| 摘要 |
A frequency synthesizer that includes an output oscillator, a difference circuit, a reference circuit, a feedback circuit and a comparator. The output oscillator generates an output signal whose frequency, FOUT, is determined by an oscillator input signal. The difference circuit generates a sampled signal having a frequency, Fsif=FOUT-FOFFSET. The reference circuit generates a reference signal having a frequency Fref. The reference circuit includes a reference oscillator for generating a high frequency reference oscillator output signal having a frequency F1 and a first division circuit for generating a signal having a frequency equal to F1 divided by R. The feedback circuit generates a feedback signal having a frequency, Ffb. The feedback signal generating circuit includes a second division circuit having a division factor chosen such that Fsif/Ffb is minimized. The comparator compares the reference signal and the feedback signal and generates the oscillator input signal. One of the first and second division circuits is a Fractional-N Divider having a division factor, N.F, greater than 10. If the second division circuit is the Fractional-N Divider, the feedback circuit includes a multiplication circuit for generating a multiplied signal having a frequency equal to X times Fsif, and the Fractional-N Divider generates the feedback signal from the multiplied signal. In the preferred embodiment of the present invention, the first division circuit is the Fractional-N Divider. In one embodiment of the invention, the input to the Fractional N Divider varies over time thereby causing the output oscillator to sweep through a corresponding frequency range.
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