Research On Key Technology Of High Performance Microwave Frequency Synthesizer And Millimeter Wave FMCW RF Front-end

Microwave and millimeter wave frequency synthesizer is an indispensable key unit in electronic systems such as radar,communication,electronic countermeasures and test and measurement equipment.Its phase noise,spur suppression and ramp linearity have great impact on the performance of the systems.As the development of RF front-ends moves toward higher frequency bands and wider bandwidths,more stringent demands have been placed on the performance of frequency synthesizers.In order to achieve high-performance microwave frequency synthesizers and millimeter-wave RF front-ends,theoretical issues and key technologies in frequency synthesizers and millimeter wave frequency modulated continuous wave(FMCW)radio frequency(RF)front-end have been studied.The developed frequency synthesizers mainly involve delta-sigma-modulator(DSM)-based wideband fractional-N phase-locked loop(PLL),direct digital frequency synthesizer(DDS),DDS-driven PLL and low phase noise mixer-PLL.An in-depth study has been carried out in many aspects,including phase noise theoretical model,measurement method of the ramp linearity,spur suppression techniques,the influence of phase noise and ramp linearity on the performance of RF front-ends,and the phase noise cancellation effect in coherent millimeter wave front-ends.The main progresses of this dissertation are as follows:1.Phase noise of DSM wideband fractional-N PLL based on high-order active loop filter in linear ramp state is theoretically modeled utilizing the linear time-invariant continuous time model(LTI-CTM).The relationship between the steady-state phase error of the PLL in linear ramp state and the loop parameters is obtained by combining the transfer function of the loop filter with the linear ramp steady-state condition.The theoretical model of the steady-state phase error of the PLL based on traditional low-order passive loop filter is extended to analyze phase noise of the wideband frequency ramp PLL based on more commonly used third-order or fourth-order active loop filter.The conditions under which the ramp slope and loop parameters should be met given a steady-state phase error are obtained.A dual frequency X-band frequency synthesizer and a C-band wideband frequency ramp synthesizer based on DSM fractional-N PLL technology are designed.Their phase noise,spur suppression and other specifications are simulated,tested and analyzed.The phase noise of the signal at the divider output of the wideband linear frequency ramp fractional-N PLL is measured under different charge pump gains and ramp slopes,which verifies the theoretical model of phase noise in the ramp state.2.A methodology for the measurement of the linearity of wideband linear ramp signals with high precision is proposed in this dissertation.It is based on a segmented sampling process combined with a signal processing scheme using the Weaver receiver architecture in digital domain.In this method,the high-frequency wideband linear ramp signal is down-converted into an intermediate ramp signal that can be directly sampled by switching the frequency of the local oscillator.The sampling process of the ramp signal with a specified time length is controlled by a ramp synchronizing signal and a delayed sampling trigger signal.The first local oscillator signal of the Weaver receiver is an ideal ramp signal obtained by linearly fitting the sampled ramp signal in digital domain.After the mixing process,a low intermediate frequency signal with its frequency approximate to a constant value is obtained.This signal processing scheme solves the problem of filtering the wideband ramp signal and suppression image interference signals.As a result,the impact of the amplitude noise and spurs on the demodulation accuracy of instantaneous frequency is reduced.The accuracy and reliability of the proposed method is demonstrated by theoretical analysis and simulation.In order to verify the measurement method experimentally,a high linearity DDS and a DDS driven wideband PLL are designed.Experimental results show that frequency step,time step and ramp slope are important factors affecting the linearity.The above method is also applied to investigate the effects of loop bandwidth,charge pump gain and ramp slope on the linearity of the DSM-based wideband fractional-N PLL.The proposed linearity measurement method provides a low-cost,simple,practical and high-precision linearity measurement technique for the development and production of linear FMCW radars.3.A comprehensive analysis of common sources of spurious signals in frequency synthesizers and RF transceivers has been carried out and a series of techniques such as optimizing frequency allocation and PLL loop bandwidth,bandpass/band-stop filter,absorptive filter,and optimizing RF schemes have been studied for improving signal purity.These techniques provide a rich and flexible technical means for the subsequent development of high-performance radar and communication systems.In order to reduce the size of traditional waveguide filters and facilitate integration with planar circuits,several novel high-performance filters and transition circuits have been designed based on air-filled substrate integrated waveguide(AFSIW).In addition,a low phase noise oscillator based on high quality factor AFSIW resonators is designed.Its phase noise is improved by approximately 10 d B than that of a conventional SIW oscillator due to the elimination of dielectric losses inside the resonator.4.The phase noise cancellation mechanism of the FMCW radar system constructed with coherent local oscillator architecture is studied in depth.A complete phase noise analytical model including the phase noise of the local oscillator and the additive phase noise of the RF chains is proposed.The phase noise characteristics at each node in the system and the cancellation effect are analyzed in detail.The impact of the additive phase noise of the system on the phase noise of the receiver output signal is discussed.The above analyses provide a theoretical basis for the design of coherent local oscillators and the estimation of phase noise of systems.For the application of short-millimeter wave human body security active imaging radar,a C-band low phase noise dual frequency synthesizer using mixer-phase-locked architecture is designed and successfully applied to the W-band FMCW transceiver front-end.The measured phase noise cancellation ratio of the W-band front-end is above 17 d B,which verifies the established phase noise model and theoretical analysis results.5.A system scheme of W-band FMCW radar RF front-end based on wideband DSM fractional-N PLL is worked out for the development of foreign object debris(FOD)detecting radar on airport runways.The key technical specifications of the RF chains such as transmitting power,receiving sensitivity,and receiving input power range are determined according to the system parameters including effective isotropic radiated power(EIRP),detection range,radar cross section(RCS)of FOD and resolution.Based on a detailed analysis of the influence of phase noise and ramp linearity on the performance of the FMCW radar,the scheme and related specifications of the local oscillator with fixed frequency and the ramp frequency synthesizer with triangular modulation manner are determined.After the detailed design of the RF front-end scheme and the development of key circuit modules,the system integration and testing are carried out.The developed RF front-end system has capabilities of self-test,status monitoring and receiving protection.The RF front-end is integrated with an antenna system to realize the FOD detection radar front-end prototype,based on which a series of FOD samples are successfully detected in the laboratory.