| The digital transmitter is a type of transmitter structure which modulates and upconverts baseband signals by digital process.A switching power amplifier is adopted as the core component to amplify RF signals in this structure.The digital transmitter is favored for its low power consumption,high integration,high efficiency and powerful anti-jamming performance.In order to maximize the high-efficiency characteristic of switching power amplifiers,the modulated RF signals should have a pulse waveform.Therefore,as the key of digital transmitter structure and switching power amplifier application,the transmitter front-end modulation technology is deeply studied in this thesis and a PWM coding scheme is proposed.The main research contents are as follows:1.This thesis is mainly concerned about transmitter structure.Principles and characteristics of super-heterodyne transmitters,zero-IF transmitters and digital transmitters are analyzed and compared.Two key technologies of digital transmitters: modulation and switching power amplifiers,are analyzed.Operation performances and advantages and disadvantages of DSM modulation and PWM modulation are described mainly,which provides theoretical basis for the design of subsequent coding schemes.2.In this thesis,based on the theory of zero-IF quadrature modulation,a new modulation architecture of digital transmitters is designed with PWM coding as the core,which directly convert baseband signals into pulse RF signals for efficient amplification of switching power amplifiers by digital operations such as quantization coding,splicing and interleaving.The mathematical derivation for the precision analysis of PWM coding and the digital up-conversion operation is carried out in detail,which provides theoretical support for the design scheme.Aiming at the design parameters of the modulation part in the transmitter,an optimization method based on multiple samples is proposed to improve the transmission quality of the digital transmitter,as well as its stability and robustness.The digital transmitter structure link optimized by this method is verified by simulation at three carrier frequencies of 1.5625 GHz,2GHz and 2.5GHz.Simulation results show that for the 64-QAM input signal with 10 MHz bandwidth,the ACPR of the RF signal from the digital transmitter front-end is-32.92 dBc,and the EVM of 6.25% is achieved.3.In this thesis,the circuit design of the digital transmitter modulation front-end is completed with FPGA as the hardware platform.Based on the previous optimized modulation structure,two PWM mapping optimization algorithms of two-division and full-division pulse width are proposed,while improving the signal transmission quality without adding any extra circuit complexity.The experimental environment of the digital transmitter front-end is set up,the prototype PWM and the two proposed algorithms are tested experimentally.The experimental results verify the feasibility and flexibility of the digital transmitter,and shows that the proposed algorithms effectively improve the signal transmission quality of the digital transmitter modulation front-end.For the 64-QAM input signal with 10 MHz bandwidth,ACPR reaches-35.65 dBc,which means an improvement of 7.37 dB compared to the prototype PWM;EVM reaches 4.99%,2.18% better than the prototype PWM.In summary,the PWM coding scheme in this thesis directly completes RF signals transmission through digital processing,and the proposed parameter optimization and algorithms improve the system transmission quality,which is of great significance to the application of digital transmitters. |
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