The use of images of DDS in the hybrid frequency synthesizers

The principle of the hybrid frequency synthesizer performed on the basis of a phase-locked loop and direct digital synthesizer is considered. The synthesizer uses images of the fundamental frequency and performs a frequency conversion with a mixer in the feedback path. The mathematical expressions for the model of power spectral density of the phase noise of the synthesizer are obtained. The results of simulation of noise curves of the synthesizer for different parameters of synthesized signals are given. A comparison of noise curves of the synthesizer, a single-loop PLL and a two-loop PLL derived from similar conditions are performed. It is shown that the phase noise level of the hybrid frequency synthesizer is significantly lower than phase noise of single-loop PLLs and two-loop PLLs.