CALLUM Linear Transmitter - Architecture and Circuit Analysis Strandberg, Roland 2004 Link to publication Citation for published version (APA): Strandberg, R. (2004). CALLUM Linear Transmitter - Architecture and Circuit Analysis. Department of Electroscience, Lund University. 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LUND UNIVERSITY PO Box 117 221 00 Lund +46 46-222 00 00 CALLUM Linear Transmitter Architecture and Circuit Analysis Roland Strandberg Lund 2004 Department of Electroscience Lund University P.O. Box 118 SE-221 00 LUND SWEDEN No. 50 ISSN 1402-8662 c Roland Strandberg 2004. Produced using LATEX Documentation System. Printed in Sweden by Tryckeriet i E-huset, Lund. December 2004. Abstract This doctoral dissertation presents a study of linear radio transmitters based on the combined analog locked loop universal modulator (CALLUM) approach. Linear ar- chitectures such as CALLUM are very attractive for power-efficient operations, since they have no fundamental limitations prohibiting a 100 % efficiency for all envelope levels, without sacrificing the potential of a linear transmission. This issue is becom- ing increasingly important in modern communication standards, where the informa- tion content is present not only in the signal phase, but in its amplitude as well. Such modulation schemes, while improving the data rate for a given signal bandwidth, pose tough demands on the linearity of the transmitter. As the power amplifier (PA) in the transmitter handles the largest signals and is the main power consumer in the radio, sufficient linearity should be achieved in conjunction with a high power efficiency of the PA, especially if the equipment is battery operated, as is the in case of mobile applications. In this work, three different CALLUM architectures (i.e., CALLUM 1, CAL- LUM 1lin, and CALLUM 2) are studied in terms of loop gain, bandwidth, stability, and frequency compensation. A simplified baseband model of a general CALLUM is presented for efficient simulation of the system, and to gain knowledge about per- formance differences between the CALLUM derivatives. The investigated CALLUM architectures make use of Cartesian feedback, as it provides better matching between the I and Q signal paths than polar feedback. From baseband simulations with different signal component generator (SCG) im- plementations, the spectral performance of CALLUM 1 and CALLUM 1lin for an EDGE modulated signal is significantly better than that of CALLUM 2, for a given maximal loop gain. It can be concluded that the radical simplifications leading to CALLUM 2 have severe effects on the spectral properties of the output signal, while the actual implementation of the SCG becomes much simpler than for example CAL- LUM 1. The effect of propagation delay in the feedback loop is also included in the model, and indeed this delay appears to be the limiting factor in the achievable closed- loop signal bandwidth. A lag-lead frequency compensation network is used to trade bandwidth for increased insensitivity to time delays. The frequency compensation is quite efficient for all CALLUM versions studied when they operate on a 3π/8-shifted 8PSK, as acceptable delays may in this case become much larger without jeopardiz- ing the stability. It is worth noting that CALLUM 1lin performs very well in terms of maximum acceptable time delay for a certain standard. Implementations of the circuits for the CALLUM 2 architecture are presented to- gether with simulation results of the fundamental blocks. A differential analog SCG realizing the control equations for CALLUM 2, and a variable-gain amplifier (VGA) were simulated together with other functional blocks to form a complete baseband- modeled CALLUM 2 transmitter. From the simulated spectral performances, the SCG and VGA implementations proved to be appropriate for an EDGE-modulated signal. iii Contents Abstract iii Contents v Preface xi Acknowledgments xv List of Acronyms xvii List of Symbols xix Linear Transmitters 1 1 Introduction .............................. 1 1.1 Motivation . ........................... 1 1.2 Communications in the Mirror ................. 2 1.3 Electronic Signal Processing . ................. 3 1.4 Signals . ........................... 4 1.4.1 Functions . ...................... 4 1.5 Communication Standards . ................. 