Research On Direct Generating System Of Wideband Radio Frequency Signal

Wideband waveform has been widely applied in radar system for the advantages of high rangeresolution, strong anti-jamming, good reliability of target detection and identification, etc. Thegeneration of the wideband waveform has become a key technology of the wideband radar becausethe performance index of the waveform directly affects the performance of the whole radar system.Therefore, it is important to do the research to design a direct-generating system of the widebandradio frequency(RF) signal.The thorough research of the wideband RF signal direct-generating system is taken in this thesisas a part of scientific research project. First, the principle of the direct digital waveformsynthesizer(DDWS)is discussed according to the linear frequency modulation(LFM) signal, which iscommonly used in radar systems, and the error model of the DDWS is also analyzed. Based on thetheoretical analysis and the technical index of the system, the wideband RF signal direct-generatingsystem is implemented by using wideband digital intermediate frequency(IF) circuit, mixer circuit andfrequencymultiplier circuit. TheLFM signal starting from50MHz and ending at550MHz is generatedby the wideband digital IF circuit using high-speed FPGA and DAC, and the implementation of thecircuit and the solution of the key problem are studied. Then,the mixer circuit and frequencymultipliercircuit are designed by RF circuit simulation software ADS. The IF signal is up converted to the2GHzto2.5GHz RF band by using the mixer circuit, filter circuit, and amplification circuit, then after thefrequencymultiplier circuit, the originalIF signal is converted into a wideband LFM signal startingfrom4GHz and ending at5GHz.Finally, the digital pre-distortion correction principle and method ofthe system errors are discussed according to the feature of the digital IF generating circuit.A wideband LFM signal with4.5GHz carrier and1GHz bandwidth is generated by the system.After pre-distortion, the spurious signals involved in the system output are lower than-55dBc, and theamplitude fluctuation in the band is below±2dB. The test results show that the system is quite stableand reliable.