A signal generator for generating a clock with lower jitter. The signal generator includes a multi-phase clock generator for generating a plurality of multi-phase reference clocks with same frequency, a multiplexer for selecting one reference clock as an output clock according to a phase selecting signal, a phase-swallow control unit having a comparator for comparing a swallow value with a reference value out of order and outputting the comparing result as a swallow control signal, and a clock selector for receiving the swallow control signal and generating the phase selecting signal. Because the reference value is provided by a counter in bit-reversed, the swallow control signal is dispersed smoothly and the jitter of the output clock is reduced.