  We present time-frequency analysis, based on the Hilbert-Huang transform
(HHT), of the evolution on the low-frequency quasi-periodic oscillations
(LFQPOs) observed in the black hole X-ray binary MAXI J1820+070. Through the
empirical mode decomposition (EMD) method, we decompose the light curve of the
QPO component and measure its intrinsic phase lag between photons from
different energy bands. We find that the QPO phase lag is negative (low energy
photons lag behind high energy photons), meanwhile the absolute value of the
lag increases with energy. By applying the Hilbert transform to the light curve
of the QPO, we further extract the instantaneous frequency and amplitude of the
QPO. Compared these results with those from the Fourier analysis, we find that
the broadening of the QPO peak is mainly caused by the frequency modulation.
Through further analysis, we find that these modulations could share a common
physical origin with the broad-band noise, and can be well explained by the
internal shock model of the jet.
