Q. How oscillations start up in an oscillator?

 The noise voltage produced due to the random motion of electrons in resistors or in active devices provides the starting voltage for an oscillator. The noise voltage contains almost all sinusoidal frequencies and this gets amplified and appears at the output of the amplifier. This amplified output drives the feedback network which is either a resonant circuit or a phase shift network. Because of this feedback voltage the feedback signal is maximum at a particular frequency or the phase shift required for the positive feedback is correct at this frequency. This particular frequency is the frequency of the oscillator. Thus although the noise voltage contains all the frequencies the output of the oscillator will contain a single sinusoidal frequency.