Design of a Broadband Frequency Synthesizer Based on a Novel DDS Extension Scheme

To enable frequency synthesizers to simultaneously possess the characteristics of broad bandwidth, high spectral purity, fine frequency steps, and fast frequency hopping, this paper designs a novel DDS (Direct Digital Synthesis) frequency expansion scheme. This scheme utilizes the spurious optimization characteristics of frequency dividers and resolves the contradiction between DDS output spurious signals and output frequency bands through multiple mixing and frequency division operations. The expanded DDS circuit simultaneously achieves the advantages of broad bandwidth, low spurious levels, fine frequency steps, and agile frequency switching. Using this frequency expansion scheme, a broadband fine-step high-spectral-purity frequency synthesizer was realized. The final implementation achieves a broadband frequency output of 1.25 ~ 20 GHz with a frequency step of micro Hz, spurious suppression better than 70 dBc, and phase noise better than-108 dBc/Hz@1 kHz (at 20 GHz).