A Novel Spur Suppression Technique Using Three-Phase Holding Pulse for High-Frequency-Output Direct Digital Synthesizer

This paper presents a novel spur suppression technique using a three-phase holding pulse for a direct digital synthesizer (DDS) with a two-phase holding digital-to-analog converter (2PH-DAC). A 2PH-DAC, which uses a reverse-sign step-function as a sampling pulse waveform instead of a commonly-used gate function of zeroth-order hold, enhances the first image of aliasing, which is of higher frequency than the fundamental. Therefore, the first image can be treated as a desired signal, while the fundamental and the second image are spurs for a DDS with a 2PH-DAC (2PH-DDS). The main problem of the 2PH-DDS is close spurs in the case that signal frequency is near Nyquist frequency or sampling frequency. This paper proposes a novel spur suppression technique for a 2PH-DDS. A configuration of a 2PH-DDS is first explained, and spectral properties are analyzed. Based on the analysis, a technique using a three-phase holding pulse to cancel spurs is proposed. Evaluated spur levels of the proposed synthesizer are from -51 to -34 dBc, and are improved by 25 dB or more by the proposed technique.