Mathematical modeling of noise properties for high-frequency signal generators based on high-speed digital-to-analog converters

Abstract

The development of current digital systems, growth of performance and operating clock frequency, as well as developments of specific designs for forming output signal has resulted in making high-speed digital-to-analog converters (DAC). They make the most important part of direct digital synthesizer (DDS) based on which digital radio-frequency signal shapers are developed. The design improvement of digital-to-analog radio frequency converters brought about development of a few new operating modes, optimal for using extraneous high-frequency components of output signal spectrum - fundamental frequency images. Using fundamental frequency images enables to increase output signal frequency of the whole shaper, and applying DAC new operating modes enables here to increase signal-to-noise ratio without using additional devices. Main operating modes used in high-speed DACs for digital forming of broadband radio signals are examined. We can single out the following among them: non-return-to-zero, return-to-zero, radio frequency, radio frequency return-to-zero and its variation as radio frequency return-to-zero 2. Previously developed methods of modeling power spectral density (PSD) of DDS phase noise when using fundamental frequency images ignore high-speed DACs new operating modes. Mathematical model of power spectral density for phase noise of signal shaper based on digital computing synthesizer, which comprises a high-speed digital-to-analog converter in special operating modes is proposed. Research findings of special operating modes’ impact of high-speed DACs on noise properties of high-frequency signal shapers are given. Transmission ratios of noise properties of high-speed digital-to-analog converters for various operating modes are derived and dependences of these ratios on duty cycle and image number are investigated. The research of developed mathematical model for power spectral density of signal shapers’ phase noise with high-speed digital-to-analogue   converters showed that using special operating modes enhances application efficiency of fundamental frequency images in direct digital synthesizers for forming radio signals and it improves signal shaper’s noise properties.