PLL with automatic frequency adjustment

A PLL frequency synthesizer that is able to automatically set a proper operating mode of the voltage controlled oscillator is given. The voltage controlled oscillator is operable in a plurality of modes, each of which specifies a different operating frequency range of the voltage controlled oscillator. The appropriate mode is selected based on an error signal which is detected by a phase / frequency detector of the PLL frequency synthesizer. A Fenstekomparator is used to switch to adjacent modes, when the error signal exceeds the predetermined upper and lower limits of error voltage or falls.