Comparative Analysis of Noise Characteristics in High-Frequency Signal Generators Using Digital-to-Analog Converters

Introduction. The advancement of modern digital technology, the increase in productivity and operating clock frequency, and the development of specialized architectures for generating output signals have led to the creation of high-speed digital-to-analog converters (DACs). These converters are a crucial component of digital computing synthesizers, which are used to develop digital radio frequency signal generators. Goal. This paper aims at comparing methods for constructing digital signal generators based on high-speed DACs and analyze their noise characteristics. Methodology. The study presents various configurations for designing digital signal generators using high-speed DACs, including configurations with a mixer, a quadrature modulator, and special operating modes that enhance the efficiency of utilizing reference frequency images. Results. Analysis of simulation results indicates that the third configuration – a digital generator based on a high-speed DAC operating in special modes – exhibits the lowest level of phase noise. In RFZ3 mode, this configuration shows a phase noise level that is 10-14 dBn/Hz lower than that of the quadrature modulator-based generator and approximately 6-8 dBn/Hz lower than the mixer-based configuration. In RFZ4 mode, when utilizing the third positive image, a reduction of 15-22 dBn/Hz below the power spectral density (PSD) phase noise of the quadrature modulator-based generator and about 15-20 dBn/Hz below the mixer-based configuration is achieved. Additionally, the highest efficiency in utilizing images is obtained in higher Nyquist zones, where the increase in the output signal spectrum's envelope in special modes surpasses that in the DAC's primary NRZ mode. Conclusion. To effectively employ a digital signal generator based on high-speed DACs, frequency planning and the selection of optimal modes for analog signal restoration according to the image number used are necessary. Practical significance. The evaluation of the noise characteristics of the considered configurations, based on the proposed models, demonstrates that the use of the special operating modes of the DAC can significantly reduce the phase noise level of signal generators. Moreover, the use of high-speed DACs allows for the exclusion of analog elements from the signal generator structure, thereby significantly enhancing energy efficiency.