Spurious signals in direct digital frequency synthesizers due tothe phase truncation

摘要

Direct digital frequency synthesizers (DDS or DDFS) are widelynused in modern communications and measurement devices. Their advantagesnare small size and power consumption together with excellent frequencynstability, high frequency resolution, and short switching times. Thendifficulties are rather low output frequencies (500 MHz at the presentnstate of the art) and a large set of the spurious signals very oftennabove the -80 dB level. One source of spurious signals in DDS is the usenof smaller number, W, of the most significant bits (MSB) applied for thenoutput sine wave reconstruction from all R bits stored in thenaccumulator. The result is a phase modulation of the output signal. Thenproblem was first solved in a rather complicated way with the resultnthat the level of the largest spurious signal is about -6 W dB below thencarrier with an increase of 3.9 dB in some instances. A simpler solutionnof the problem of spurious signal level due to the phase truncation innDDS was found earlier. However, no attention was paid to the validity ofnthe corrections suggested. In this paper we will be concerned with thisnproblem and investigate the validity and correctness of these generallyncited results and provide a simple way for finding positions, levels,nand numbers of these spurious signals generated by truncation to W bitsnof the phase information stored in the DDS accumulator memory of R bitsn(W