Patent ID: 6842049

Claim:
An apparatus for detecting a difference between frequencies, the apparatus comprising: a beat waveform generating unit which generates a beat waveform signal having a frequency which is equal to a difference between frequencies of a reference clock signal and a target clock signal; a reference-clock dividing unit which divides the reference clock signal by N, where N is an integer, to generate a divided reference-clock signal; and a frequency comparing unit which compares frequencies of the beat waveform signal and the divided reference-clock signal, and generates a step out alarm signal which is a binary signal depending upon a polarity of a difference between the frequencies of the beat waveform signal and the divided reference-clock signal, wherein the frequency comparing unit includes, a first reference-clock dividing unit which divides the divided reference-clock signal by 2 to generate a divided-divided reference-clock signal; a first frequency comparing unit which compares frequencies of the beat waveform signal and the divided-divided reference-clock signal, and generates a first binary signal depending upon a polarity of a difference between the frequencies of the beat waveform signal and the divided-divided reference-clock signal; a second frequency comparing unit which compares frequencies of the beat waveform signal and the divided reference-clock signal, and generates a second binary signal depending upon a polarity of a difference between the frequencies of the beat waveform signal and the divided reference-clock signal; and a set-reset type flip-flop unit which controls generation and output of the step out alarm signal by receiving one of the first and second binary signals at a set terminal and receiving a remaining one of the first and second binary signals at a reset terminal.