Software Defined Radios 長庚電機通訊組 碩一 張晉銓 指導教授 : 黃文傑博士

Outline Digital Generation of Signal DDS –Direct Digital Synthesis Comparison of DDS with Analog Signal Synthesis Approaches to DDS Phase truncation Conclusions

Digital Generation of Signal SDR needed reconfiguration ， adaptability for hardware The development of digital IC Synthesis of waveforms DDS (Direct digital synthesis)

DDS –Direct Digital Synthesis 1970s for generating audio signal (ROM) DDS’s chips now provide several hundreds of megahertz output Advantages: Precision Fast switching capability Arbitrary wave shapes Small and low-power Fewer components

Comparison of DDS with Analog Signal Synthesis Analog Signal Synthesis: Direct analog synthesis (DAS) and PLL method High purity,fast switch about.1 to 20us Size, cost, and power consumption High-speed testing, medical, radar imaging, spectroscopy, radar and electronic warfare simulation and frequency hopping comm.

Comparison of DDS with Analog Signal Synthesis Summary of DDS properties

Comparison of DDS with Analog Signal Synthesis

Approaches to DDS PO DDS (Pulse output DDS) Use a phase accumulator Generates pulse, sawtooth or rectangular waveform High levels of spurs and phase jitter

PO DDS (Pulse output DDS)

Approaches to DDS ROM LUT (ROM look-up table) The sampled values of the sine waveform are stored in ROM and output through a DAC and LPF It is advantageous to perform digital filtering before the DAC to compensate for distortion of the non-ideal analog filters The problems of ROM size Arbitrary waveform is to be generated

ROM LUT (ROM look-up table)

Phase truncation If the maximum output =2.5MHz,the turning step=1Hz,Fclk=10Mhz, N=24=16Mbit W are used to address the ROM while returning the number of bits used in the accumulator

Phase truncation

ROM LUT (ROM look-up table)

Conclusions DDS provides the flexibility that a SDR needs to support various modulation formats Have many benefits ex: fine frequency resolution ;fast response time ; easy of manufacturing and testing ;robustness….. Issues in DDS design,such as spectral purity.

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