Dds Spread Spectrum Technology

After direct digital frequency synthesizer (DDS) technology was put forward in1970s, it develops very rapidly owing to its high frequency-switching speed, goodoutput phase noise performance and excellent frequency resolution capability. However,because of its all-digital structure, it has some disadvantages such as low outputfrequency, narrow bandwidth and bad spur level. Especially when output bandwidth isincreased further, spur level will become worse. In this paper, the theory of direct digitalfrequency synthesizer (DDS) and phase lock loop (PLL) is introduced in detail, onwhich a new PLL scheme with low phase noise is proposed. After the analysis andcomparison of several DDS plus PLL schemes, the DDS high hopping scheme is finallypresented by the author. The new scheme aims to enhance the output bandwidth of theDDS, increase output frequency and better spur level while output phase noise andfrequency switching time are kept unaffected.The new scheme includes DDS high-speed hopping technology, PLL fastswitching technology, low phase noise lock technology, DDS real time controltechnology, difference modulator technology, etc. In this paper, profound theoreticaland experimental analyses are carried out on those technologies. After the feasibilityanalysis on the new DDS high-speed hopping scheme, the final result shows that everyindex of the scheme reaches and even exceeds former expectation.At last through the test of the system the paper gives the results of such system asfollowing: output frequency range 750MHz～769MHz, SFDR: --77dBc, Harmonic:--72 dBc, phase noise-:--126dBc/Hz@10kHz, hopping time: 100 ns, hopping step1MHz.