Design Of Direct Digital Radio Frequency Modulator

Direct digital radio frequency modulator is a modulator architecture which can directly modulate and quantize baseband analog signals into band-limited digital radio frequency signals.It is widely favored due to its advantages of high integration,low power consumption,reconfigurability,and low cost.Limited by structural issues,direct digital radio frequency modulators can usually only transmit at a single frequency point,and the signal spectrum spreading under large bandwidth limits the modulator performance;in a dual-band modulation system,the difficulty of dual-band noise suppression results in the inability to balance transmission efficiency and modulation quality.To improve the transmission frequency flexibility and transmission efficiency of the direct digital radio frequency modulator,this thesis proposes a multi-band reconfigurable direct digital radio frequency modulator based on the Delta-Sigma scheme.The main research works are as follows:1.This thesis loriefty explained the basic architecture and application advantages of the all-digital modulation scheme,as well as the principle discussion and structure classification of PWM modulation technology and Delta-Sigma modulation technology.This thesis focuses on the analysis of noise,and discusses the suppression characteristics of key design parameters such as oversampling rate,modulator order,and quantizer bits.On this basis,it is further promoted to the band-pass Delta-Sigma modulator,and its design method and working characteristics are discussed in detail to pave the way for the design of direct digital radio frequency modulators.2.Based on the structural improvements,bandpass and dual-band modulation structures are proposed,which expand the frequency range of the single-band system and greatly compress the in-band noise of the dual-band system.By reasonably adding a feedback branch inside the traditional low-pass modulation structure,the zero point distribution of the noise transfer function is adjusted to achieve multi-frequency coverage.The designed band-pass modulation structure transmission frequency range can be expanded to 1.5GHz ? 2.5GHz.The two bandpass modulators share the quantizer and feedback branch to construct the double stopband noise transfer function.The simulation shows that the dual-band noise suppression effect can reach-70 d B.This modulation structure not only inherits the wide coverage of the band-pass structure,but also supports independent modulation and transmission of dual-mode signals.3.To sovle the problem of unstable signal quality of direct digital radio frequency modulators,a multi-parameter optimization strategy based on multi-sample statistical feature extraction is proposed.Through nuclear density analysis,the optimal parameters are extracted to improve system stability.After optimizing the simulation,the ACPR index of the band-pass modulation system has been improved by 5d B ? 10 d B,up to-72 d B,and the ACPR index of the dual-band modulation system is increased by 4d B ?8d B,up to-68 d B.After FPGA design is implemented,an experimental environment is set up to test the specifications of the three modulators designed respectively.The results show that the three modulation systems all meet the design specifications,with ACPR of-35.33 d B ?-51.09 d B and EVM of 3.03% ? 8.67%,meeting the signal transmission requirements.In sum up,the direct digital radio frequency modulator designed in this thesis can modulates baseband signal to digital radio frequency signal into all digital well.By improving the Delta-Sigma algorithm and optimizing system parameters,the transmission efficiency of the system is improved and the performance is stable,which is of great significance to the development and application of direct digital radio frequency modulators.