Close-in Phase Noise of a Digitally Tuned VCO
Here we present analysis and simulation of the close-in phase noise of a digitally tuned VCO, tunable only in discrete frequency steps. By rapidly tuning between two closely spaced discrete frequencies with a variable duty cycle any average frequency can be achieved. This process will result in incr...
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Published in | 2006 49th IEEE International Midwest Symposium on Circuits and Systems Vol. 1; pp. 51 - 55 |
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Main Authors | , |
Format | Conference Proceeding |
Language | English |
Published |
IEEE
01.08.2006
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Subjects | |
Online Access | Get full text |
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Summary: | Here we present analysis and simulation of the close-in phase noise of a digitally tuned VCO, tunable only in discrete frequency steps. By rapidly tuning between two closely spaced discrete frequencies with a variable duty cycle any average frequency can be achieved. This process will result in increased phase noise and spurs if a strictly periodic duty cycle modulation is used. Ideally a randomly varying digital control signal with the appropriate duty cycle should be used to eliminate spurs and smooth the close-in phase noise. Here we use continuous-phase-frequency-shifting-key (CPFSK) modulation results from communication theory to analytically predict the effect of a random modulation at 50% duty cycle. Next the use of a sigma-delta modulator to produce a randomly varying digital signal with continuously variable duty cycle is simulated and compared to our analytical result. The use of a simple repetitive duty cycle modulation produced by a phase detector controlling the VCO in a PLL is also investigated. We present a comparison of the close-in phase noise for these possible approaches to the realization of an all digital PLL and demonstrate the improvement possible with the use of sigma-delta modulation. |
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ISBN: | 1424401720 9781424401727 |
ISSN: | 1548-3746 1558-3899 |
DOI: | 10.1109/MWSCAS.2006.381992 |