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

clkrr

clkNr

out

T

FF

:

2

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

Nclk

clkr

FF

F

2

4/

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.

End

Thank you for your attention