DDS Synthesis

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Our final year project is DDS, which was first proposed by J.Tierney in 1971. Direct Digital Synthesis (DDS) is a method used for creating arbitrary digital waveforms and frequencies from a single frequency source. As the VLSI technology is developing also the requirement of the various modern communication systems, direct digital synthesizers have been widely in use like wireless transceivers since the 1980’s. Direct Digital Frequency Synthesis (DDS), consists of a Numerically Controlled Oscillator (NCO), which is a technique which uses digital-data and mixed or analog-signal processing blocks which generate signal waveforms that are repetitive. The DDS achieves very fast frequency switching in very few frequency steps, over a wide band. Also it is used for providing linear phase and frequency shifting with good spectral purity. The DDS is used especially for a high frequency, precise and a phase tunable output. A standard DDS architecture consists of an accumulator, a ROM /lookup table, a DAC and few reconstruction filters. DDS solutions have been implemented in LSI (large-scale integration) and they play a major role in digital waveform and clock generation. One of the major advantage of a direct digital synthesizer is that the output frequency, amplitude or phase can be precisely and rapidly manipulated under the control of the digital processor . Other important DDS attributes include the ability to tune with extremely precise and fine frequency and phase resolution. Different modulation capabilities can be very easily included in the DDS by using digital signal processing methods, because the signal is in the digital form. If we program the DDS, adaptive channel bandwidths, modulation formats, frequency hoppin... ... middle of paper ... ... quantized. Frequency is the rate of change in phase, so obviously the frequency is also quantized. Because of the phase quantization, for a given clock frequency, there are certain frequencies that cannot be exactly achieved. The frequency tuning word (FTW) is calculated using the equation: Fig 1.3 DDS Output Waveform 1.4 POPULAR USES FOR DDS Applications which uses DDS-based waveform generation are divided into two principal categories: Designers of communications systems which require agile (which immediately responds) frequency sources with an excellent phase noise and should have low spurious performance therefore we often choose DDS for its combination of spectral performance and frequency resolution .Some of the applications are which uses a DDS for modulation, as a reference for a PLL to enhance overall frequency, as a local oscillator (LO).
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