A Direct-Sampling Digital-Downconversion Technique for a Flexible, Low-Bias GNSS RF Front-end

J. York, J. Little, D. Munton

Abstract:	Many GNSS receivers utilize an analog heterodyne ap- proach to convert ranging signals from L-band down to a intermediate or baseband frequency for digital sam- pling. This approach, while often simple and power- e_cient, is reliant on analog circuitry both for local os- cillator generation and for _ltering. Typically much of this analog circuitry must typically be duplicated per frequency band, thereby limiting future upgradabil- ity and introducing greater relative uncertainty in the receiver bias terms. While the technique of severely under-sampling to directly sample the L-band signal is well-known in academic literature, this scheme re- lies upon clever assumptions about signal frequencies and high-Q, band-speci_c, analog _lters to avoid suf- fering from aliased signal and noise power. Thus, this approach does not ful_ll much of the promise of direct- sampling. In this paper, we demonstrate an alternate approach enabled by recent advances in integrated circuit hard- ware: direct sampling of the L-band at approximately 2 Gigasamples per second, followed by real-time di- rect digital down conversion inside of an Field Pro- grammable Gate Array (FPGA). The single analog signal path is shared by all L-band signals, and thus hardware biases are minimized and the entire L-band from approximately 1 to 2 GHz is sampled without aliasing ambiguity, thereby allowing the addition of new signal bands within this range with a _rmware upgrade. The paper begins with a discussion the sig- nal processing and hardware capabilities required for this technique, followed by detailed algorithmic dis- cussion. The paper concludes by discussing the de- sign and initial data from a 1U rack mount hardware implementing this strategy referred to as the L-Band Digitizing Front End (DFE).
Published in:	Proceedings of the 23rd International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GNSS 2010) September 21 - 24, 2010 Oregon Convention Center, Portland, Oregon Portland, OR
Pages:	1905 - 1918
Cite this article:	York, J., Little, J., Munton, D., "A Direct-Sampling Digital-Downconversion Technique for a Flexible, Low-Bias GNSS RF Front-end," Proceedings of the 23rd International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GNSS 2010), Portland, OR, September 2010, pp. 1905-1918.
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