We investigate on the wideband phase-modulation to amplitude-modulation (PM-AM) conversion based on the chromatic dispersion in fiber. To overcome the shortcomings of the single-tone or dual-tone modulation-based model in previous researches, we present a more intuitive time-frequency analysis method for the propagation of phase-modulated signals in dispersive fibers, and give the physical picture for the temporal waveform changes. By analyzing the amplitude variation near the transition zone, we establish a bit-by-bit correspondence between the pulse waveforms and the actual modulated data, and realized the non-return-to-zero (NRZ) differential phase-shift keying (DPSK) demodulation. Furthermore, the effect of fiber length and bit rate on PM-AM conversion is also investigated quantitatively and experimentally.