L-band Digital Microwave Transmitter Rf Front-end Study

Transmitter is the equipment which transmits electromagnetic signal. It determines the performance of the electronic equipments, plays the most important role in our lives. Along with the development of modern military, national defence and wireless communication, many electronic systems such as mobile communication, radars and guided weapons set increasingly higher requirements on the performance of transmitters. Modern communication system lays particular emphasis on high pure spectrum and linearity of transmitters.This project is exactly based on the research and realization of the digital microwave transmitter which can transmit digital signals. The digital signal is converted to intermediate frequency(IF) signal after source coding, channel coding and QPSK modulation. Then the IF signals is changed to radio frequency(RF) signal by way of being mixed twice with the local oscillation(LO) signal. A part of the RF signal which comes from the directional coupler is amplified and demodulated to direct current(DC) signal to control the gain of the variable gain amplifier(VGA). So the system may produce stable signal to transmit. The driving amplifier enlarges signal to reach an adaptive level in order to drive the power amplifier. The enlarged signal is delivered to a filter and transmitted through an antenna.In this paper, the structure of the wireless transmitter is introduced at first. Subsequently, the system design and frequency synthesizer design of the transmitter are introduced. Then the scheme of this transmitter, the realization of each functional module, and some questions which must be paid attention to in the system design are presented. Finally, the testing results are given in the paper.