  The spectrum of a CW field whose phase experiences a periodic sawtooth
modulation is analyzed. Two types of the sawtooth phase modulation are
considered. One is created by combining many harmonics of the fundamental
frequency. The second is produced by electro-optic modulator fed by the
relaxation oscillator, which generates a voltage slowly rising during charging
the energy storage capacitor and dropping fast due to discharge by a short
circuit. It is proposed to filter out the main spectral component of the
sawtooth phase modulated field. This filtering produces short pulses from CW
phase modulated field. Several filters are proposed to remove the selected
spectral component. It is shown that such a filtering is capable to produce a
train of short pulses. Duty cycle of this train is equal to the modulation
period and duration of the pulses can vary from $10^{-1}$ to $10^{-2}$ of the
period. Depending on the modulation frequency, the proposed method is capable
to produce pulses with duration ranging from nanoseconds to a fraction of
picosecond.
