Frequency characteristics of hybrid frequency synthesizer with phase distortion autocompensation

Abstract

Methods and devices of direct digital frequency synthesis based on direct digital synthesizers (DDS) and methods of indirect synthesis based on phase-locked loop (PLL) systems are widely used in current radio equipment. Using hybrid synthesis enables to combine advantages of both methods. However, hybrid frequency synthesizers (HFS) have retained a number of shortcomings native to DDS and PLL-based synthesizers including signal phase fluctuations caused by external actions, design and schematic-level solutions. Schematic-level and design methods of phase noise control do not always prove to be effective. To suppress HFS phase noise, this paper proposes to use noise autocompensation diagram through antiphased phase control of output signal. The article proposes and presents HFS block diagram with noise autocompensation. The principle of operation of the diagram and functionality of its constituent blocks are examined. The diagram implements hybrid synthesis method and is based on PLL system with DDS. The diagram contains two paths: control and autocompensation ones. Compensatory phase control of output signal is done in controllable phase shifter. The control path uses phase deviation data provided by PLL phase detector. The autocompensation path makes possible extra suppression of phase noise arising in DDS. The output signals of both paths generate control signal of controllable phase shifter. HFS structural model for four types of destabilizing effects is constructed. Transfer functions of hybrid synthesizer with autocompensation diagram are obtained. The synthesizer stability with diagram different parameters is investigated based on frequency-response method for three inertia types of component elements. HFS research of its amplitude-frequency and phase-frequency characteristics is performed. Conditions for maximum phase noise compensation are defined.