121 5.3.1 Noise Reduction Ratio r ............................... 121 5.3.2 Synchronisation Level......................... .. .. 123 5.3.3 Limiter Characteristics .............................. 124 5.3.4 Noise Convergency ....................................... 129 5.4 Timing Jitter Characteristics .................................. 132 5.4.1 Pulse Pattern Dependency .............................. 134 Chapter Page 5.4.2 Number of Regenerations....................................137 5.4.3 Signal-to-Noise Ratio .................................. 138 5.5 Error Rate......................................................... 139 5.5.1 Pulse Pattern Dependency .................................. 142 5.5.2 Number of Regenerations....................................145 5.5.3 Signal-to-Noise Ratio .................................. 146 5.6 Mistuning ........................................................ 150 5.6.1 Timing Jitter ............................................ 150 5.6.2 Error Rate ................................................ 151 5.7 Phase Discontinuities ........................................... 154 5.8 Conclusions........................................................ 156 6 CONCLUSIONS AND RECOMMENDATIONS ...................................... 158 BIBLIOGRAPHY ................................................................ 165 APPENDIX 1 170 2 172 3 175 PUBLICATIONS 181 1 CHAPTER 1 INTRODUCTION 1.1 Area of Research The most important advantage of transmitting information in digital- pulse form results from the fact that these pulses can be regenerated each time the information passes through a repeater. The function of each re­ generative repeater is to remove undesired amplitude effects