The feed back factor in the oscillator refers to the output which is usually sinusoidal.
sinusoidal vs non sinusoidal
no limit
An oscillator requires a phase shift filter and an amplifier. There are several different designs, based on the requirements, such as sinusoidal or pulse train output. Please restate the question and provide more information as to the type of oscillator required.
A crystal oscillator is more commonly used for very small signals, such as those used in radio signals. Noisy AC is more commonly associated with power supplies. There are commercial units available for providing a more stable and uniform power supply. They generally consist of a rectifier unit supplying a solid state inverter. The sinusoidal output of these inverters are usually controlled by computer, which is in turn, controlled by a crystal oscillator.