4 1.5.1 Terrestrial Trunked Radio, TETRA . ........ 4 1.5.2 Enhanced Data Rates for GSM Evolution, EDGE . 6 1.5.3 Wideband Code-Division Multiple-Access, W-CDMA . 9 2 Linear Amplification Techniques ................... 11 2.1 Overview of Linearization Techniques ............. 11 2.2 Predistortion ........................... 13 2.2.1 Analog Predistortion . ................. 13 2.2.2 Digital Predistortion . ................. 15 2.2.3 Adaptation of the Predistortion Circuit . ........ 16 2.3 Feedforward ........................... 17 2.4 Feedback . ........................... 18 2.4.1 The Asymptotic-Gain Model . ............. 19 2.4.2 Direct and Modulation Feedback ............ 21 2.4.3 Cartesian Modulation Feedback ............. 21 2.4.4 Phase Adjustment . ................. 22 2.4.5 Polar Modulation Feedback . ............. 23 2.5 Linear Transmitter Architectures . ............. 24 2.5.1 Envelope Elimination and Restoration . ........ 24 2.5.2 Linear Amplification with Nonlinear Components . 25 v vi Contents 2.5.3 Combined Analog Locked Loop Universal Modulator . 28 3 Linear Amplification with Nonlinear Components .......... 31 3.1 LINC and the Signal Decomposition . ............ 31 3.2 Signal Component Separator . ................. 32 3.2.1 Phase-Modulation Method ................ 32 3.2.2 In-Phase and Quadrature-Phase Method . ....... 34 3.2.3 Spectral Regrowth . ................. 34 3.3 Efficiency Issues . ...................... 35 3.3.1 Power Combining . ................. 37 3.3.2 Efficiency Enhancement by Supply Voltage Adjustment 41 4 The Combined Analog Locked Loop Universal Modulator ..... 43 4.1 CALLUM . ........................... 43 4.2 The Signal Component Generator ................ 44 4.3 Approximation of the Control Signal . ............ 48 4.3.1 Control Equations for CALLUM 1 ........... 48 4.3.2 CALLUM 1 with Linearized Denominator ....... 51 4.3.3 CALLUM 2 . ...................... 52 4.3.4 CALLUM 3 . ...................... 52 4.3.5 Vector-Locked Loop . ................. 53 4.4 Baseband Modeling of CALLUM ................ 56 4.5 Loop Properties Using Linearized Control Equations . 57 4.5.1 Signal Representation . ................. 58 4.5.2 Loop Gain Calculations Based on Linearized Equations 59 4.5.3 Linearization of the Control Equations . ....... 62 4.5.4 Loop Gain Characteristics ................ 63 4.5.5 Comparison of Loop Gain Characteristics . ....... 66 4.5.6 Frequency Dependency of the Loop Gain . ....... 69 4.6 The Asymptotic-Gain Model Applied on CALLUM ...... 70 4.7 Bandwidth Estimation of a CALLUM Transmitter ....... 72 4.8 Frequency Compensation of the CALLUM System ...... 74 4.8.1 Simulated Frequency Response for CALLUM 2 . 75 4.8.2 Phantom Zero Compensation . ............ 77 4.9 Loop Properties with Delay . ................. 80 4.9.1 Non-Minimum-Phase System . ............ 82 4.9.2 Generalization of Time Delay . ............ 84 4.9.3 Bandwidth Limitation Due to Time Delay . ....... 85 4.9.4 Lag-Lead Compensation ................. 86 4.10 Estimation of Dead Time in the Loop . ............ 88 4.10.1 Delay from Wave Propagation . ............ 88 4.10.2 Delay from the Charge Transport Mechanism ...... 88 4.11 Spectrum Widening of Internal Signals in CALLUM ...... 90 4.12 Spectral Performance ...................... 92 Contents vii 4.12.1 CALLUM and Spectrum Emission Mask for EDGE . 92 4.12.2 CALLUM and Spectrum Emission Mask for W-CDMA 94 4.13 Accounting for Time Delay in the Loop ............ 95 4.13.1 Acceptable Loop Delay in CALLUM . ........ 96 4.13.2 Loop Bandwidth Reduction . ............. 97 4.13.3 Acceptable Loop Delay after Compensation . 98 4.14 Comparison of CALLUM Derivatives ............. 99 5 Implementation Aspects of CALLUM ................ 103 5.1 Circuit Orientated CALLUM Implementation . ........ 103 5.2 Signal Component Generator . ................. 105 5.2.1 Summation . ...................... 106 5.2.2 Differential Analog SCG Implementation of CALLUM 2 107 5.3 The Variable Gain Amplifier . ................. 110 5.3.1 Variable Gain Techniques . ............. 110 5.3.2 VGA with Common Mode Control . ........ 111 5.4 The Voltage Controlled Oscillator . ............. 113 5.4.1 Phase Noise Complications in the Transceiver . 114 5.4.2 VCO Inductor ...................... 114 5.5 Design of the PLL for Frequency