A built-in self-test circuit for testing a voltage controlled oscillator comprises a voltage controlled oscillator, a buffer having an input coupled to an output of the voltage controlled oscillator and a radio frequency peak detector coupled to the output of the buffer. The radio frequency peak detector is configured to receive an ac signal from the voltage controlled oscillator and generate a dc value proportional to the ac signal at an output of the radio frequency peak detector. Furthermore, the output of the radio frequency peak detector generates a dc value proportional to an amplitude of the ac signal from the voltage controlled oscillator when the voltage controlled oscillator functions correctly. On the other hand, the output of the radio frequency peak detector is at zero volts when the voltage controlled oscillator fails to generate an ac signal.