A low noise PLL-based frequency synthesiser for X-band radar
Thesis (MScEng (Electrical and Electronic Engineering))--Stellenbosch University, 2008.
This thesis discusses the design, development and measured results of a phase-locked loop based frequency synthesiser for X-band Doppler radar. The objective is to obtain phase noise comparable or lower than that typically achieved with direct analogue frequency synthesis techniques. To meet this objective, a theoretical study of the noise contributions of individual components of the synthesiser and their effect on the total phase noise within and outside the loop bandwidth of the PLL is performed. 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-frequency detector reference frequency, loop bandwidth, adequate phase margin, and suitable components are chosen for optimal phase noise performance. The total phase noise at the output of the synthesiser is calculated 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 frequency multiplier, as well as to a combline and interdigital bandpass filter required for the frequency synthesiser. In the first case, a piecewise linear circuit model is used to model the behaviour of the nonlinear multiplier circuit. Fourier theory is used to calculate the large-signal driving point input and output impedances of the nonlinear circuit, enabling the computation of the circuit elements for the input and output matching networks. The measured response of the frequency multiplier under various different operating conditions is presented and discussed. The design of the microwave bandpass filters is based on the theory of coupling and external quality factors. To aid in the verification and optimisation of the design, a software simulation tool is used. The presented S-parameter measurements 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 components under 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 investigated and discussed. High power spurious components at some output frequencies are indentified and reduced. The feasibility of using the developed prototype phase-locked loop based frequency synthesiser for coherent X-band Doppler radar is discussed and demonstrated.