Method and apparatus for improving the frequency resolution...

Electrical computers: arithmetic processing and calculating – Electrical digital calculating computer – Particular function performed

Reexamination Certificate

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Reexamination Certificate

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07606849

ABSTRACT:
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 F1and F2such that F1is present for time T1and F2is 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 |F1−F2|<<π/T must be satisfied. The time-weighted average frequency Favg=(T1·F1+T2·F2)/(T+T2). By an appropriate choice of T1and T2, Favg can be set to any frequency between these two frequencies.

REFERENCES:
patent: 5694377 (1997-12-01), Kushnick
patent: 5898325 (1999-04-01), Crook et al.
patent: 7034624 (2006-04-01), Gentile
patent: 2005/0003785 (2005-01-01), Jackson et al.
patent: 0 443 242 (1991-08-01), None
patent: 0 493 057 (1992-07-01), None
Analog Devices data sheet; AD9954; 400 MSPS 14Bit, 1.8 V CMOS—Direct Digital Synthesizer; 2003 (cover sheet plus pp. 1-36).

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