Spectrograms have been used for time-resolved analysis of Fourier power spectra in various fields. We have performed spectrogram analysis for two neighboring modes and a low frequency mode changing gate width of the time gate function. The obtained spectrogram of the former showed both amplitude and frequency modulations between two neighboring modes. A contamination appearing on the spectrogram of the latter reflects an artifact showing beat between the signal mode and a negative frequency mode whose sign was inverted from the signal mode. Surprising results from the amplitude and phase modulations were observed in the simulation even in the case of single mode. This puzzling result was explained in terms of the interference between the single mode and the "negative" frequency composing the single mode with the corresponding "positive frequency."