  Simple analytically solvable models are proposed exhibiting 1/f spectrum in
wide range of frequency. The signals of the models consist of pulses (point
process) which interevent times fluctuate about some average value, obeying an
autoregressive process with very small damping. The power spectrum of the
process can be expressed by the Hooge formula. The proposed models reveal
possible origin of 1/f noise, i.e., random increments of the time intervals
between pulses or interevent time of the process (Brownian motion in the time
axis).
