METHOD AND APPARATUS FOR IMPROVING THE FREQUENCY RESOLUTION OF A DIRECT DIGITAL SYNTHESIZER

摘要

The present invention is directed to the use of a DDS to generate a high purity reference signal with high frequency resolution by switching a frequency tuning word (FTW) between particular values for particular time durations to produce two or more closely spaced frequencies that appear at the DDS output as a single frequency. Given a DDS switching between F1 and F2 such that F1 is present for time T1 and F2 is present for time T2, with the total period of the repeating pattern being T=T1+T2, in order for the output of the DDS to produce a single high-purity frequency that is the time-weighted average of the alternating frequencies, the condition ïF1-F2ï<< pi/T must be satisfied. The time-weighted average frequency Favg = (T1 F1 + T2 F2)/(T + T2). By an appropriate choice of T1 and T2, Favg can be set to any frequency between these two frequencies.