The Study On Mini-SAR Signal Generation

Benefits from small size, light weight, and low power consumption, Mini SAR can be installed in a small UAV which greatly reduces the cost. The Mini SAR can be widely used in the fields of reconnaissance, topographic mapping and other areas. Mini SAR adopts the signal of FMCW. The signal generator is an extremely important part in the FMCW SAR. It is required to generate signal with wide bandwidth and long pulse width by the generator with small volume, which is quite difficult to be realized in the traditional way. Around the FMCW signal generating, we discussed digital wideband signal synthesis technology, analog wideband signal synthesis technology, phase error compensation of FMCW, as well as the echo signal simulator in-depth, and have achieved some valuable research results. The main achievements and contributions of the dissertation are as follows:1. We proposed the method of band picking to depress the harmonics in DDS and pre-distortion to compensate the phase error in DDWS. The experiments showed that the methods proposed in the paper depress the harmonics and compensate the phase error effectively. The DDS gets the advantages of high frequency resolution, fast switching speed and continuous phase. But there are serious harmonics, limited bandwidth and some other issues. By the analysis of the DDS, we listed the distribution of the harmonics and proposed the method of band picking. We realized a signal generator of DDS and took some tests. The tests showed that the harmonics are low. The scheme of DDWS not only possesses the advantages in DDS, but also has the characteristics of output flexible waveform. Moreover, the usage of orthogonal modulation brings leakage and mirror frequency for the imbalance amplitude and phase in baseband signals. The leakage and mirror frequency depress the signal quality seriously. We proposed pre-distortion to compensate the amplitude and phase errors. We designed and implemented a DDWS signal generator. With the pre-distortion, the amplitude and phase errors, as well as the systematic errors were compensated and the pulse compression was as good as the ideal one.2. Based on the analysis of PLL and hybrid frequency synthesis scheme, we proposed the Sweep Voltage DDS-PLL(SVDP) frequency synthesis technique. DDS was employed to generate the reference signal of PLL, and the voltage of VCO was preset by the sweep voltage circuit. The signal generated with this method had large frequency modulate rate, and fast acquisition time. A SVDP test circuit was built up. The test results showed that the switching speed is fast, the frequency linearity of output signal is greatly improved with the help of pre-distortion, the pulse compression is excellent, and the coherent is good. This structure is very suitable for the application of Mini SAR.3. To compensate the range-dependent phase error in the FMCW, we proposed a compensation method based on the RVPC operation. The compensation is validated to the range-dependent phase error. The Mini SAR employs the Dechirp to process the echo, which can reduce the pressure of sampling and processing effectively, so as to control the volume and weight. The Dechirp approach brings in a problem, which is range-dependent phase error contained in the echo reflected from the target. The range-dependent phase error is related to the target distance. We built the phase error model in the output signal. The compensation approach was executed in two steps: first, the phase error introduced by the transmitted signal was compensated after Dechirp; second, the phase error introduced by the received signal was compensated after RVPC. The two compensation functions were calculated previously with the phase error extracted from the transmitted signal. Both the simulation and the real data experiments suggested that the proposed algorithm is effective to compensate the phase error, improved the pulse compression without weight and calculation increasing.4. The echo simulator based on the DDWS was developed with the storage of 1TB and bandwidth of 200 MHz. The echo simulator output the simulate echo and formed the semi-physical simulate system with the signal processing module. The semi-physical simulate system simulates the radar working state in the laboratory. The present echo simulator used NAND Flash to realize the mass storage. The parallel memory, bad block management, ECC, and USB2.0 were employed to overcome the difficulties in the implementation. The echo simulator has the ability of good versatility and practicality with Mass storage, fast data transfer, three type signals output.The proposed methods and ideas in this article are all verified with the real hardware system. By the comprehensive test, it indicated that the study have excellent theoretical and practical value.