Design Methods of Modern Ultra-Low Noise Synthesizers

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  • Author: Farron L. Dacus

This is the full length version of the first of a four-article series on low noise synthesizer design using the latest synthesizer IC’s and supporting parts. Recent years have seen major changes in the frequency synthesis art, driven by a combination of advancing RFIC design and higher frequency crystal references, that are fundamentally changing the frequency sources field. Ultra-low noise discrete VCO’s, which have been the heart of low noise synthesizers for decades, now find themselves challenged by integrated VCO’s on the same die as the synthesizer.

The best discrete VCO’s still enjoy a 20-30dB phase noise superiority over the best integrated VCO’s, but IC companies are conducting an asymmetric battle to dominate the market with full integration based not on the best VCO noise, but on architectural innovations that often render free running VCO noise less important. This is achieved by putting good if not great VCO’s on die with SiGe BiCMOS processes and MEMS inductor resonators, and then suppressing that fairly good noise down to a very low level via feedback. This article will review simple historical and modern more advanced design methods in order to set the stage.