Coarse tuning for fractional-N synthesizers having reduced period comparison error
First Claim
1. A fractional-N frequency synthesizer comprising:
- a) a phase lock loop (PLL) adapted to provide a CO output signal having a frequency determined by a reference signal and a divide value, the PLL comprising;
i) a controlled oscillator (CO) adapted to provide the CO output signal; and
ii) a first divider adapted to divide the CO output signal by the divide value to provide a divided CO signal;
b) coarse tuning circuitry comprising a second divider adapted to divide the reference signal by a factor M to provide a divided reference signal; and
c) synchronization circuitry adapted to synchronize the first and second dividers based on a clock signal derived from the CO output signal.
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Abstract
An improved coarse tuning process for fractional-N frequency synthesizers is provided. In general, a coarse tuning circuit controls a phase lock loop (PLL) of a frequency synthesizer. During coarse tuning, a reference signal used to control an output frequency of the PLL is provided to the coarse tuning circuitry and is divided by a factor M to provide a divided reference signal. A controllable oscillator (CO) output signal from a CO in the PLL is divided by an N divider in the PLL to provide a divided CO signal. The periods or, equivalently, frequencies of the divided CO signal and the divided reference signal are compared, and the result is used to select an appropriate tuning curve for the CO. In order to reduce a period comparison error, synchronization circuitry operates to synchronize the N divider of the PLL and an M divider of the coarse tuning circuit.
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Citations
26 Claims
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1. A fractional-N frequency synthesizer comprising:
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a) a phase lock loop (PLL) adapted to provide a CO output signal having a frequency determined by a reference signal and a divide value, the PLL comprising; i) a controlled oscillator (CO) adapted to provide the CO output signal; and ii) a first divider adapted to divide the CO output signal by the divide value to provide a divided CO signal; b) coarse tuning circuitry comprising a second divider adapted to divide the reference signal by a factor M to provide a divided reference signal; and c) synchronization circuitry adapted to synchronize the first and second dividers based on a clock signal derived from the CO output signal. - View Dependent Claims (2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17)
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18. A method for improving coarse tuning of a fractional-N frequency synthesizer comprising:
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a) generating a controlled oscillator (CO) output signal based on a reference signal and a divide value; b) dividing the CO output signal by the divide value to provide a divided CO signal; c) dividing the reference signal by a factor M to provide a divided reference signal; d) synchronizing dividing steps (b) and (c) based on a clock signal derived from the CO output signal; and e) providing coarse tuning of a CO providing the CO output signal based on a comparison of the divided reference signal and the divided CO signal. - View Dependent Claims (19, 20, 21, 22, 23, 24, 25, 26)
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Specification