Patent ID: 7429896

Claim:
A frequency stability measuring apparatus for measuring a frequency fluctuation of a signal outputted from a given signal source, the apparatus comprising: a reference signal output unit that outputs a reference frequency signal; a mixer that mixes a signal outputted from the signal source with the reference frequency signal outputted from the reference signal output unit and outputs a resultant signal; a filter that allows a low-frequency component of the signal outputted from the mixer to pass therethrough; a waveform-shaping circuit that shapes a waveform of a signal outputted from the filter; a counter that counts a number of pulses of a signal shaped by the waveform-shaping circuit; and a control unit that calculates average frequencies of the signal outputted from the filter within respective measurement periods of a predetermined length, and calculates and outputs a standard deviation of the average frequencies calculated, wherein the waveform-shaping circuit includes an amplifier that amplifies a waveform of the signal outputted from the filter and an amplitude limiter circuit that limits an amplitude of a signal outputted from the amplifier, wherein a slew rate of the signal inputted to the counter in a predetermined gate time is set close to a saturation point of a frequency resolution of the counter, and wherein the slew rate of the waveform-shaping circuit is set in a range of 0.1 V/μs to 100 V/μs.