One embodiment of the present invention provides a synthesizer. The synthesizer includes one or more tunable oscillators, a frequency-dividing circuit coupled to the tunable oscillators, and a multiplexer coupled to the frequency-dividing circuit. The frequency-dividing circuit includes a number of frequency dividers, and is configured to generate a number of frequency-dividing outputs. At least one frequency-dividing output has a different frequency division factor. The multiplexer is configured to select a frequency-dividing output.