Research On The Method Of Baseband Simulation Of Nonideal Characteristic Of Quadrature Modulator And Corresponding Module Design

The baseband signal is the core of vector signal source and it is widely used in the fields of electronic countermeasures,electronic measurement,signal excitation and so on.In the actual communications,quadrature modulator will be used to modulate the baseband of low frequency to the adapted channel.And then radiofrequency signal can be transmitted.However,the inherent nonideal characteristics of the quadrature modulator will be added to the transmitting modulated signal.How to use the baseband signal to simulate the nonideality of the quadrature modulator,it is of great significance to the performance test of the receiving instrument.In addition,it is a key point that how to make effective pre-compensation for these nonideal characteristics to reduce the influence caused by the various distortion of the modulated signal.This paper divides the nonideal characteristics of the quadrature modulator into linear and nonlinear distortion.And conduct the research on the baseband simulation and pre-compensation of two distortions respectively.Finally,the derivation of the algorithm,the building of the model and the design of the signal generating module are completed.The main research contents are as follows:1.Firstly,comprehensive consider the characteristics of linear distortion and nonlinear distortion synthetically.Among them,the analysis of linear distortion mainly includes carrier leakage,amplitude imbalance and phase imbalance.And describe quantitatively the effect of linear distortion on the modulated signal by introducing edge band suppression ratio and local oscillator suppression ratio.Additionally,the analysis objects of nonlinear distortion include Rapp model,Saleh model and complex coefficient polynomial model.Secondly,sum up the distortion parameters which can cause two types of distortion,and then use the corresponding algorithm to convert all distortion parameters to the baseband signal to simulate the distortion of the baseband signal.Finally,the linear distortion model and the nonlinear distortion model are established according to the derivation,and the correctness of the model is tested by the simulation.2.The compensation algorithm for distortion is studied based on the linear distortion and nonlinear distortion model established in the previous chapter.For linear distortion compensation,different methods are used respectively according to the different bandwidth of input signal.Use distortion parameter derivation to construct the compensation model for single frequency signal and design FIR digital filter to compensate wideband signal which has distortion.For nonlinear distortion compensation,this paper focuses on the derivation of the AM/AM characteristic function and AM/PM characteristic function.And introduce the curve fitting method to estimate the expression of the characteristic function.The correctness of the compensation model is proved by the change of the simulated distortion data after the pre-compensation model.3.Based on the established distortion model and compensation model,complete the design of corresponding module in FPGA by using the waveform playback method and the real time data generation method.Analyze the the DDR3 SDRAM control module and pre-distortion module which are the core module of the waveform playback method and real time data generation method.The DDR3 SDRAM control module combines serial and parallel conversion,read and write control state machine and the control of MIG core,realize the waveform playback with the maximum rate of 200 MHz and maximum storage capacity of 4GB.The pre-distortion module uses multiphase structure design to realize the real-time data filtering and the maximum 32 times interpolation function.After real-time data testing,the designed module can simulate the distortion signal and precompensate the distortion simultaneously.