A new NCO model for tracking loop design in GNSS software receivers

Traditional NCO models in PLL update the local initial phase estimation at every update interval based on the assumption that signal frequency estimation is accurate. The waveform generated by these models will be discontinuous at the update instant. We propose a new NCO model which adjusts both the frequency and phase of the local carrier to maintain waveform continuity. We denote the traditional method as phase feedback NCO, while our proposed method as frequency feedback NCO. Simulation results show that our proposed frequency feedback NCO model has wider pull-in range and larger equivalent bandwidth when compared with traditional phase feedback NCO model. The relationship between bandwidth and filter parameters is also derived. The formulas can be used to guide the selection of the filter parameters given desired bandwidth.