Software for design of hybrid frequency synthesizer based on direct analog and direct digital synthesis
Abstract
Hybrid frequency synthesizers based on direct analog and direct digital synthesis methods (hybrid digital-analog synthesizers) provide a small step and high frequency tuning speed, support various types of modulation, and have a low level of phase noise. The basic principle of constructing such synthesizers is transferring a small step of frequency tuning of the synthesizer of a direct digital synthesis method – a digital synthesizer (DDS) to a higher frequency range by mixing with signals generated by reference frequency generators. This paper describes the principle of operation of a hybrid synthesizer. A block diagram and mathematical model of the power spectral density (PSD) of phase noise are presented. Algorithms for structural design and determination of optimal parameters are proposed. A special software tool for structural design of hybrid frequency synthesizers based on direct analog and direct digital methods of synthesis is presented. Software automatically calculated optimal frequency ratios in the synthesizer structure provided the lowest level of phase noise. An example of structural design and analysis of the noise characteristics of a hybrid synthesizer based on DDS chips: AD9858 and AD9915. The optimal frequency ratios in the synthesizer structure are determined. The contributions of synthesizer components to the resulting level of phase noise are analyzed. The hybrid synthesizer has a gain in the level of phase noise by 7-8 dB at tuning frequencies above 10 kHz when it based on AD9915 chip.
