Publisher Summary This chapter explains radio frequency oscillators and frequency synthesizers. Radio frequency oscillators produce radio frequency signals without an input signal. Frequency synthesizers are used to control and vary the frequency of an oscillator very precisely. An oscillator is a device which produces an output signal without requiring an external input signal. An amplifier can be made into an oscillator if its output signal is fed back into its own input terminals to provide an input signal of the correct amplitude and phase. In addition, the configuration of popular oscillators such as Wien, phase shift, Hartley, Colpitts, Clapp, crystal and the phase locked loop are discussed. The criteria for choosing an oscillator depends on frequency stability, amplitude stability, low noise, low power consumption, and size. Frequency stability is important because it enables narrow-band communication systems to be accurately fixed within a frequency band. An unstable frequency oscillator also behaves like an unstable f.m. modulator and produces unwanted f.m. noise. An amplitude unstable oscillator behaves like amplitude modulated modulator because it produces unwanted a.m. modulation noise. Even if the oscillation frequency and amplitude can be held precisely, it is inevitable that noise is produced in an oscillator because of transistor noise which includes flicker noise, shot noise and i/frequency noise. In other words, oscillator noise is inevitable but it should be kept as low as possible. Low power consumption and small size are important in portable equipment.