Research Of HF Modem’s Hardware Design Based On Software-Defined Radio

High Frequency (HF) communication has a long history and reaches its heyday in the1950s.US military use it as an important telecommunication mean in the war since1980s. HF communication devices have small and lightweight appearance, can be transferred and networking quickly. It can be used for long-distance communications with low power cost, which makes it important in the military communications area and has always been one of the hotspots around the world.HF data modem is the equipment which using variety of techniques to convert digital signals to analog signals that suit for short wave channel transmission. It is an important component for short-wave communications and the main telecommunication equipment in the military field. HF data modem has been widely used and the number is enormous. The traditional modem has low data rate and no strong anti-interference. In the days the digital information develops so rapidly, they can no longer meet the needs of digital telecommunications. There is a large gap between our short-wave communications technology with United States and other developed countries. The modems which we use generally have low data transfer rate and poor reliability, and it is difficult to realize the cooperative communication with so many kinds of models which use different communication standard.To solve these problems in current modem, a new type of modem design is introduced in this paper. This modem which uses software defined radio idea to design a common hardware platform can realize digital signal processing and reduce the loss of signal and noise; high-speed data converters and digital signal processor can achieve real-time processing of voice, images and other signals. Interface and software in accordance with MIL-STD-188-110C and STANAG5066standard also make the modem has higher transmission efficiency and compatibility.The main work and achievements of this paper includes the following aspects:(1) A new design of modem with software defined radio structure. US military communications standard MIL-STD-188-110C provides much effective means to improve the modem’s data transfer such as better interweave and coding technologies. NATO standard STANAG5066can improve the compatibility of HF communication devices. According to these standards, the system design combined with software defined radio technology and digital signal processing technology can achieve high speed digital signal processing.(2) The hardware circuit design. Digital signal processing is the core of digital modem and responsible for data processing and algorithms that using a high performance DSP TMS320C6748. DDR2, Flash, power supply circuit are designed according to the needs, the same as communication and control interface such as serial port and Ethernet port; as a bridge between the analog and digital signals, signal sampling and conversion module using Sigma-Delta ADC/DAC with high anti-aliasing and resolution, which greatly decreases the noise. McBSP can achieve high-speed data transmission between ADC/DAC and DSP.(3) PCB design. PCB design often affects the performance and EMI of the equipment. Because the modem is a powerful system and the chips inside is quite a lot, the high speed signal is very complex. So the design of PCB should consider many factors from chips selection to the circuit design and layout. In this paper lots of manners are adopted such as design of PCB layers, impedance control to guarantee signal transmission quality. The noise’s generation and transmission is also be controlled by power supply chips selection and capacitor placing. The simulation results show that these designs have a good effect.(4) Circuit debugging and testing. After hardware circuit debugging, power on PCB board and testing DSP, memory and interface on the platform of CCS. The results show that the board works well and verify the feasibility.