Close-in Phase Noise of a Digitally Tuned VCO

• Published in: 2006 49th IEEE International Midwest Symposium on Circuits and Systems Vol. 1; pp. 51 - 55
• Main Authors: Yang, Long, Pedrotti, Ken
• Format: Conference Proceeding
• Language: English
• Published: IEEE 01.08.2006
• Subjects: Analytical models; Delta-sigma modulation; Digital control; Digital modulation; Frequency; Phase locked loops; Phase modulation; Phase noise; Tuning; Voltage-controlled oscillators
• ISBN: 1424401720; 9781424401727
• ISSN: 1548-3746; 1558-3899
• DOI: 10.1109/MWSCAS.2006.381992

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.