  We study the frequency of the plasma oscillations of electron-positron pairs
created by the vacuum polarization in an uniform electric field with strength E
in the range 0.2 Ec < E < 10 Ec. Following the approach adopted in [1] we work
out one second order ordinary differential equation for a variable related to
the velocity from which we can recover the classical plasma oscillation
equation when E -> 0. Thereby, we focus our attention on its evolution in time
studying how this oscillation frequency approaches the plasma frequency. The
time-scale needed to approach to the plasma frequency and the power spectrum of
these oscillations are computed. The characteristic frequency of the power
spectrum is determined uniquely from the initial value of the electric field
strength. The effects of plasma degeneracy and pair annihilation are discussed.
