In The High-speed Data Transmission System, Direct Digital Rf Modulation / Demodulation Techniques

The topic of this paper is high-speed data transmission system, which is a part of an item of the 10th Institute of Electronic Science and Technology Corporation Group: space borne Ka-band 300Mbps data transmission system and technology. It is mainly concerned in this paper the schematic of modulation and demodulation.The paper adopted advanced RF direct digital modulation scheme. The modulation circuit utilized the vector modulation technology based on fourth-harmonic mixer. The LO frequency is only quarter of carrier frequency. Direct high speed QPSK digital modulation of millimeter wave signal can be realized with fairly simplified LO circuit. Carrier frequency leakage less than -40dB can be attained since good agreement of the two diodes antiparalleled in even-harmonic mixer is guaranteed inherently. Fourth-harmonic mixers are adopted in carrier recover in the modulation millimeter wave QPSK signal. In the part of modulation and demodulation, we analyzed the principles of harmonic mixer, detectors, power dividers/combiners and branch-line couplers. Especially, The characteristic of even-harmonic mixer is analyzed and design technique to control the phase characteristic of the mixer is presented. The mixer circuits are designed; the results of simulation and experiment test are compared and analyzed. Theory analyze of base-band shaping filter is delivered and adaptive compensation of imperfections in modulators are discussed. FPGA is used as platform of base-band shaping filter .The VHDL program and simulation of FIR shaping filter is given in this paper. In the modulation/demodulation circuits, CPLD is selected as platform of the digital logic part, which includes series-shunt/shunt- series transform, difference coding and sample verdict. In order to test the feasibility of the project, desktop test of modulation and demodulation as a whole is performed, the test report and characteristic evaluation are presented also. The test results demonstrated the feasible of the scheme. The technique researched in this paper will largely simplify the equipment of modulation and demodulation, and may find its application in millimeter wave in high-speed data communication link.