Improving tracking performance of PLL in high dynamic applications
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AbstractThe Phase-locked loop (PLL) is used in GPS receivers to track an incoming signal and to provide accurate carrier phase measurements. However, the PLL tracking performance and measurement accuracy are affected by a number of factors, such as signal-to-noise power ratio, Doppler frequency shift, the GPS receiver's jitter caused by vibration, and the Allan deviation. Among these factors, the thermal noise and Doppler shift are the most predominant and have a large influence on the design of the PLL. In high dynamic situations, the conflict between improving PLL tracking performance and the ability to track the signal necessitates some compromises in PLL design. This thesis investigates the strategies to resolve this conflict. Three methods are investigated to improve PLL tracking performance in high dynamic applications: a Kalman filter-based tracking algorithm, application of a wavelet denoising technique in PLL, and an adaptive bandwidth algorithm. The Kalman filter-based tracking algorithm makes use of a carrier phase dynamic model and a measurement from the output of the discriminator to estimate the phase difference between the incoming signal and the Numerical Controlled Oscillator (NCO) output, Doppler frequency and the change rate of Doppler frequency. The wavelet de-noising technique effectively decreases the noise level and allows broadening of the PLL bandwidth to track high dynamics signals. The adaptive bandwidth PLL algorithm adapts the bandwidth of the PLL according to the estimation of the incoming signal dynamics and noise level. The performance is evaluated in terms of signal-to-noise ratios and dynamic variations using simulating signals. The first two methods are found to produce better improvements when the signal-to-noise ratio is low and the signal dynamic is high. The third method works well under high signal-to-noise ratios and less random dynamic variations. The results show that these methods outperform the ordinary PLL under high dynamic conditions and the resulting carrier phase measurement is more accurate.
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