Design And Implementation Of Look-ahead Delta-sigma Modulator In Digital Transmitter

In recent years,the service demand of wireless communication has increased explosively with the increase of mobile devices.Efficient and flexible use of frequency resources is the key factor to solve the supply of wireless communication services and user demand,which drives the transformation of the theory and implementation of wireless communication system architecture.The digital transmitter based on FPGA(Field-Programmable Gate Array)has become an important carrier of software radio research because of its flexibility,reconfigurability and high hardware integration.Therefore,it is of great significance to study the digital RF transmitter based on FPGA.This paper first analyzes the basic structure of the digital transmitter,and introduces the advantages of the digital transmitter compared with the traditional analog transmitter.Then the digital signal processing part involved in the digital transmitter is explained.The principles,advantages and disadvantages of different quantization modulation schemes such as Deltasigma modulation,PWM modulation,DFT-PWM modulation are analyzed and compared,and the Delta-sigma modulation is determined as the quantization modulation scheme of the digital transmitter designed in this paper.From the view of implementation of digital circuit,the quantization modulator is analyzed and optimized,and a new architecture of quantization modulator is proposed.At the same time,a method to improve the performance of RF signal is proposed,which is verified and tested on the hardware platform.The main work of this paper is as follows:1.A Delta-sigma modulator structure based on look-ahead operation is proposed,which effectively reduces the logic level depth of time-interleaved Delta-sigma modulator and improves the working frequency of Delta-sigma modulator.A four branch second-order Deltasigma modulator is designed based on this structure,and the system link simulation of digital transmitter based on this modulator is carried out.The experimental results show that the 20 MHz bandwidth LTE signal,which passing the digital transmitter link designed in this paper,the ACLR of the RF signal can reach-43 d Bc,and the working frequency of the Delta-sigma modulator designed in this paper can work at 1GHz.2.In order to optimize the performance of RF signal,a combined filtering schem is designed to achieve near-end noise suppression of the RF output signal.The phase control module and amplitude control module are designed to realize the independent phase and amplitude adjustment of multiple output signals,and then the way of combining multiple high-speed digital interfaces is adopted.The simulation results show that the ACLR of the combined output RF signal is increased to-49 d Bc.3.The digital transmitter designed in this paper is implemented and verified on the hardware platform.Using the hardware description language Verilog to design the digital signal processing part of the digital RF transmitter,including polyphase interpolator,polyphase digital mixing,Delta-sigma modulator based on forward-looking operation,amplitude phase control and other modules.The transmission signal after digital signal processing is finally connected to the power combiner through the high-speed transceiver on the FPGA to output.Finally,the performance of the digital RF transmitter is tested to verify the feasibility of the design.