  We analyze the power spectral density of a signal composed of nonoverlapping
rectangular pulses. First, we derive a general formula for the power spectral
density of a signal constructed from the sequence of nonoverlapping pulses.
Then we perform a detailed analysis of the rectangular pulse case. We show that
pure $1/f$ noise can be observed until extremely low frequencies when the
characteristic pulse (or gap) duration is long in comparison to the
characteristic gap (or pulse) duration, and gap (or pulse) durations are
power-law distributed. The obtained results hold for the ergodic and weakly
nonergodic processes.
