A digital transmitter includes: a plurality of adjustable delay lines arranged to delay a plurality of digital input signals by a plurality of delay times to generate a plurality of delayed digital input signals respectively; a plurality of converting devices arranged to convert the plurality of delayed digital input signals into a plurality of converting signals respectively; and a calibration device arranged to adjust a delay time of at least one adjustable delay line in the plurality of adjustable delay lines to make the plurality of converting devices convert the plurality of delayed digital input signals at respective desire time points.