| With the rapid development of software radio technology,the receiver technology has been very mature at home and abroad,however,the research of broadband reconfigurable digital radio frequency transmitter is relatively small.The reconfigurable digital broadband RF transmitter using direct digital frequency synthesizer and a mixer,minimalist using analog part and the debugging of the circuit design caused by the complexity and make the wireless communication system more stable.This paper proposes a scheme to realize the overall reconstruction of digital broadband RF transmitter,and mainly analyzes the specific scheme of the digital signal synthesis based on FPGA in the transmitter,which includes the digital up conversion and the specific frequency signal synthesis.Meanwhile the module program involves such as the low pass increment modulation,polar coordinates and phase modulation techniques are studied in detail in the paper.The main results of this study are as follows:1)To research and design the low pass Sigma Delta technology,which has the ability of noise reduction and anti interference.The SIMULINK modeling of the one order and two order modulators respectively based on MATLAB,the over sampling rate of OSR take 128,1bit quantization.Finally through the simulation analysis that draw the signal to noise ratio of the two order modulator is about 99 d B,and the first order is about 73 d B,which is the same as the value of directing using SNR formula to calculate.At the same time compared to the two output signal spectrum contours concluded that the noise reduction effect of the two order modulator is obviously better than that of the first order.The signal to noise ratio is also significantly improved by about 26 d B.2)The polar coordinate conversion technology is studied and designed to realize the polar coordinate conversion of I and Q signals.Firstly,the cosine and sine wave of I and Q signals are set at the frequency of 5 Hz.Then using the MATLAB to program and simulate,the results show that the polar diameter r is a constant.Meanwhile the polar angle q is a saw tooth wave which could change with periodic.3)To research and design the involved DDS digital phase modulation technology in order to synthesize the signal of the required frequency and phase modulation.Firstly,to establish the SIMULINK model,let the clock frequencycf to take 125 MHz,the phase accumulator number N to take 20 and the frequency control word to take215 ?213 ?211 to carry on the simulation to obtain the corresponding output signal spectrum char.From the output signal spectrum char that we can obtain the central frequency of is2.45*107 rad/s,6.14*106 rad/s,1.53*106 rad/s,meanwhile the results meet the calculation formulaf0=K·fc/2Nof the output signal frequency of DDS system.It further illustrates that only to change the frequency control word can adjust the frequency of the output signal of DDS system.Secondly,to realize the key control method and the simulation method of the common 2PSK modulation technology by SIMULINK modeling and Simulating,lay the foundation for the simulation of DDS digital phase modulation technology verification.Finally,to establish the SIMULINK model of DDS digital phase modulation technology,DDS system parameters set with the same as before,only take the frequency control word K take182 as an example to simulate.By setting a fixed cycle phase signal to output a 2PSK signal,which proves the DDS system has the function of digital phase modulation.4)The whole scheme of up conversion and signal synthesis in the transmitter is implemented in FPGA.Finally downloaded to the FPGA development chip,and call the software to observe the various key output waveform,finally downloaded to the development of the chip,and call CHIPSCOPE software to observe the various key output waveform.The calculated output signal frequency is about 21.4 MHz.This is similar to the frequency control word K take764504178 corresponding frequency.So as to verify the correctness of the whole project. |
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