The method of Synchronized Quantum Beat Spectroscopy has been improved by introducing the amplitude modulation on the excitation pulse train. By using an amplitude-modulated train of picosecond pulses from a cw mode-locked dye laser, we obtain resonance enhancement of the substate coherence (quantum beat) at f 0 = n f p + n ' f m ( n , n ' : integer, f 0 : sublevel splitting, f p : pulse repetition frequency, f m : modulation frequency), which can be detected by monitoring the optical anisotropy due to the substate coherence. Resonances can be obtained in the frequency domain by sweeping f m . Pressure shift coefficients of the hyperfine splitting in Na 3 2 S 1/2 are measured to be 82, 61, -1 and -47 Hz/Torr for He, N 2 , Ar and Kr, respectively, within an accuracy of ±3 Hz/Torr.