Fractional-N-PLL frequency synthesizer and phase error canceling method therefor

摘要

A fractional-N-PLL frequency synthesizer (100) that reduces the spurious level of a frequency signal (fvco) caused by a phase error is provided. A reference phase error is determined from a plurality of phase errors (Δt0 - Δt7) generated between a reference signal (fr) and a comparison (fp) signal when the fractional-N-PLL frequency synthesizer is locked. Then, any phase error equal to or smaller than the reference phase error is canceled. The fractional-N-PLL frequency synthesizer may comprise a first phase comparator (11) for generating a phase difference signal by comparing a reference signal with a comparison signal, a charge pump (14) for receiving the phase difference signal from the phase comparator and converting the phase difference signal into a voltage signal, a low-pass filter (15), connected to the charge pump, for smoothing the voltage signal to generate a voltage control signal, a voltage controlled oscillator (16), connected to the low-pass filter, for generating a frequency signal having a frequency according to the voltage control signal, and a variable frequency divider (17a), connected to the voltage controlled oscillator, for frequency-dividing the frequency signal to generate the comparison signal. A plurality of phase errors including a predetermined reference phase error are generated between the reference signal and the comparison signal when the fractional-N-PLL frequency synthesizer is locked. A canceling circuit (10, 12, 13b) is connected to the variable frequency divider, for canceling any of the plurality of phase errors equal to or smaller than the predetermined reference phase error.