A low noise PLL-based frequency synthesiser for X-band radar

[摘要] This thesis discusses the design, development and measured results of a phase-locked loop basedfrequency synthesiser for X-band Doppler radar. The objective is to obtain phase noise comparableor lower than that typically achieved with direct analogue frequency synthesis techniques. To meetthis objective, a theoretical study of the noise contributions of individual components of the synthesiserand their effect on the total phase noise within and outside the loop bandwidth of the PLL isperformed. The effect of different phase margins on the closed-loop frequency response of the PLL,and hence the total phase noise, is investigated. Based on the results, an optimal phase-frequencydetector reference frequency, loop bandwidth, adequate phase margin, and suitable components arechosen for optimal phase noise performance. The total phase noise at the output of the synthesiser iscalculated and it is shown that the phase noise specification can be met.A significant part of this thesis is devoted to the design, modelling and characterisation of a frequencymultiplier, as well as to a combline and interdigital bandpass filter required for the frequencysynthesiser. In the first case, a piecewise linear circuit model is used to model the behaviour of thenonlinear multiplier circuit. Fourier theory is used to calculate the large-signal driving point inputand output impedances of the nonlinear circuit, enabling the computation of the circuit elements forthe input and output matching networks. The measured response of the frequency multiplier undervarious different operating conditions is presented and discussed. The design of the microwavebandpass filters is based on the theory of coupling and external quality factors. To aid in the verificationand optimisation of the design, a software simulation tool is used. The presented S-parametermeasurements of the filters show how well the theory matches with what is obtained in practice.The measured spectral and phase noise response of various components comprising the synthesiser,are discussed. These measurements provide insight into the response of individual componentsunder different operating conditions and show the behaviour of important subsystems of the synthesiser.The thesis culminates in the presentation of the measured phase noise of the complete synthesiser.It is shown how well the measured phase noise correlates with the calculated phase noise.In addition, the measured spectral content and transient behaviour of the synthesiser are investigatedand discussed. High power spurious components at some output frequencies are indentifiedand reduced. The feasibility of using the developed prototype phase-locked loop based frequencysynthesiser for coherent X-band Doppler radar is discussed and demonstrated.

[发布机构] Stellenbosch University