Determining Specifications for an External GPS Reference Oscillator

Avrarn K. Tetewsky, Jeffery B. Lozow, and Karl. W. Flueckiger

Abstract:	There are many test and development situations in which a GPS receiver is operated with an external frequency reference that is phase locked to a GPS simulator or to a second receiver. Although one might intuitively predict that receiver performance could be enhanced by using a high quality commercial frequency source, the application of this source can actually degrade performance as evidenced by reduced signal to noise ratio (SNR) measurements, increased bit detection errors, and related operational failures. The cause of this degradation may be the frequency synthesizer that converts the reference frequency to the unique master ikx-pency required by a specific receiver. These degradations are generally the result of residual high frequency phase noise introduced by tuneable frequency synthesizers. In this paper, we quantitatively model the effects that reference frequency phase noise has on the receiver’s master timing chain, specifically. L band to base-band down-conversion errors and user clock errors. These timing chain errors then impact GPS receiver phase tracking, bit detection, and SNR estimation. Although a GPS receiver can compensate for “low frequency” noises by calculating the oscillator’s bias and drift-rate, it can’t compensate for excess high frequency phase noise. Using the ardaysis presented here, we can then understand why high frequency side-band performance or small time difference Allan variance performance typical of a high quality reference standard is needed, and why care is needed in selecting a general purpose synthesizer. Measurements with a MITELJPlessy GPS Builder-2 kit and P(Y) COTS receiver driven by an external oscillator will be presented to illustrate the analysis. The main contributions of this work are to: 1) derive the industry rule-of-thumb of maintaining -80 to -100 dBc sidebands over the GPS front-end bandwidth and the oscillator’s bandwidth fh; 2) understand how the upper bandwidth of the oscillator noise spectrum, fh, enters into the problem, 3) translate oscillator specifications in Allan variance and other domains into the sideband domain that many circuit designers seem to prefer.
Published in:	Proceedings of the 12th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GPS 1999) September 14 - 17, 1999 Nashville, TN
Pages:	339 - 352
Cite this article:	Tetewsky, Avrarn K., Lozow, Jeffery B., Flueckiger, Karl. W., "Determining Specifications for an External GPS Reference Oscillator," Proceedings of the 12th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GPS 1999), Nashville, TN, September 1999, pp. 339-352.
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