Design And Implementation Of TT&C Transponder Digital Baseband Of Microsatellite

With the development of microsattellite technology, the TT&C transponder of microsattellite is becoming digital, integrated and multifunctional. The current mainstream transponders are almost the structure of intermediate frequency digitization, while taking samples from the IF signal directly and useing the digital signal processing technology to achieve the main parts of the baseband signal. This kind of TT&C transponder needs less hardware resource, and has simpler structure, smaller volume and lighter weight. Additionally, the TT&C transponder can change function by software design and the hardware structure need not to be changed.In this thesis, the development of TT&C transponder is reviewed, and the digital baseband of TT&C transponder is analyzed and designed according to the demand of the microsatellite communication. Then the digital baseband is implemented in a FPGA chip.TT&C transponder can modulate the downlink telemetry data, so the designed transmitter digital baseband of TT&C transponder mainly includes data modulation and spread spectrum modulation. An appropriate modulation technology which is commonly used in satellite communication is chosen. Spread spectrum modulation uses direct sequence spread spectrum method. Some kinds of pseudo-random sequence is discussed. And the most suitable pseudo-random sequence is chosen as the spread spectrum code.TT&C transponder can demodulate the uplink tele-command signal. The key techniques needed by the receiver to demodulate successfully are analyzed from the expression of the received signal. The technology of acquisition and tracking of the spread spectrum code, bandpass sampling theorem and the technology of carrier tracking are analyzed and discussed. On the basis of theoretical analysis, the receiver’s digital baseband is designed.On the basis of the theory and algorithm analysis, considering the hardware condition, the digital baseband of the transmitter and the receiver is implemented in an Altera FPGA chip, and the data conversion modules are also designed. Fianlly, the digital baseband was verified through the test result of the system.