Digital Electronics: Principles, Devices and Applications

688 Digital ElectronicsFrequencyConstantΣReferenceClockRegisterPhaseAccumulatorSine Look up Table withInterpolation BetweenStored EntriesD/AConverterLow PassFilterO/PFigure 16.29Direct digital synthesis.of the memory would correspond to phase differences of 36, 72, 108, 144, 180, 216, 252, 288, 324and 360 to complete one cycle of output waveform. The 10 samples will be produced in 10 clockcycles. Therefore, the output frequency will be one-tenth of the clock frequency. In general, the outputfrequency is given by/2 × f clock (16.1)where is the phase increment in radians.The digital samples are converted into their analogue counterparts in a D/A converter and theninterpolated to construct the waveform. The interpolator here is a low-pass filter. Relevant waveformsare shown in Fig. 16.30.This method of synthesis derives its accuracy from the fact that both the phase increment informationand the time in which the phase increment occurs can be computed to a very high degree of accuracy.With the frequency being equal to the rate of change in phase, the resulting waveform is highlyThe IdealWaveform to beSynthesisedOutput ofLPFOutput ofDACFigure 16.30Direct digital synthesis – relevant waveforms.