Digital Phase Locked Loop with Frequency Rate Feedback

P.L. Kazemi , C. O'Driscoll, G. Lachapelle

Abstract:	In a traditional loop at each update interval the frequency of the numerically controlled oscillator (NCO) is set to its new value and this frequency remains unchanged during the integration time. As a result, the continuously changing frequency of the signal tracked is followed by the NCO in a staircase fashion. The frequency mismatch between the locally generated signal and the incoming signal attenuates the apparent received power. This is especially the case in loops with high integration times where the update rate of the loop is low. One way of reducing these Doppler-related losses is to compensate for the Doppler effect at each update interval using some kind of frequency-rate estimator. A combination of Kalman Filter (KF) and Digital Phase Lock Loop (DPLL) is proposed where the KF estimates the frequency rate and feeds this information back to the NCO to change the frequency of the locally generated signal even during each update interval. The additional feedback of the frequency rate to the NCO raises stability issues in high integration times; hence the poles of the frequency rate estimator should be compensated to ensure stability. The NCO transfer function and the effect of the frequency rate estimator in are modeled precisely in the Z-domain and the loop filter is designed to optimize the overall loop transfer function and to overcome the stability issue raised by high integration time and additional feedback to the loop. The performance of these approaches is demonstrated with line of sight live GPS signals and for coherent integration times of up to one second. . It is shown that the phase mismatch between the locally generated signal and the incoming signal is reduced significantly in high integration times, hence the coherent integration time in a closed-loop tracking architecture can be increased without any significant correlation loss.
Published in:	Proceedings of the 22nd International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GNSS 2009) September 22 - 25, 2009 Savannah International Convention Center Savannah, GA
Pages:	201 - 208
Cite this article:	Kazemi, P.L., O'Driscoll, C., Lachapelle, G., "Digital Phase Locked Loop with Frequency Rate Feedback," Proceedings of the 22nd International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GNSS 2009), Savannah, GA, September 2009, pp. 201-208.
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