Low-Noise Hybrid Frequency Synthesizers Based on Direct Digital and Direct Analog Synthesis

Hybrid frequency synthesizers based on direct methods of digital and analog synthesis were examined. The possibilities of applying synthesizers as heterodynes of modern spectrum analyzers built on the superheterodyne plan were studied. The possibility of using a hybrid synthesizer as the heterodyne of the first cascade of spectrum analyzers was analyzed. The requirements for heterodynes of the first cascades of the mixing of spectrum analyzers are provided. A structural diagram of a broadband heterodyne generating frequencies in the range 4000–8000 MHz with a step not exceeding 1 Hz is presented. Formulas to calculate the fundamental frequency relations in the heterodyne structure were developed. A mathematical model of the power spectrum density of phase noises as a function of the offset frequency from the carrier was developed. With the aid of the created model, the noise characteristics of the proposed layout was examined. It is shown that at offset frequencies over 1 kHz from the carrier, a hybrid synthesizer based on direct digital and analog methods of synthesis has an advantage at a phase noise level 5–30 dB over low-noise heterodynes of modern spectrum analyzers. The advantages of the suggested layout by which the hybrid synthesizer is capable of improving the technical characteristics of modern spectrum analyzers are defined.