  In the present paper a self-consistent theory, explaining shape of the
observed phase-averaged radio spectrum in the frequency range from 100MHz to
10GHz is presented. The radio waves are assumed to be generated near the light
cylinder through the cyclotron resonance. The cyclotron instability provides
excitement of the electron-positron plasma eigen-waves, which come in radio
domain when the resonant particles are the most energetic primary beam
electrons. It is widely accepted that the distribution function of relativistic
particles is one-dimensional at the pulsar surface. The generated waves react
back on the resonant particles causing their diffusion in the perpendicular
direction to the magnetic field and violating the one-dimensionality, which
switches on the synchrotron radiation process. The synchrotron emission of the
beam electrons provides generation of high-energy $\gamma$-rays simultaneously
with the radio emission, that explains the observed pulse phase-coincidence in
these energy domains. The theory provides a power-law radio spectrum with the
spectral index equal to $3.7$, coming in agreement with the observations.
