A frequency synthesizer based on a direct digital synthesizer (DDS) developed by Emhiser Research, Inc. is presented to obtain fast frequency hopping while maintaining small output frequency step size. The design proposes to eliminate the tradeoff in frequency synthesizer design between frequency hopping time and output frequency step size. A basic review of traditional phase-locked loop (PLL) and frequency synthesizer design is presented and performance characteristics are defined. The fundamental operation of the DDS is reviewed and the effects of inserting the DDS in the synthesizer loop are discussed. The major advantages to this approach are faster hopping times and smaller output frequency step sizes. The disadvantage of inserting the DDS in the loop is the creation of an image signal that must be attenuated for the loop to obtain lock. A method of DDS image cancellation without the use of sharp cutoff, narrowband bandpass filters is presented. The design should find wide application for agile synthesizer design and high-speed data communications.
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