  Accurate calculation of the phase lags of quasi-periodic oscillations (QPOs)
will provide insight into their origin. In this paper we investigate the phase
lag correction method which has been applied to calculate the intrinsic phase
lags of the QPOs in MAXI J1820+070. We find that the traditional additive model
between BBN and QPOs in the time domain is rejected, but the convolution model
is accepted. By introducing a convolution mechanism in the time domain, the
Fourier cross-spectrum analysis shows that the phase lags between QPOs
components in different energy bands will have a simple linear relationship
with the phase lags between the total signals, so that the intrinsic phase lags
of the QPOs can be obtained by linear correction. The power density spectrum
(PDS) thus requires a multiplicative model to interpret the data. We briefly
discuss a physical scenario for interpreting the convolution. In this scenario,
the corona acts as a low-pass filter, the Green's function containing the noise
is convolved with the QPOs to form the low-frequency part of the PDS, while the
high-frequency part requires an additive component. We use a multiplicative PDS
model to fit the data observed by Insight-HXMT. The overall fitting results are
similar compared to the traditional additive PDS model. Neither the width nor
the centroid frequency of the QPOs obtained from each of the two PDS models
were significantly different, except for the r.m.s. of the QPOs. Our work thus
provides a new perspective on the coupling of noise and QPOs.
