Direct digital synthesiser with improved spectrum at low frequencies

Abstract

In this work, we suggest a new method for obtaining lower distortion of the output spectrum of a direct digital synthesiser (DDS) at lower sub ranges of its operating frequency range. In DDS, the output sine wave is synthesised from discrete steps. At the higher frequency sub range of DDS, the duration of each step is equal to one period of the clock signal. When this discrete sine wave is passed through a low pass filter (LPF), on its output is obtained a continuous, near ideal sine wave. At lower sub ranges of DDS, the duration of each step is more than one period of the clock signal. When this signal is passed through the same LPF the output signal is not sine wave. To avoid this discontinuity, we replace the step approximation with linear one. For this purpose, we use adaptive multilevel pulse-width modulation. The period of this signal is equal to the period of the clock. After filtering this signal through the same LPF, we obtain a continuous sine wave at lower sub range of the DDS. The distortion of this signal is less than the distortion of the standard DDS signal at the same frequency. The improvement of the output spectrum is a function of number of bits of used phase accumulator, sine lookup table and digital-to-analog converter (DAC).