FFH/BFSK 시스템 송신부용 DDS 설계 및 성능평가에 관한 연구

Abstract

The global trends of mobile communication system is moving toward digitizing, high-speed, and large-capacity. To utilize the limited frequency -resource efficiently, in addition, the spread spectrum system is mainly used. The spread spectrum systems are clasified into direct-sequence and frequency-hopping. This thesis is concerned with frequency synthesizer in the FFH(Fast Frequency Hopping) system. The FFH system requires the characteristics of high-speed and low-noise. Most of the frequency synthesizers have adopted the PLL(Phase Lock Loop), which has many problems such as phase-noise, high-cost, and size. Because the VCO is the analog device, it is the main cause of noise in the PLL. Since the DDS(Direct Digital Synthesizer) is a digital device, it is possible to minimize the disadvantage of the PLL.

In the FH system using the PLL, the main lobe of the dehopped carrier spectrum is twice hopping carrier bandwidth by the balanced modulation. In this thesis, therefore, by adopting the direct modulation system using the DDS, the dehopped carrier bandwidth became equal to the hopping carrier bandwidth. Thus, the available number of the channels are increased in the infinite RF bandwidth and the performance of the bit error rate is improved.

Furthermore, the frequency synthesizer for the transmitter of the FFH system was designed using DDS and the system was experimented. The RF bandwidth is 5 MHz, the hopping rate is over 1 Mhps, frequency step is 25 KHz, and the data rate is 1 Mbps. Then, it was confirmed that the parameters can be easily changed by programming the control data of DDS without degrading the performance. These results may be expected to be applied to the FFH systems directly.