Research On The Minitype TT&C Transponder

As the key subsystem of a satellite, the TT&C transponder provides the main communication channel between the satellite and the ground, which determines whether or not the satellite missions would be success. With the development of small satellites, more and more new characteristics are needed for the TT&C transponder, such as low power-consumption, little volume, light weight and full functions. But our traditional TT&C transponder can't fit these requirements.Based on the alalog TT&C transponder which is already designed by our team, a new minitype TT&C transponder structure is proposed in this paper, which include the receiver module, transmitter module, baseband processing modules, demodulation module and power module. The final minitype TT&C transponder is 70g and 210cm3, which has 3.5W power consumption,-92dBm sensitivity and 250mW transmission power. It meets all the requirements of small satellistes. However, its sensitivity is not very high, which also limits its use.Based on the analysis of the receiver structure, the main factor limiting its sensitivity is found in this paper, the carrier tracking loop. Meanwhile, a new receiver structure is proposed in this paper, which mixes digital frequency synthesizer with traditional PLL. Thus without increasing power consumption and size, the sensitivity of the minitype TT&C transponder can be easily imporved.By calculation and simulation of the system function of nested loop, a new method that can quickly determine the bandwidths of frequency synthesizers is proposed in this paper. Using this method, the complex nested loop system function can be equivalent to a simple second-order PLL function. Thus by using the equivalent functions, loop noise, lock time and sensitivity of this PLL can be easily analysed. A new signal simulation method is also proposed which can show every signal of the PLL after loop parameters have been determined, and thereby there is no 'need to debug the actual circuit. The hardware design of this new struction is described in this paper. The loop parameters are designd by using Matlab, and the result can be quickly implemented in hardware. The result shows that the simulation and the experiment are perfectly matched. This new structure can not only greatly improve the sensitivity of the minitype TT&C transponder, to nearly-110dBm; but also can make the whole circuit meet the small size, low power requirements of small satellites.