  Context. Solar-like oscillations exhibit a regular pattern of frequencies.
This pattern is dominated by the small and large frequency separations between
modes. The accurate determination of these parameters is of great interest,
because they give information about e.g. the evolutionary state and the mass of
a star.
  Aims. We want to develop a robust method to determine the large and small
frequency separations for time series with low signal-tonoise ratio. For this
purpose, we analyse a time series of the Sun from the GOLF instrument aboard
SOHO and a time series of the star KIC 5184732 from the NASA Kepler satellite
by employing a combination of Fourier and Hilbert transform.
  Methods. We use the analytic signal of filtered stellar oscillation time
series to compute the signal envelope. Spectral analysis of the signal envelope
then reveals frequency differences of dominant modes in the periodogram of the
stellar time series.
  Results. With the described method the large frequency separation $\Delta\nu$
can be extracted from the envelope spectrum even for data of poor
signal-to-noise ratio. A modification of the method allows for an overview of
the regularities in the periodogram of the time series.
